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The biggest challenges to humans on long-term spaceflights


Historic in its own right, the Crew-1 mission will also demonstrate the rapid progress private spaceflight companies, especially SpaceX, have made in recent years. And though still far off on the horizon, regular human spaceflights beyond low-Earth orbit look inevitable over the coming decades.

To prepare for that eventuality, scientists, engineers, and medical professionals are tirelessly working to identify and mitigate the new challenges interplanetary travel poses to human health and well-being.

Researchers don’t know every possible issue that astronauts will encounter during an extended stay in lunar orbit, a roundtrip flight to Mars , or a journey to Jupiter’s icy moon Europa . But space medicine researchers are taking the data they do have — from analog long-term missions carried out on the ISS and human isolation experiments carried out on Earth, as well as animal studies — to prepare astronauts for what breaking the bonds of Earth will do to both their bodies and minds.

Here are just a few problems (and solutions) experts are considering to keep astronauts happy and health on their way to the Moon, Mars, and beyond.

Spaceflight Associated Neuro-ocular Syndrome (SANS)

On Earth, our bodies are constantly fighting gravity to pump fluids up to our heads. However, when you take gravity out of the equation, you get all pump and no dump — the cranial fluid doesn’t fully drain. The result is fluid pooling in both the face and the skull. This is known as Spaceflight Associated Neuro-ocular Syndrome , or SANS for short.

Puffiness in the face during the first few days of the mission quickly resolves itself as the body acclimates to microgravity. But for the cerebrospinal fluid that surrounds the brain, things don’t normalize as fast.

“We do know that after long-term spaceflight, it takes months to years for some of the brain changes to reverse,” Donna Roberts, a medical doctor and professor at the Medical University of South Carolina who specializes in the condition, tells Astronomy.

This additional pressure on the brain pushes it upwards against the skull, causing the fluid filled ventricles, or cavities, within the brain to expand. The pressure also pushes against astronauts’ eyeballs, causing vision changes that require glasses .

At present, though, researchers don’t fully understand all symptoms of SANS, or how to effectively combat them.

“In preliminary data, we have found some associations between changes in brain structure in astronauts with changes in posture, balance control, and changes in reaction times.” Roberts says. However, “these need to be studied in larger groups of astronauts.”

We still have plenty to learn about what impact SANS-induced cognitive changes will have on a crewed mission to Mars. But several ideas for preventative measures have been floated, such as creating artificial gravity or having astronauts wear special negative-pressure suits around their legs to draw fluids from their heads.


Radiation risks

Here on terra firma, we’re protected from cosmic radiation by Earth’s magnetic shield, which includes the Van Allen belts . Outside this zone, however, highly charged particles streaming from the Sun (or beyond) can slice through the bodies of astronauts , damaging cells.

This causes a whole host of issues, such as radiation sickness and increased lifetime risk of cancer. Right now, we mitigate the risk of radiation by shielding spacecraft and closely monitoring radiation exposure.

But research also suggests that cosmic radiation during a Mars trip might cause more immediate issues. Radiation damage to tissue may affect astronauts’ behavior, cognition, and general health while on the Moon or Mars, so it’s vital to shield a lunar or martian crew from as much of it as possible.

Some potential solutions including building habitats with an insulating layer of water, which effectively blocks radiation. Or by shielding habitats with a layer of readily available regolith . Or, perhaps, the answer is to tweak our own genomes.

Professor Nesrin Sarigul-Klijn, a specialist in aerospace engineering, believes that the best way to protect astronauts from radiation is a multi-pronged approach. “The most exciting countermeasures, in our eyes, are the ones that seek to unite hybrid approaches to radiation mitigation.”

Though the exact countermeasures that will be used during a Moon or Mars journey are still up for debate, Sarigul-Klign argues that combining both spacecraft shielding and biological methods, such as gene editing , will provide the most protection from hazardous space radiation

Hazards to the microbiome

Many aspects of our health, from good digestion, to stable mood, to healthy skin, are maintained by our microbiome — the complex cultures of bacteria, fungi, protozoa, and viruses that live on and inside us .

In space, however, the balance of these tiny helpers changes from what you’d expect on Earth. Studies following astronauts who have recently stayed on the ISS show that microbial diversity in the gut increases upon return to Earth, whereas in other areas of the body, such as the nose, it decreases. These microbiome changes may relate to common astronaut health issues, such as skin rashes and episodes of hypersensitivity .

While researchers are getting a grip on these changes in low-Earth orbit, some experts believe that we’re likely to run into different alterations of the microbiome during extended trips to other worlds.

“Cosmic radiation, for instance, may increase the mutation rate of commensal bacteria,” explains Hernan Lorenzi, an expert on the astronaut microbiome and profess at the J. Craig Venter Institute. These bacteria help instigate protective immune responses in the human body. So, if they mutate more often, it “may potentially lead to new bacterial behaviors and the way they interact with the human host,” adds Lorenzi.

But some of the best solutions for keeping spacefarers’ microbiomes healthy seem deceptively simple.

“Maintaining a healthy, balanced diet will be very important,” says Lorenzi. “Diet could be complemented with the use of prebiotics and probiotics.”But he also notes significant challenges, such as an astronaut needing antibiotics. Treating an astronaut with antibiotics can deplete the natural supply of probiotics in their microbiome.

However, according to a paper co-authored by Lorenzi, the answer to that problem might be fecal transplants : “One option for emergency in-flight antibiotic use may be to package astronauts’ stool into probiotic capsules while they are healthy on Earth. This would allow an astronaut to refresh their GI microbiome with a diverse set of organisms from their own healthy GI tract. Regardless of the source, one or more methods of rebalancing a damaged microbiome will be essential for long-duration space missions.”


Maintaining crew cohesion

Like any long-haul journey, an extended trip to the Moon, Mars, or beyond will be a lot more bearable if the crew members get along. And part of ensuring travel harmony is a good crew selection process.

Assembling a cohesive crew isn’t just a case of cherry-picking excellent astronauts; it requires considering the group’s dynamic as a whole. And to study the complex nature of isolated groups, researchers often turn to analog missions , or simulations of living on the Moon or Mars that are hosted elsewhere, like on Earth. By utilizing both analog missions and extensive social sciences research, space agencies across the world have honed their own unique, highly secretive methods for selecting the right astronaut crews.

But even if there existed a perfect crew selection method, some conflicts are unavoidable.

“Naturally there will be times of tension. But well-chosen individuals and groups can tackle these constructively,” explains Konstanin Chterev, Space Psychology Lead for the upcoming Lunark analog mission .

There’s only so much that can be done to iron out the frustrations of being confined in an inherently stressful environment with only a few other individuals for months on end. But one innovative solution may lie in having astronauts sleep through transit in a hibernation-like, or ‘torpor,’ state . “

At least in the relative near-term, we believe passengers and crews will undergo torpor with 10- to 20-day periods of inactivity [through] suppressed metabolism and body cooling, followed by short periods of activity [at] normal metabolic rate and body temperature,” says John Bradford, President and CEO of SpaceWorks Enterprises , which specialises in solving complex spaceflight challenges.

There is some evidence — from people surviving after “drowning” in frozen lakes to regularly used medical cooling techniques — that a hibernation-like state may well be achievable for humans. And as of right now, research is ongoing. But if confirmed — and controlled — such a stasis-based strategy may be vital to crews staying sane during extremely long spaceflights.

But as enthralling as the idea of hibernation may be, it’s doubtful it will stave off any major astronaut conflicts. And according to Chterev, that’s probably a good thing.

“It’s important to note that conflict can be constructive and useful, as well as destructive, depending on how it is handled,” he says, citing findings from analog missions. “Some crew come out of disagreements stronger than before.”

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Astronaut Bruce McCandless II approaches his maximum distance from the Earth-orbiting Space Shuttle Challenger in this 70mm photo from Feb. 7, 1984.

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The 12 Greatest Challenges for Space Exploration

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Humanity began in Africa. But we didn’t stay there, not all of us—over thousands of years our ancestors walked all over the continent, then out of it. And when they came to the sea, they built boats and sailed tremendous distances to islands they could not have known were there. Why?

Probably for the same reason we look up at the moon and the stars and say, “What’s up there? Could we go there? Maybe we could go there.” Because it’s something human beings do.

March 2016

Space is, of course, infinitely more hostile to human life than the surface of the sea; escaping Earth’s gravity entails a good deal more work and expense than shoving off from the shore. But those boats were the cutting-edge technology of their time. Voyagers carefully planned their expensive, dangerous journeys, and many of them died trying to find out what was beyond the horizon. So why keep doing it?

I could tell you about spin-off technologies, ranging from small products of convenience to discoveries that might feed millions or prevent deadly accidents or save the lives of the sick and injured.

I could tell you that we shouldn’t keep all our eggs in this increasingly fragile basket—one good meteor strike and we all join the non-avian dinosaurs. And have you noticed the weather lately?

I could tell you that it might be good for us to unite behind a project that doesn’t involve killing one another, that does involve understanding our home planet and the ways we survive on it and what things are crucial to our continuing to survive on it.

I could tell you that moving farther out into the solar system might be a good plan, if humanity is lucky enough to survive the next 5.5 billion years and the sun expands enough to fry the Earth.

I could tell you all those things: all the reasons we should find some way to live away from this planet, to build space stations and moon bases and cities on Mars and habitats on the moons of Jupiter. All the reasons we should, if we manage that, look out at the stars beyond our sun and say, “Could we go there? Maybe we could go there.”

It’s a huge, dangerous, maybe impossible project. But that’s never stopped humans from bloody-mindedly trying anyway.

Humanity was born on Earth. Are we going to stay here? I suspect—I hope—the answer is no. — Ann Leckie

Ann Leckie is the Hugo- and Nebula-award-winning author of Ancillary Justice .


problem: takeoff

Getting off Earth is a little like getting divorced: You want to do it quickly, with as little baggage as possible. But powerful forces conspire against you—specifically, gravity. If an object on Earth’s surface wants to fly free, it needs to shoot up and out at speeds exceeding 25,000 mph.

That takes serious oomph—read: dollars. It cost nearly $200 million just to launch the Mars Curiosity rover, about a tenth of the mission’s budget, and any crewed mission would be weighed down by the stuff needed to sustain life. Composite materials like exotic-metal alloys and fibered sheets could reduce the weight; combine that with more efficient, more powerful fuel mixtures and you get a bigger bang for your booster.

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But the ultimate money saver will be reusability. “As the number of flights increases, economies of scale kick in,” says Les Johnson, a technical assistant at NASA’s Advanced Concepts Office. “That’s the key to getting the cost to drop dramatically.” SpaceX’s Falcon 9 , for example, was designed to relaunch time and again. The more you go to space, the cheaper it gets. —Nick Stockton

problem: propulsion

Hurtling through space is easy. It’s a vacuum, after all; nothing to slow you down. But getting started? That’s a bear. The larger an object’s mass, the more force it takes to move it—and rockets are kind of massive. Chemical propellants are great for an initial push, but your precious kerosene will burn up in a matter of minutes. After that, expect to reach the moons of Jupiter in, oh, five to seven years. That’s a heck of a lot of in-flight movies. Propulsion needs a radical new method. Here’s a look at what rocket scientists now have, or are working on, or wish they had. —Nick Stockton


problem: space junk

Congratulations! You’ve successfully launched a rocket into orbit. But before you break into outer space, a rogue bit of broke-ass satellite comes from out of nowhere and caps your second-stage fuel tank. No more rocket.

This is the problem of space debris , and it’s very real. The US Space Surveillance Network has eyes on 17,000 objects—each at least the size of a softball—hurtling around Earth at speeds of more than 17,500 mph; if you count pieces under 10 centimeters, it’s closer to 500,000 objects. Launch adapters, lens covers, even a fleck of paint can punch a crater in critical systems.

Whipple shields—layers of metal and Kevlar—can protect against the bitsy pieces, but nothing can save you from a whole satellite. Some 4,000 orbit Earth, most dead in the air. Mission control avoids dangerous paths, but tracking isn’t perfect.

space junk

Pulling the sats out of orbit isn’t realistic—it would take a whole mission to capture just one. So starting now, all satellites will have to fall out of orbit on their own. They’ll jettison extra fuel, then use rocket boosters or solar sails to angle down and burn up on reentry. Put decommissioning programs in 90 percent of new launches or you’ll get the Kessler syndrome: One collision leads to more collisions until there’s so much crap up there, no one can fly at all. That might be a century hence—or a lot sooner if space war breaks out. If someone (like China?) starts blowing up enemy satellites, “it would be a disaster,” says Holger Krag, head of the Space Debris Office at the European Space Agency. Essential to the future of space travel: world peace. —Jason Kehe

problem: navigation

The Deep Space Network, a collection of antenna arrays in California, Australia, and Spain, is the only navigation tool for space. Everything from student-project satellites to the New Horizons probe meandering through the Kuiper Belt depends on it to stay oriented. An ultraprecise atomic clock on Earth times how long it takes for a signal to get from the network to a spacecraft and back, and navigators use that to determine the craft’s position.

But as more and more missions take flight, the network is getting congested. The switchboard is often busy. So in the near term, NASA is working to lighten the load. Atomic clocks on the crafts themselves will cut transmission time in half, allowing distance calculations with a single downlink. And higher-bandwidth lasers will handle big data packages, like photos or video messages.

The farther rockets go from Earth, however, the less reliable this method becomes. Sure, radio waves travel at light speed, but transmissions to deep space still take hours. And the stars can tell you where to go, but they’re too distant to tell you where you are. For future missions, deep-space navigation expert Joseph Guinn wants to design an autonomous system that would collect images of targets and nearby objects and use their relative location to triangulate a spaceship’s coordinates—no ground control required. “It’ll be like GPS on Earth,” Guinn says. “You put a GPS receiver on your car and problem solved.” He calls it a deep-space positioning system—DPS for short. —Katie M. Palmer


problem: radiation

Outside the safe cocoon of Earth’s atmosphere and magnetic field, subatomic particles zip around at close to the speed of light. This is space radiation, and it’s deadly. Aside from cancer, it can also cause cataracts and possibly Alzheimer’s.

When these particles knock into the atoms of aluminum that make up a spacecraft hull, their nuclei blow up, emitting yet more superfast particles called secondary radiation. “You’re actually making the problem worse,” says Nasser Barghouty, a physicist at NASA’s Marshall Space Flight Center.

radiation dose

A better solution? One word: plastics. They’re light and strong, and they’re full of hydrogen atoms, whose small nuclei don’t produce much secondary radiation. NASA is testing plastics that can mitigate radiation in spaceships or space suits.

Or how about this word: magnets. Scientists on the Space Radiation Superconducting Shield project are working on a magnesium diboride superconductor that would deflect charged particles away from a ship. It works at –263 degrees Celsius, which is balmy for superconductors, but it helps that space is already so damn cold. —Sarah Zhang

problem: food and water

Lettuce got to be a hero last August. That’s when astronauts on the ISS ate a few leaves they’d grown in space for the first time. But large-scale gardening in zero g is tricky. Water wants to float around in bubbles instead of trickling through soil, so engineers have devised ceramic tubes that wick it down to the plants’ roots. “It’s like a Chia pet,” says Raymond Wheeler, a botanist at Kennedy Space Center. Also, existing vehicles are cramped. Some veggies are already pretty space-efficient (ha!), but scientists are working on a genetically modified dwarf plum tree that’s just 2 feet tall. Proteins, fats, and carbs could come from a more diverse harvest—like potatoes and peanuts.

All that’s for naught, though, if you run out of water. (On the ISS, the pee-and-water recycling system needs periodic fixing, and interplanetary crews won’t be able to rely on a resupply of new parts.) GMOs could help here too. Michael Flynn, an engineer at NASA Ames Research Center, is working on a water filter made of genetically modified bacteria. He likens it to how your small intestine recycles what you drink. “Basically you are a water recycling system,” he says. “with a useful life of 75 or 80 years.” This filter would continually replenish itself, just like your innards do. —Sarah Zhang

problem: bone and muscle wasting

Weightlessness wrecks the body: It makes certain immune cells unable to do their jobs, and red blood cells explode. It gives you kidney stones and makes your heart lazy. Astronauts on the ISS exercise to combat muscle wasting and bone loss, but they still lose bone mass in space, and those zero-g spin cycles don’t help the other problems. Artificial gravity would fix all that.

In his lab at MIT, former astronaut Laurence Young is testing a human centrifuge: Victims lie on their side on a platform and pedal a stationary wheel as the whole contraption spins around. The resulting force tugs their feet—just like gravity, but awkward.

Young’s machine is too cramped to use for more than an hour or two a day, though, so for 24/7 gravity, the whole spacecraft will have to become a centrifuge. A spinning spaceship could be shaped like a dumbbell, with two chambers connected by a truss. As it gets easier to send more mass into space, designers could become more ambitious—but they don’t have to reinvent the wheel. Remember the station in 2001: A Space Odyssey ? The design has been around since 1903. —Sarah Zhang

problem: mental health

When physicians treat stroke or heart attack, they sometimes bring the patient’s temperature way down, slowing their metabolism to reduce the damage from lack of oxygen. It’s a trick that might work for astronauts too. Which is good, because to sign up for interplanetary travel is to sign up for a year (at least) of living in a cramped spacecraft with bad food and zero privacy—a recipe for space madness . That’s why John Bradford says we should sleep through it. President of the engineering firm SpaceWorks and coauthor of a report for NASA on long missions, Bradford says cold storage would be a twofer: It cuts down on the amount of food, water, and air a crew would need and keeps them sane. “If we’re going to become a multiplanet species,” he says, “we’ll need a capability like human stasis.” Sleep tight, voyagers. —Sarah Zhang


Planet, ho! You’ve been in space for months. Years, maybe. Now a formerly distant world is finally filling up your viewport. All you have to do is land. But you’re careening through frictionless space at, oh, call it 200,000 mph (assuming you’ve cracked fusion). Oh yeah, and there’s the planet’s gravity to worry about. If you don’t want your touchdown to be remembered as one small leap for a human and one giant splat for humankind, follow these simple steps. —Nick Stockton


problem: resources

When space caravans embark from Earth, they’ll leave full of supplies. But you can’t take everything with you. Seeds, oxygen generators, maybe a few machines for building infrastructure. But settlers will have to harvest or make everything else.

Luckily, space is far from barren. “Every planet has every chemical element in it,” says Ian Crawford, a planetary scientist at Birbeck, University of London, though concen­trations differ. The moon has lots of aluminum. Mars has silica and iron oxide. Nearby asteroids are a great source of carbon and platinum ores—and water, once pioneers figure out how to mine the stuff. If blasters and drillers are too heavy to ship, they’ll have to extract those riches with gentler techniques: melting, magnets, or metal-digesting microbes. And NASA is looking into a process that can 3-D-print whole buildings —no need to import special equipment.

In the end, a destination’s resources will shape settlements, which makes surveying the drop zone critical. Just think of the moon’s far side. “It’s been pummeled by asteroids for billions of years,” says Anita Gale, a space shuttle engineer. “Whole new materials could be out there.” Before humanity books a one-way ticket to Kepler-438b, it’ll have to study up. —Chelsea Leu


Dogs helped humans colonize Earth, but they’d survive on Mars about as well as we would. To spread out on a new world, we’ll need a new best friend: a robot.


See, settling takes a lot of grunt work, and robots can dig all day without having to eat or breathe. Theoretically, at least. Current prototypes— bulky, bipedal bots that mimic human physiognomy—can barely walk on Earth. So automatons will have to be everything we aren’t—like, say, a lightweight tracked bot with backhoe claws for arms. That’s the shape of one NASA machine designed to dig for ice on Mars: Its two appendages spin in opposite directions, keeping it from flipping over as it works.

Still, humans have a big leg up when it comes to fingers. If a job requires dexterity and precision, you want people doing it—provided they have the right duds. Today’s space suit is designed for weightlessness, not hiking on exoplanets. NASA’s prototype Z-2 model has flexible joints and a helmet that gives a clear view of whatever delicate wiring needs fixing. When the job’s done, just hop on an autonomous transporter to get home. Attaboy, Rover. —Matt Simon


problem: space is big

The fastest thing humans have ever built is a probe called Helios 2. It’s dead now, but if sound traveled in space, you’d hear it screaming as it whips around the sun at speeds of more than 157,000 miles per hour. That’s almost 100 times faster than a bullet, but even at that velocity it would take some 19,000 years to reach Earth’s first stellar neighbor, Alpha Centauri. It’d be a multigenerational ship, and nobody dreams of going to space because it’s a nice place to die of old age.

To beat the clock, you need power—and lots of it. Maybe you could mine Jupiter for enough helium-3 to fuel nuclear fusion—after you’ve figured out fusion engines. Matter-antimatter annihilation is more scalable, but smashing those pugilistic particles together is dangerous. “You’d never want to do that on Earth,” says Les Johnson, technical assistant for NASA’s Advanced Concepts Office, which works on crazy starship ideas. “You do that in deep space, so if you have an accident, you don’t destroy a continent.” Too intense? How about solar power? All you’d need is a sail the size of Texas.


Far more elegant would be hacking the universe’s source code—with physics. The theoretical Alcubierre drive would compress space in front of your craft and expand space behind it so the stuff in between—where your ship is—effectively moves faster than light. Tweaking the Alcubierre equations gets you a Krasnikov tube, an interstellar subway that shortens your return trip.

All aboard? Not quite. Humanity will need a few more Einsteins working at places like the Large Hadron Collider to untangle all the theoretical knots. “It’s entirely possible that we’ll make some discovery that changes everything,” Johnson says. “But you can’t count on that breakthrough to save the day.” If you want eureka moments, you need to budget for them. That means more cash for NASA— and the particle physicists. Until then, Earth’s space ambitions will look a lot like Helios 2: stuck in a futile race around the same old star. —Nick Stockton


A couple decades back, sci-fi author Kim Stanley Robinson sketched out a future utopia on Mars built by scientists from an overpopulated, overextended Earth. His Mars trilogy made a forceful case for colonization of the solar system. But, really, other than science, why should we go to space?

The need to explore is built into our souls, goes one argument—the pioneer spirit and manifest destiny. But scientists don’t talk about pioneers anymore. “You did hear that frontier language 20, 30 years ago,” says Heidi Hammel, who helps set exploration priorities at NASA. But since the New Horizons probe passed by Pluto last July, “we’ve explored every type of environment in the solar system at least once,” she says. Humans could still go dig in the dirt to study distant geology—but when robots can do it, well, maybe not.

As for manifest destiny? Historians know better. Western expansion was a vicious land grab, and the great explorers were mostly in it for resources or treasure. Human wanderlust expresses itself only in the service of political or economic will.

Of course, Earth’s impending destruction could provide some incentive. Deplete the planet’s resources and asteroid-belt mining suddenly seems reasonable. Change the climate and space provides room for humanity (and everything else).

But that’s a dangerous line of thinking. “It creates a moral hazard,” Robinson says. “People think if we fuck up here on Earth we can always go to Mars or the stars. It’s pernicious.” His latest book, Aurora , again makes a forceful case about settlement beyond the solar system: You probably can’t. As far as anyone knows, Earth is the only habitable place in the universe. If we’re going to leave this planet, let’s go because we want to—not because we have to. —Adam Rogers

This article appears in the March 2016 issue .

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Almost half of Moon missions fail. Why is space still so hard?

difficulties with space travel

Senior Lecturer in Physics, RMIT University

Disclosure statement

Gail Iles does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.

RMIT University provides funding as a strategic partner of The Conversation AU.

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In 2019, India attempted to land a spacecraft on the Moon – and ended up painting a kilometres-long streak of debris on its barren surface. Now the Indian Space Research Organisation has returned in triumph, with the Chandrayaan-3 lander successfully touching down near the south pole of Earth’s rocky neighbour.

India’s success came just days after a spectacular Russian failure , when the Luna 25 mission tried to land nearby and “ceased to exist as a result of a collision with the lunar surface”.

Read more: India's Chandrayaan-3 landed on the south pole of the Moon − a space policy expert explains what this means for India and the global race to the Moon

These twin missions remind us that, close to 60 years after the first successful “soft landing” on the Moon, spaceflight is still difficult and dangerous. Moon missions in particular are still a coin flip, and we have seen several high-profile failures in recent years.

Why were these missions unsuccessful and why did they fail? Is there a secret to the success of countries and agencies who have achieved a space mission triumph?

An exclusive club

The Moon is the only celestial location humans have visited (so far). It makes sense to go there first: it’s the closest planetary body to us, at a distance of around 400,000 kilometres.

Yet only four countries have achieved successful “soft landings” – landings which the spacecraft survives – on the lunar surface.

The USSR was the first. The Luna 9 mission safely touched down on the Moon almost 60 years ago, in February 1966. The United States followed suit a few months later, in June 1966, with the Surveyor 1 mission.

China was the next country to join the club, with the Chang'e 3 mission in 2013. And now India too has arrived, with Chandrayaan-3 .

Missions from Japan, the United Arab Emirates, Israel, Russia, the European Space Agency, Luxembourg, South Korea and Italy have also had some measure of lunar success with fly-bys, orbiters and impacts (whether intentional or not).

Crashes are not uncommon

On August 19 2023, the Russian space agency Roscosmos announced that “communication with the Luna 25 spacecraft was interrupted”, after an impulse command was sent to the spacecraft to lower its orbit around the Moon. Attempts to contact the spacecraft on August 20 were unsuccessful, leading Roscosmos to determine Luna 25 had crashed.

Despite more than 60 years of spaceflight experience extending from the USSR to modern Russia, this mission failed. We don’t know exactly what happened – but the current situation in Russia, where resources are stretched thin and tensions are high due to the ongoing war in Ukraine, may well have been a factor.

Read more: Russia has declared a new space race, hoping to join forces with China. Here's why that's unlikely

The Luna 25 failure recalled two high-profile lunar crashes in 2019.

In April that year, the Israeli Beresheet lander crash-landed after a gyroscope failed during the braking procedure, and the ground control crew was unable to reset the component due to a loss of communications. It was later reported a capsule containing microscopic creatures called tardigrades, in a dormant “cryptobiotic” state, may have survived the crash.

difficulties with space travel

And in September, India sent its own Vikram lander down to the surface of the Moon – but it did not survive the landing. NASA later released an image taken by its Lunar Reconnaissance Orbiter showing the site of the Vikram lander’s impact. Debris was scattered over almost two dozen locations spanning several kilometres.

Space is still risky

Space missions are a risky business. Just over 50% of lunar missions succeed . Even small satellite missions to Earth’s orbit don’t have a perfect track record, with a success rate somewhere between 40% and 70%.

We could compare uncrewed with crewed missions: around 98% of the latter are successful , because people are more invested in people. Ground staff working to support a crewed mission will be more focused, management will invest more resources, and delays will be accepted to prioritise the safety of the crew.

Read more: I'm training to become Australia's first woman astronaut. Here's what it takes

We could talk about the details of why so many uncrewed missions fail. We could talk about technological difficulties, lack of experience, and even the political landscapes of individual countries.

But perhaps it’s better to step back from the details of individual missions and look at averages, to see the overall picture more clearly.

The big picture

Rocket launches and space launches are not very common in the scheme of things. There are around 1.5 billion cars in the world, and perhaps 40,000 aeroplanes . By contrast, there have been fewer than 20,000 space launches in all of history.

Plenty of things still go wrong with cars, and problems occur even in the better-regulated world of planes, from loose rivets to computers overriding pilot inputs. And we have more than a century of experience with these vehicles, in every country on the planet.

So perhaps it’s unrealistic to expect spaceflight – whether it’s the launch stage of rockets, or the even rarer stage of trying to land on an alien world – to have ironed out all its problems.

We are still very much in the early, pioneering days of space exploration.

Monumental challenges remain

If humanity is ever to create a fully fledged space-faring civilisation, we must overcome monumental challenges .

To make long-duration, long-distance space travel possible, there are a huge number of problems to be solved. Some of them seem within the realm of the possible, such as better radiation shielding, self-sustaining ecosystems, autonomous robots, extracting air and water from raw resources, and zero-gravity manufacturing. Others are still speculative hopes, such as faster-than-light travel, instantaneous communication, and artificial gravity.

Read more: New warp drive research dashes faster than light travel dreams – but reveals stranger possibilities

Progress will be little by little, small step by slightly larger step. Engineers and space enthusiasts will keep putting their brainpower, time and energy into space missions, and they will gradually become more reliable.

And maybe one day we’ll see a time when going for a ride in your spacecraft is as safe as getting in your car.

Correction: a typing error in the original version of this article put the Surveyor 1 mission in 1996, rather than its actual year of 1966.

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  • Published: 05 November 2020

Red risks for a journey to the red planet: The highest priority human health risks for a mission to Mars

  • Zarana S. Patel   ORCID: orcid.org/0000-0003-0996-6381 1 , 2 ,
  • Tyson J. Brunstetter 3 ,
  • William J. Tarver 2 ,
  • Alexandra M. Whitmire 2 ,
  • Sara R. Zwart 2 , 4 ,
  • Scott M. Smith 2 &
  • Janice L. Huff   ORCID: orcid.org/0000-0003-4236-7698 5  

npj Microgravity volume  6 , Article number:  33 ( 2020 ) Cite this article

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  • Cardiovascular diseases
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NASA’s plans for space exploration include a return to the Moon to stay—boots back on the lunar surface with an orbital outpost. This station will be a launch point for voyages to destinations further away in our solar system, including journeys to the red planet Mars. To ensure success of these missions, health and performance risks associated with the unique hazards of spaceflight must be adequately controlled. These hazards—space radiation, altered gravity fields, isolation and confinement, closed environments, and distance from Earth—are linked with over 30 human health risks as documented by NASA’s Human Research Program. The programmatic goal is to develop the tools and technologies to adequately mitigate, control, or accept these risks. The risks ranked as “red” have the highest priority based on both the likelihood of occurrence and the severity of their impact on human health, performance in mission, and long-term quality of life. These include: (1) space radiation health effects of cancer, cardiovascular disease, and cognitive decrements (2) Spaceflight-Associated Neuro-ocular Syndrome (3) behavioral health and performance decrements, and (4) inadequate food and nutrition. Evaluation of the hazards and risks in terms of the space exposome—the total sum of spaceflight and lifetime exposures and how they relate to genetics and determine the whole-body outcome—will provide a comprehensive picture of risk profiles for individual astronauts. In this review, we provide a primer on these “red” risks for the research community. The aim is to inform the development of studies and projects with high potential for generating both new knowledge and technologies to assist with mitigating multisystem risks to crew health during exploratory missions.

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Spaceflight is a dangerous and demanding endeavor with unique hazards and technical challenges. Ensuring the overall safety of the crew—their physical and mental health and well-being—are vital for mission success. These are large challenges that are further amplified as exploration campaigns extend to greater distances into our solar system and for longer durations. The major health hazards of spaceflight include higher levels of damaging radiation, altered gravity fields, long periods of isolation and confinement, a closed and potentially hostile living environment, and the stress associated with being a long distance from mother Earth. Each of these threats is associated with its own set of physiological and performance risks to the crew (Fig. 1a ) that must be adequately characterized and sufficiently mitigated. Crews do not experience these stressors independently, so it is important to also consider their combined impact on human physiology and performance. This “space exposome” is a unifying framework that reflects the interaction of all the environmental impacts on the human body (Fig. 1b ) and, when combined with individual genetics, will shape the outcomes of space travel on the human system 1 , 2 .

figure 1

a The key threats to human health and performance associated with spaceflight are radiation, altered gravity fields, hostile and closed environments, distance from Earth, and isolation and confinement. From these five hazards stem the health and performance risks studied by NASA’s Human Research Program. b The space exposome considers the summation of an individual’s environmental exposures and their interaction with individual factors such as age, sex, genomics, etc. - these interactions are ultimately responsible for risks to the human system. Images used in this figure are courtesy of NASA.

The NASA Human Research Program (HRP) aims to develop and provide the knowledge base, technologies, and countermeasure strategies that will permit safe and successful human spaceflight. With agency resources and planning directed toward extended missions both within low Earth orbit (LEO) and outside LEO (including cis-lunar space, lunar surface operations, a lunar outpost, and exploration of Mars) 3 , HRP research and development efforts are focused on mitigation of over 30 categories of health risks relevant to these missions. The HRP’s current research strategy, portfolio, and evidence base are described in the HRP Integrated Research Plan (IRP) and are available online in the Human Research Roadmap, a managed tool used to convey these plans ( https://humanresearchroadmap.nasa.gov/ ). To determine research priorities, NASA uses an evidence-based risk approach to assess the likelihood and consequence (LxC), which gauges the level of each risk for a set of standard design reference missions (Fig. 2 ) 4 . Risks are assigned a rating for their potential to impact in-mission crew health and performance and for their potential to impact long-term health outcomes and quality of life. “Red” risks are those that are considered the highest priority due to their greatest likelihood of occurrence and their association with the most significant risks to crew health and performance for a given design reference mission (DRM). Risks rated “yellow” are considered medium level risks and are either accepted due to a very low probability of occurrence, require in-mission monitoring to be accepted, or require refinement of standards or mitigation strategies in order to be accepted. Risks rated “green” are considered sufficiently controlled either due to lower likelihood and consequence or because the current knowledge base provides sufficient mitigation strategies to control the risk to an acceptable level for that DRM. Milestones and planned program deliverables intended to move a risk rating to an acceptable, controlled level are detailed in a format known as the path to risk reduction (PRR) and are developed for each of the identified risks. The most recent IRP and PRR documents are useful resources for investigators during the development of relevant research approaches and proposals intended for submission to NASA HRP research announcements ( https://humanresearchroadmap.nasa.gov/Documents/IRP_Rev-Current.pdf ).

figure 2

NASA uses an evidence-based approach to assess likelihood and consequence for each documented human system risk. The matrix used for classifying and prioritizing human system risks has two sets of consequences—the left side shows consequences for in-mission risks while the right side is used to evaluate long-term health consequences (Romero and Francisco) 4 .

This work reviews HRP-defined high priority “red” risks for crew health on exploration missions: (1) space radiation health effects that include cancer, cardiovascular disease, and cognitive decrements (2) Spaceflight-Associated Neuro-ocular Syndrome (3) behavioral health and performance decrements, and (4) inadequate food and nutrition. The approaches used to address these risks are described with the aim of informing potential NASA proposers on the challenges and high priority risks to crew health and performance present in the spaceflight environment. This should serve as a primer to help individual proposers develop projects with high potential for generating both new knowledge and technology to assist with mitigating risks to crew health during exploratory missions.

Space radiation health risks

Outside of the Earth’s protective magnetosphere, crews are exposed to pervasive, low dose-rate galactic cosmic rays (GCR) and to intermittent solar particle events (SPEs) 5 . Exposures from GCR are from high charge (Z) and energy (HZE) ions, high-energy protons, and secondary protons, neutrons, and fragments produced by interactions with spacecraft shielding and human tissues. The main components of an SPE are low-to-medium energy protons. In LEO, the exposures are from GCR modulated by the Earth’s magnetic field and from trapped protons in the South Atlantic Anomaly. The absorbed doses for crews on the International Space Station (ISS) on 6- to 12-month missions range from ~30 to 120 mGy. Outside of LEO, without the protection offered by the Earth’s magnetosphere, absorbed radiation doses will be significantly higher. Estimates for a 1 year stay on the lunar surface range from 100 to 120 mGy, and 300 to 450 mGy for an ~3-year Mars mission (transit and surface stay) 6 . The exact dose a crewmember will receive is highly dependent on exact parameters of a given mission, such as detailed vehicle and habitat designs, and mission location and duration 7 . Time in the solar cycle is also a large factor contributing to crew exposure, with highest GCR exposure occurring during periods of minimum solar activity. The lowest GCR exposures occur during periods of maximum solar activity when the heightened magnetic activity of the Sun diverts some cosmic rays; however, during maximum solar activity, the probability of an SPE is higher 8 , 9 . SPEs, which vary in the magnitude and frequency, will obviously also contribute to total mission doses so it is important to note that total mission exposures are only estimates. Further information on the space radiation environment that astronauts will experience is discussed in Simonsen et al. 5 and Durante and Cucinotta 10 .

An important consideration for risk assessment is that the types of radiation encountered in space are very different from the types of radiation exposure we are familiar with here on Earth. HZE ions, although a small fraction of the overall GCR spectrum compared to protons, are more biologically damaging. They differ from terrestrial forms of radiation, such as X-rays and gamma-rays, in both the amount (dose) of exposure as well as in the patterns of DNA double-strand breaks and oxidative damage that they impart as they traverse through tissue and cells (Fig. 3 ) 5 . The highly energetic HZE particles produce complex DNA lesions with clustered double-stranded and single-stranded DNA breaks that are difficult to repair. This damage leads to distinct cellular behavior and intracellular signaling patterns that may be associated with altered disease outcomes compared to those for terrestrial sources of radiation 11 , 12 , 13 . As an example, persistently high levels of oxidative damage are observed in the intestine from mice examined 1 year after exposure to 56 Fe-ion radiation compared to gamma radiation and unirradiated controls 14 , 15 . The higher levels of residual oxidative damage in HZE ion-irradiated tissue is significant because of the association of oxidative stress and damage with the etiology of many human diseases, including cancer, cardiovascular and late neurodegenerative disorders. These types of alterations are believed to contribute to the higher biological effectiveness of HZE particles 10 , 11 .

figure 3

a HZE ions produce dense ionization along the particle track as they traverse a tissue and impart distinct patterns of DNA damage compared to terrestrial radiation such as X-rays. γH2AX foci (green) illuminate distinct patterns of DNA double-strand breaks in nuclei of human fibroblast cells after exposure to b gamma-rays, with diffuse damage, and c HZE ions with single tracks. Image credits: NASA ( a ) and Cucinotta and Durante 97 ( b and c ).

Within the HRP, the Space Radiation Element (SRE) has developed a research strategy involving both vertical translation and horizontal integration, as well as products focused on mitigating space radiation risks across all phases of a mission. Vertical translation involves the integration of benchtop research with preclinical studies and clinical data. Horizontal integration involves a multidisciplinary approach that includes a range of expertize from physicians to clinicians, epidemiologists to computational modelers 16 . The suite of tools includes computational models of the space radiation environment, mission design tools, models for risk projection, and tools and technologies for accurate simulation of the space radiation environment for radiobiology investigations. Ongoing research is focused on radiation quality, age, sex, and healthy worker effects, medical countermeasures to reduce or eliminate space radiation health risks, understanding the complex nature of individual sensitivity, identification and validation of biomarkers (translational, surrogate, predictive, etc.) and integration of personalized risk assessment and mitigation approaches. Owing to the lack of human data for heavy ion exposure on Earth and the complications of obtaining reliable data for space radiation health effects from flight studies, SRE conducts research at the NASA Space Radiation Laboratory (NSRL) at Brookhaven National Laboratory. The NSRL is a ground-based analog for space radiation, where a beamline and associated experimental facilities are dedicated to the radiobiology and physics of a range of ions from proton and helium ions to the typical GCR ions such as carbon, silicon, titanium, oxygen, and iron 5 , 17 , 18 .

Radiation carcinogenesis

Central evidence for association between radiation exposure and the development of cancer and other non-cancer health effects comes from epidemiological studies of humans exposed to radiation 19 , 20 , 21 , 22 . Scaling factors are used by NASA and other space agencies in the analysis of cancer (and other risks) to account for differences between terrestrial radiation exposures and cosmic radiation exposures 23 . The risk of radiation carcinogenesis is considered a “red” risk for exploration-class missions due to both the high likelihood of occurrence, as well as the high potential for detrimental impact on both quality of life and disease-free survival post flight. The major cancers of concern are epithelial in origin (particularly cancers of the lung, breast, stomach, colon, and bladder), as well as leukemias ( https://humanresearchroadmap.nasa.gov/Evidence/reports/Cancer.pdf ). Owing to the lack of human epidemiology directly relevant to the types of radiation found in space, current research utilizes a translational approach that incorporates rodent and advanced human cell-based model systems exposed to space radiation simulants along with comparison of molecular pathways across these systems to the human.

A key question that impacts risk assessment and mitigation is how HZE tumors compare to either radiogenic tumors induced by ground-based radiation or spontaneous tumors. As a unifying concept, NASA studies have sought to examine how space radiation exposure modifies the key genetic and epigenetic modifications noted as the hallmarks of cancer (Fig. 4 ) 24 , 25 , 26 , 27 . This approach provides data for development of translational scaling factors (relative biological effectiveness values, quality factors, dose-rate effectiveness factor) to relate the biological effects of space radiation to effects from similar exposures to ground-based gamma- and X-rays and extrapolation of results to large human epidemiology cohorts. It also supports acquisition of mechanistic information required for successful identification and implementation of medical countermeasure strategies to lower this risk to an acceptable posture for space exploration, and it is relevant for the future development of biologically based dose-response models and integrated systems biology approaches 25 . Cancer is a long-term health risk and although it is rated as “red”, most research in this area is currently delayed, as HRP research priorities focus on in-mission risks.

figure 4

Shown are the enabling characteristics and possible mechanisms of radiation damage that lead to these changes observed in all human tumors. (Adapted from Hanahanand Weinberg) 24 .

Risk of cardiovascular disease and other degenerative tissue effects from radiation exposure and secondary spaceflight stressors

A large number of degenerative tissue (non-cancer) adverse health outcomes are associated with terrestrial radiation exposure, including cardiovascular and cerebrovascular diseases, cataracts, digestive and endocrine disorders, immune system decrements, and respiratory dysfunction ( https://humanresearchroadmap.nasa.gov/Evidence/other/Degen.pdf ). For cardiovascular disease (CVD), a majority of the evidence comes from radiotherapy cohorts receiving high-dose mediastinal exposures that are associated with an increased risk for heart attack and stroke 28 . Recent evidence shows risk at lower doses (<0.5 Gy), with an estimated latency of 10 years or more 29 , 30 , 31 . For a Mars mission, preliminary estimates suggest that circulatory disease risk may increase the risk of exposure induced death by ~40% compared to cancer alone 32 . NASA is also concerned about in-flight risks to the cardiovascular system ( https://humanresearchroadmap.nasa.gov/Evidence/other/Arrhythmia.pdf ), when considering the combined effects of radiation exposure and other spaceflight hazards (Fig. 5 ) 33 . The Space Radiation Element is focused on accumulating data specific to the space radiation environment to characterize and quantify the magnitude of the degenerative disease risks. The current efforts are on establishing dose thresholds, understanding the impact of dose-rate and radiation quality effects, uncovering mechanisms and pathways of radiation-associated cardiovascular and cerebrovascular diseases, and subsequent risk modeling for astronauts. Uncovering the mechanistic underpinnings governing disease processes supports the development of specific diagnostic and therapeutic approaches, is a necessary step in the translation of insights from animal models to humans, and is the basis of personalized medicine approaches.

figure 5

In blue are the known risk factors for CVD and in black are the other spaceflight stressors that may also contribute to disease development. Image used in this figure is courtesy of NASA.

This information will provide a means to reduce the uncertainty in current permissible exposure limits (PELs), quantify the impact to disease-free survival years, and determine if additional protection or mitigation strategies are required. The research portfolio includes evaluation of current clinical standard-of-care biomarkers for their relevance as surrogate endpoints for radiation-induced disease outcomes. Studies are also addressing the possible role of chronic inflammation and increased oxidative stress in the etiology of radiation-induced CVD, as well as identification of key events in disease pathways, like endothelial dysfunction, that will guide the most effective medical countermeasures. Products include validated space radiation PELs, models to quantify the risk of CVD for the astronaut cohort, and countermeasures and evidence to inform development of appropriate recommendations to clinical guidelines for diagnosis and mitigation of this risk.

Elucidating the role that radiation plays in degenerative disease risks is problematic because multiple factors, including lifestyle and genetic influences, are believed to play a major role in the etiology of these diseases. This confounds epidemiological analyses, making it difficult to detect significant differences from background disease without a large study population 34 . This issue is especially significant in astronaut cohorts because those studies have small sample sizes 35 . There is also a general lack of experimental data that specifically addresses the role of radiation at low, space-relevant doses 36 . Selection of experimental models needs to be carefully considered and planned to ensure that the cardiovascular disease mechanisms and study endpoints are clinically relevant and translatable to humans 37 , 38 . Combined approaches using data from wildtype and genetically modified animal models with accelerated disease development will likely be necessary to elucidate mechanisms and generate the body of knowledge required for development of accurate permissible exposure limits, risk assessment models, and to develop effective mitigation approaches.

Risk of acute (in-flight) and late CNS effects from space radiation exposure

The possibility of acute (in-flight) and late risks to the central nervous system (CNS) from GCR and SPEs are concerns for human exploration of space ( https://humanresearchroadmap.nasa.gov/Evidence/reports/CNS.pdf ). Acute CNS risks may include altered neurocognitive function, impaired motor function, and neurobehavioral changes, all of which may affect human health and performance during a mission. Late CNS risks may include neurological disorders such as Alzheimer’s disease, dementia, or accelerated aging. Detrimental CNS changes from radiation exposure are observed in humans treated with high doses of gamma-rays or proton beams and are supported by a large body of experimental evidence showing neurocognitive and behavioral effects in animal models exposed to lower doses of HZE ions. Rodent studies conducted with HZE ions at low, mission-relevant doses and time frames show a variety of structural and functional alterations to neurons and neural circuits with associated performance deficits 39 , 40 , 41 , 42 , 43 , 44 . Fig. 6 shows an example of changes in dendritic spine density following HZE ion radiation. However, the significance and relationship of these results to adverse outcomes in astronauts is unclear, as similar decrements are not seen with comparable doses of terrestrial radiation. Therefore, scaling to human epidemiology data, as is done for cancer and cardiovascular disease, is not possible. It is also important to note that to date, no radiation-associated clinically significant operational or long-term deficits have been identified in astronauts receiving similar doses via long-duration ISS missions. It is clear that further development of standardized translational models, research paradigms, and appropriate scaling approaches are required to determine significance in humans 45 , 46 . In addition, elucidation of how space radiation interacts with other mission hazards to impact neurocognitive and behavioral health and performance is critical to defining appropriate PELs and countermeasure strategies. The current research approach is a combined effort of SRE, the human factors and behavioral performance element, and the human health countermeasures element in support of an integrated CBS (CNS/behavioral medicine/sensorimotor) plan ( https://humanresearchroadmap.nasa.gov/Risks/risk.aspx?i=99 ). Further information on this risk area is presented below in the Behavioral Health and Performance section and can also be found at the Human Research Roadmap.

figure 6

Representative digital images of 3D reconstructed dendritic segments (green) containing spines (red) in unirradiated (0 cGy) and irradiated (5 and 30 cGy) mice brains. Multiple comparisons show that total spine numbers (left bar chart) and spine density (right bar chart) are significantly reduced after exposure to 5 or 30 cGy of 16 O particles. Data are expressed as mean ± SEM. * P  < 0.05, ** P  < 0.01 versus control; ANOVA. Adapted from Parihar et al. 39 . Permission to reproduce open-source figure per the Creative Commons Attribution 4.0 International License. https://creativecommons.org/licenses/by/4.0 .

To summarize, the health risks posed by the omnipresent exposure to space radiation are significant and include the “red” risks of cancer, cardiovascular diseases, and cognitive and behavioral decrements. While research on the late health risk of cancer is currently delayed, research on the in-flight effects of radiation on the cardiovascular system and CNS within the context of the space exposome are considered the highest priority and are the focus of investigations. Major knowledge gaps include the effects of radiation quality, dose-rate, and translation from animal models to human systems and evaluation of the requirement for medical countermeasure approaches to reduce the risk.

Spaceflight-Associated Neuro-ocular Syndrome

The Risk of Spaceflight-Associated Neuro-ocular Syndrome (SANS), originally termed the Risk of Vision Impairment Intracranial Pressure (VIIP), was first discovered about 15 years ago. VIIP was the original name used because the syndrome most noticeably affects a crewmember’s eyes and vision, and its signs can appear like those of the terrestrial condition idiopathic intracranial hypertension (IIH; which is due to increased intracranial pressure). Over time, it was realized that the VIIP name required an update. Most notably, SANS is not associated with the classic symptoms of increased intracranial pressure in IIH (e.g., severe headaches, transient vision obscurations, double vision, pulsatile tinnitus), and it has never induced vision changes that meet the definition of vision impairment, as defined by the National Eye Institute. In 2017, VIIP was renamed to SANS, a term that welcomes additional pathogenesis theories and serves as a reminder that this syndrome could affect the CNS well beyond the retina and optic nerve.

SANS presents with an array of signs, as documented in the HRP Evidence Report ( https://humanresearchroadmap.nasa.gov/evidence/reports/SANS.pdf ). Primarily, these include edema (swelling) of the optic disc and retinal nerve fiber layer (RNFL), chorioretinal folds (wrinkles in the retina), globe flattening, and refractive error shifts 47 . Flight duration is thought to play a role in the pathogenesis of SANS, as nearly all cases have been diagnosed during or immediately after long-duration spaceflight (i.e., missions of 30 days duration or longer), although signs have been discovered as early as mission day 10 48 . Because of SANS, ocular data are nominally collected during ISS missions. For most ISS crewmembers, this testing includes optical coherence tomography (OCT), retinal imaging, visual acuity, a vision symptom questionnaire, Amsler grid, and ocular ultrasound (Fig. 7 ).

figure 7

Image courtesy of NASA.

From a short-term perspective (e.g., a 6-month ISS deployment), SANS presents four main risks to crewmembers and their mission: optic disc edema (ODE), chorioretinal folds, shifts in refractive error, and globe flattening 49 . Approximately 69% of the US crewmembers on the ISS experience a > 20 µm increase in peripapillary retinal thickness in at least one eye, indicating the presence of ODE. With significant levels of ODE, a crewmember can experience an enlargement of his/her blind spots and a corresponding loss in visual function. To date, blind spots are uncommon and have not had an impact on mission performance.

If chorioretinal folds are severe enough and located near the fovea (the retina associated with central vision), a crewmember may experience visual distortions or reduced visual acuity that cannot be corrected with glasses or contact lenses, as noted in the SANS Evidence Report. Despite a prevalence of 15–20% in long-duration crewmembers, chorioretinal folds have not yet impacted astronauts’ visual performance during or after a mission. An on-orbit shift in refractive error is due to a shortening of the eye’s axial length (distance between the cornea and the fovea), and it occurs in about 16% of crewmembers during long-duration spaceflight. This risk is mitigated by providing deploying crewmembers with several pairs of “Space Anticipation Glasses” (or contact lenses) of varying power. On-orbit, the crewmember can then select the appropriate lenses to restore best-corrected visual acuity. Approximately 29% of long-duration crewmembers experience a posterior eyeball flattening, which is typically centered around the insertion of the optic nerve into the globe. Globe flattening can induce chorioretinal folds and shifts in refractive error, posing the respective risks described above.

From a longer-term perspective, SANS presents two main risks to crewmembers: ODE and chorioretinal folds. It is unknown if a multi-year spaceflight (e.g., a Mars mission) will be associated with a higher prevalence, duration, and/or severity of ODE compared to what has been experienced onboard the ISS. Since the retina and optic nerve are part of the CNS, if ODE is severe enough, the crewmember risks a permanent loss of optic nerve and RNFL tissue and thus, a permanent loss of visual function. It should be stressed that no SANS-related permanent loss of visual function has yet been discovered in any astronauts.

For choroidal folds, improvement generally occurs post-flight in affected crewmembers; however, significant folds can persist for 10 or more years after long-duration missions. Using MultiColor Imaging and autofluorescence capabilities of the latest OCT device, it was discovered recently that one crewmember’s longstanding (>5 years) post-flight choroidal folds have induced disruption to its overlying retinal pigment epithelium (RPE) 50 . The RPE is a monolayer of pigmented cells located between the vascular-rich choroid and the photoreceptor outer segments. This layer forms the posterior blood-brain barrier for the retina and is essential for maintaining the health of the posterior retina via the transport of nutrients and fluids, among other key functions. If the RPE is damaged, it could potentially lead to a degeneration of the local retina and progress to vision impairment.

Recent long-duration head-down tilt studies have shown potential for recreating SANS signs in terrestrial cohorts 51 . However, SANS is considered a pathology unique to spaceflight. In microgravity, fluid within the body is free to redistribute uniformly. This means that much of the fluid that normally pools in a person’s feet and legs due to gravity can transfer upward towards the head and cause a general congestion of the cerebral venous system. The central pathogenesis theories of SANS are based on these facts, but the actual cause(s) and pathophysiology of SANS are yet unknown 49 . The most publicized theory for SANS has been that cerebral spinal fluid outflow might be impeded, causing an overall increase in intracranial pressure (ICP) 47 , 52 . Other potential mechanisms (see Fig. 8 ) include cerebral venous congestion or altered folate-dependent 1-carbon metabolism via a cascade of mechanisms that may ultimately increase ICP or affect the response of the eye to fluid shifts 53 , 54 . Potential confounding variables for SANS pathogenesis include resistive exercise, high-sodium dietary intake, and high carbon dioxide levels.

figure 8

Image created with BioRender.com.

Discovering patterns and trends in the SANS population has been difficult due to the relatively low number of crewmembers who have completed long-duration spaceflight. This is especially true for female astronauts. However, there is now enough evidence to state—emphatically—that SANS is not a male-only syndrome. OCT has been utilized onboard the ISS since late 2013, and it has revolutionized NASA’s ability to objectively detect and monitor SANS and build a high-resolution database of retinal and optic nerve head images. Through this technology, it has been recently discovered that that a majority of long-duration astronauts (including females) present with some level of ODE and engorgement of the choroidal vasculature 48 , 55 . The trends and patterns of these ocular anatomical changes may hold the key to deciphering the pathophysiology of SANS 48 , 55 .

Beginning in 2009 in response to SANS, all NASA crewmembers receive pre- and post-flight 3 Tesla magnetic resonance imaging of the brain and orbits. Based on these images, there is growing evidence that brain structural changes also occur during long-duration spaceflight. Most notably, a 10.7–14.6% ventricular enlargement (i.e., approximately a 2–3 ml increase) has been detected in astronauts and cosmonauts by multiple investigators 56 , 57 , 58 , 59 . On-orbit and post-flight cognitive testing have not revealed any systemic cognitive decrements associated with these anatomical changes. Moreover, additional research is required to determine if spaceflight-associated brain structural changes are related to ocular structural changes (i.e., SANS) or if the two are initiated by a common cause. Thus, until a relationship is established, SANS will be defined by ocular signs.

Future SANS medical operations, research, and surveillance will focus on: 1) determining the pathogenesis of the syndrome, 2) developing small-footprint diagnostic devices for expeditionary spaceflight, 3) establishing effective countermeasures, 4) monitoring for any long-term health consequences, and 5) discovering what factors make certain individuals more susceptible to developing the syndrome.

In summary, SANS is a top risk and priority to NASA and HRP. The primary SANS-related risk is ODE, due to the possibility of permanent vision impairment; however, choroidal folds also present a short- and long-term risk to astronaut vision. Shifts in refractive error are relatively common in long-duration missions, but crewmembers do not experience a loss of visual acuity if adequate correction is available. SANS affects female astronauts, not just males, although it is not yet known if SANS prevalence is equal between the sexes. There are no terrestrial pathologies identical to SANS, including IIH. Long-duration spaceflight is also associated with brain anatomical changes; however, it is not yet known whether these changes are related to SANS. Finally, the pathogenesis of SANS remains elusive; however, the main theories are related to increased intracranial pressure, ocular venous congestion, and individual anatomical/genetic variability.

Behavioral health and performance

The Risk of Adverse Cognitive and Behavioral Conditions and Psychiatric Disorders (BMed) focuses on characterizing and mitigating potential decrements in performance and psychological health resulting from multiple spaceflight hazards, including isolation and distance from earth. Spaceflight radiation is also recognized as contributing factor, particularly relative to a deep space planetary mission. The potential of additive or synergistic effects on the CNS resulting from simultaneous exposures to radiation, isolation and confinement, and prolonged weightlessness, is also of emerging concern ( https://humanresearchroadmap.nasa.gov/Risks/risk.aspx?i=99 ).

The official risk statement in the BMed Evidence Report notes, “ given the extended duration of future missions and the isolated, confined and extreme environments, there is a possibility that (a) adverse cognitive or behavioral conditions will occur affecting crew health and performance; and (b) mental disorders could develop should adverse behavioral conditions be undetected and unmitigated ” ( https://humanresearchroadmap.nasa.gov/Evidence/reports/BMED.pdf ). Primary outcomes for this risk include decrements in cognitive function, operational performance, and psychological and behavioral states, with the development of psychiatric disorders representing the least likely but one of the most consequential outcomes crew could experience in extended spaceflight. BMed is considered a “red” risk for planetary missions, given the long-duration of isolation, extended confinement, and exposure to additional stressors, including increased radiation exposure. The Human Factors and Behavioral Performance Element within HRP utilizes a research strategy that incorporates flight studies on astronauts, research in astronaut-like individuals and teams in ground analogs, and works with the Space Radiation Element to use animal models supporting research on combined spaceflight stressors.

While astronauts successfully accomplish their mission objectives and report very positive experiences living and working in space, some anecdotal accounts from current and past astronauts suggest that psychological adaptation in the long-duration spaceflight environment can be challenging. However, clinically significant operational decrements have not been documented to date, as noted in the BMed Evidence Report. Discrete events that have been documented include accounts of adverse responses to workload by Shuttle payload specialists, and descriptions of ‘hostile’ and ‘irritable’ crew in the 84-day Skylab 4 mission, as well as symptoms of depression reported on Mir by 2 of the 7 NASA astronauts.

Currently, potential stressors affiliated with missions to the ISS include extended periods of high workload and/or schedule shifting, physiological adaptation including fluid shifts caused by weightlessness and possibly, exposure to other environmental factors such as elevated carbon dioxide (see the BMed Evidence Report). While still physically isolated from home, the presence of the ISS in LEO facilitates a robust ground behavioral health and performance support team who offer services such as bi-weekly private psychological conferences and regular delivery of novel goods and surprises from home in crew care packages. Coupled with the relatively ample volume in the ISS, near-constant real-time communication with Earth, new crewmembers rotating periodically throughout missions, and relatively low levels of radiation exposure, —it is expected that behavioral challenges experienced today do not represent those that future crews will face during exploration missions.

Nevertheless, the few completed behavioral studies on the ISS suggest that subjective perceptions of stress increase over time for some crewmembers, as shown by an in-flight study collecting subjective ratings of well-being and objective measures of fatigue 60 . Notably, it was found that astronaut ratings of sleep quality and sleep duration (also measured through visual analog scales) were found to be inversely related to ratings of stress. Another in-flight investigation seeking to characterize behavioral responses to spaceflight is the “Journals” study by Stuster 61 . This investigation provided a systematic approach to examining a rich set of qualitative data by evaluating astronaut journal entries for temporal patterns of across different behavioral states over the course of a mission (Fig. 9 ). Based on findings, some categories suggest temporal patterns while other categories of outcomes do not suggest a pattern relative to time, which may be due to no temporal relationship between outcomes and time, and/or various contextual factors within missions that negate the presence of such a relationship (e.g., visiting crew). An overall assessment by Stuster of negative comments relative to positive comments over time suggests evidence of a third quarter phenomenon in Adjustment alone, a category which reflects individual morale 61 .

figure 9

Example bar graph showing distribution of journal entries related to general adjustment to the spaceflight enivronment during each quarter of an ISS mission 61 .

Other in-flight investigations support and expand upon contributors to increased stress on-orbit, including studies documenting reductions in sleep duration 62 , 63 and evaluation of crew responses to habitability and human factors during spaceflight 64 . While no studies have assessed potentially relevant mechanisms for behavioral or other reported symptoms, a recently completed investigation suggests neurostructural changes may be occurring in the spaceflight environment 56 . Magnetic resonance imaging scans were conducted on astronauts pre- and post-flight on both long-duration missions to the ISS or short-duration Shuttle missions. Assessments from a subgroup of participants ( n  = 12) showed a slight upward shift of the brain after all long-duration flights but not after short-duration flights ( n  = 6), and they also showed narrowing of cerebral spinal fluid spaces at the vertex after all long-duration flights ( n  = 6) and in 1 of 6 crew after short-duration flights. A retrospective analysis of free water volume in the frontal, temporal, and occipital lobes before versus after spaceflight suggests alterations in free water distribution 65 . Whether there is a functionally relevant outcome as a result of such changes remains to be determined. Hence, while certain aspects of the spaceflight environment have been shown to increase some behavioral responses (e.g., reduced sleep owing to workload), the direct role of spaceflight-specific factors (such as fluid shifts and weightlessness) on behavioral outcomes or functional performance has not yet been established.

Future long-duration missions will pose threats to behavioral health and performance, such as extreme confinement in a small volume and communication delays, that are distinct from what is currently experienced on missions to the ISS. Analog research is concurrently underway to help further characterize the likelihood and consequence of an adverse behavioral outcome, and the effectiveness of potential countermeasures. Ground analogs, such as the Human Exploration Research Analog (HERA) at NASA Johnson Space Center, provide a test bed where controlled studies of small teams for periods up to 45 days, can be implemented (Fig. 10 ). HERA can be used to provide scenarios and environments analogous to space (e.g., isolation and confinement, communication delays, space food, and daily tasks and schedules) to investigate their effects on behavioral health, human factors, exploration medical capabilities, and communication and autonomy. Research in locations such as Antarctica also offer a unique opportunity to conduct research in less controlled but higher fidelity conditions. In general, these studies show an increased risk in deleterious effects such as decreased mood and increased stress, and in some instances, psychiatric outcomes (see the BMed Evidence Report).

figure 10

HERA is used to simulate environments and mission scenarios analogous to spaceflight to investigate a variety of behavioral and human factors issues. Images courtesy of NASA.

In 2014, Basner and colleagues 62 completed an assessment of crew health and performance in a 520-day mission at an isolation chamber in Moscow at the Institute for Biomedical Problems (IBMP). During this simulated mission to Mars, the crew of six completed behavioral questionnaires and additional testing weekly. One of six (20%) crew reported depressive symptoms based on the Beck Depression Inventory in 93% of mission weeks, which reached mild-to-moderate levels in >10% of mission weeks. Additional indications of changes in mood were observed via the Profile of Mood States. Additionally, two crewmembers who had the highest ratings of stress and physical exhaustion accounted for 85% of the perceived conflicts, and other crew demonstrated dysregulation in their circadian entrainment and sleep difficulties. Two of the six crewmembers reported no adverse behavioral symptoms during the missions 62 . Building on this work, the NASA HRP and the IBMP have ongoing studies in the SIRIUS project, a series of long-duration ground-analog missions for understanding the effects of isolation and confinement on human health and performance ( http://www.nasa.gov/analogs/nek/about ).

Finally, more recent research in the HERA analog at Johnson Space Center is underway to assess not only individual, psychiatric outcomes but also changes in team dynamics and team performance over time (Fig. 10 ). A recent publication reported that conceptual team performance (e.g., creativity) seems to decrease over time, while performance requiring cognitive function and coordinated action improved 66 . While results from additional team studies in HERA are currently under review, the Teams Risk Evidence Report ( https://humanresearchroadmap.nasa.gov/Evidence/reports/Team.pdf ) provides a thorough overview of the evidence surrounding team level outcomes.

In summary, evidence from spaceflight and spaceflight analogs suggests that the BMed Risk poses a high likelihood and high consequence risk for exploration. Given the possible synergistic effects of prolonged isolation and confinement, radiation exposure, and prolonged weightlessness, mitigating such enhanced risks faced by future crews are of highest priority to the NASA HRP.

Inadequate food and nutrition

Historically, nutrition has driven the success—and often the failure—of terrestrial exploration missions. For space explorers, nutrition provides indispensable sustenance, provides potential countermeasures to some of the negative effects of space travel on human physiology, and also presents a multifaceted risk to the health and safety of astronauts ( https://humanresearchroadmap.nasa.gov/Evidence/other/Nutrition-20150105.pdf ).

At a minimum, the need to prevent nutrient deficiencies is absolute. This was proven on voyages during the Age of Sail, where scurvy—caused by vitamin C deficiency— killed more sailors than all other causes of death. On a closed (or even semi-closed) food system, the risk of nutrient deficiency is increased. On ISS missions, arriving vehicles typically bring some fresh fruits and/or vegetables to the crew. While limited in volume and shelf-life, these likely provide a valuable source of nutrients and phytochemicals every month or two. One underlying concern is that availability of these foods may be mitigating nutrition issues of the nominal food system, and without this external source of nutrients on exploration-class missions, those issues will be more likely to surface.

As a cross-cutting science, nutrition interfaces with many, if not all, physiological systems, along with many of the elements associated with space exploration, including the spacecraft environment (Fig. 11 ). Thus, beyond the basics of preventing deficiency of specific nutrients, at best, nutrition can serve as a countermeasure to mitigate risks to other systems. Conversely, at worst, diet and nutrition can exacerbate risks to other physiological systems and crew health. For example, many of the diseases of concern as related to space exploration are nutritionally modifiable on Earth, including cancer, cardiovascular disease, osteoporosis, sarcopenia, and cataracts.

figure 11

Many of the physiological systems and performance characteristics that are touched by nutrition are shown in white text, while the unique elements of spacecraft and space exploration are shown in red text.

The NASA Nutritional Biochemistry Laboratory approaches astronaut health with both operational and research efforts. These efforts aim to keep current crews healthy while working to understand and define optimal nutrition for future crews, to maximize performance and overall health while minimizing damaging effects of spaceflight exposure.

A Clinical Nutrition Assessment is conducted for ISS astronauts dating back to ISS Expedition 1 67 , 68 , which includes pre- and post-flight biochemical analyses conducted on blood and urine samples, along with in-flight monitoring of dietary intake and body mass. The biochemical assessments include a wide swath of nutritional indicators such as vitamins, minerals, proteins, hematology, bone markers, antioxidant markers, general chemistry, and renal stone risk. These data are reported to the flight surgeon soon after collection for use in the clinical care of the astronaut. Initial findings from the Clinical Nutritional Assessment protocol identified evidence of vitamin D deficiency, altered folate status, loss of body mass, increased kidney stone risk, and more 69 , 70 . These initial findings led to several research efforts (described below), including the Nutritional Status Assessment flight project, and research in the Antarctic on vitamin D supplementation 71 , 72 .

In addition to in-flight dietary intake monitoring, research to understand the impact and involvement of nutrition with other spaceflight risks such as bone loss and visual impairments, and interaction with exercise and spacecraft environment, are performed by the Nutrition Team using both flight and ground-analog research efforts. Tracking body mass is a very basic but nonetheless indispensable element of crew health 73 . Loss of body mass during spaceflight and in ground analogs of spaceflight is associated with exacerbated bone and muscle loss, cardiovascular degradation, increased oxidative stress, and more 70 , 73 , 74 . Historically, it was often assumed that some degree of body mass loss was to be expected, and that this was a typical part of adaptation to microgravity. Fluid loss is often assumed to be a key factor, but research has documented this to be a relatively small contributor, of approximately 1% of weight loss being fluid 74 , 75 . While on average, crewmembers on ISS missions have lost body mass over the course of flight, not all do 74 . Importantly, those that did not lose body mass managed to maintain bone mineral density (discussed below) 76 .

Bone loss has long been a concern for space travelers 77 , 78 , 79 , 80 , 81 . It has been shown that an increase in bone resorption was the likely culprit and that bone formation was largely unchanged in microgravity or ground analogs 77 , 78 , 79 . The search for a means to counteract this bone loss, and this hyper-resorptive state specifically, has been extensive. The potential for nutrition to mitigate this bone loss was identified early but studies of increasing intakes of calcium, or fluoride, or phosphate, were unsuccessful 74 , 77 , 79 , 82 , 83 , 84 .

Exercise provides a multisystem countermeasure, and heavy resistive exercise specifically provides for loading of bone to help mitigate weightlessness-induced bone loss.

In evaluating the data from astronauts using the first “interim” resistive exercise device (iRED) on ISS compared to a later, “advanced” resistive exercise device (ARED) (Fig. 12 ), it was quickly realized that exercise was not the only difference in these two groups of astronauts. ARED crews had better dietary intakes (as evidenced by maintenance of body mass) and better vitamin D status as a result of increased dose of supplementation and awareness of the importance of these supplements starting in 2006 76 . Bone mineral density was protected in these astronauts 76 , proving that diet and exercise are a powerful countermeasure combination. Follow-on evaluations showed similar results and further that the effects of microgravity exposure on bone health in men and women were similar 85 despite differences in pre-flight bone mass.

figure 12

Sunita Williams exercising on the iRED ( a ), and on a later mission, Sandy Magnus exercises on the much improved ARED device ( b ). Images courtesy of NASA.

From a purely nutrition perspective, ISS and associated ground analog research has identified several specific dietary effects on bone health. Fish intake, likely secondary to omega-3 fatty acid intake, is beneficial for bone health 86 . Conversely, high intakes of dietary protein 87 , 88 , iron 89 and sodium 90 are detrimental to bone. The mechanism of the effect of protein and sodium on bone are likely similar, with both contributing to the acidogenic potential of the diet, leading to bone dissolution 91 , 92 . This effect was recently documented in a diet and bone health study on ISS, where the acidogenic potential of the diet correlated with post-flight bone losses 93 . The data from terrestrial research, along with the more limited spaceflight research, clearly identifies nutrition as important in maintenance of bone health and in the mitigation of bone loss. While initial evaluations of dietary quality and health are underway at NASA, much work remains to document the full potential of nutrition to mitigate bone loss and other disease processes in space travelers.

Another health risk with nutrition underpinnings is SANS, which was described earlier. When this issue first arose, an examination of data from the aforementioned ISS Nutrition project was conducted. This analysis revealed that affected crewmembers had significantly higher circulating concentrations of homocysteine and other one-carbon pathway metabolites when compared to non-cases and that these differences existed before flight 53 . Many potential confounding factors were ruled out, including: sex, kidney function, vitamin status, and coffee consumption, among others. After identifying differences in one-carbon biochemistry, the next logical step was to examine the genetics—single-nucleotide polymorphisms (SNPs)—involved in this pathway as possible causes of the biochemical differences, but perhaps also their association with the astronaut ocular pathologies. An initial study examined a small set of SNPs—five to be exact—and when the data were statistically modeled, it was found that B-vitamin status and genetics were significant predictors of many of the observed ophthalmic outcomes in astronauts 94 . Interestingly, the same SNPs identified in astronauts to be associated with ophthalmic changes after flight were associated with greater changes in total retina thickness after a strict head-down tilt with 0.5% CO 2 bed rest study 54 . A follow-on study is underway to evaluate a much broader look at one-carbon pathway and associated SNPs, potentially to help better characterize this relationship.

A hypothesis was developed to plausibly link these genetics and biochemical differences with these ophthalmic outcomes, as there is no existing literature regarding such a relationship. This multi-hit hypothesis posits that one-carbon pathway genetics is an indispensable factor, and that the combination with one or more other factors (e.g., fluid shifts, carbon dioxide, radiation, endocrine effects) lead to these pathologies. This has been detailed in a hypothesis paper 95 and in a recent review 96 . In brief, the hypothesis is that genetics and B-vitamin status contribute to endothelial dysfunction, as folate (and other B-vitamins) play critical roles in nitric oxide synthesis and endothelial function. A disruption in nitric oxide synthesis can also lead to an activation of matrix metalloproteinase activation, increasing the turnover and breakdown of structural elements of the sclera, altering retinal elasticity and increasing susceptibility to fluid shifts to induce ophthalmic pathologies like optic disc edema and choroidal folds 54 . This is likely exacerbated cerebrally due to limitations of transport of B-vitamins across the blood-brain barrier. In or around the orbit, endothelial dysfunction, oxidative stress, and potentially individual anatomical differences contribute to leaky blood vessels, and subsequent edema. This can impinge on cerebrospinal fluid drainage from the head, increasing those fluid pressures, which can impinge upon the optic nerve and eye itself, yielding the aforementioned ophthalmic pathologies. These are hypotheses proposed as starting points for further research. Given the irrefutable biochemical and genetic findings to date, this research should be a high priority to either prove or dismiss these as contributing factors in SANS to mitigate that “red” risk.

Another intriguing element from this research is that there is a clinical population that has many of the same characteristics of affected astronauts (or characteristics that they are purported to have), and that is women with polycystic ovary syndrome (PCOS) 95 , 96 . Women with PCOS have higher circulating homocysteine concentrations (as do their siblings and fathers), and also have cardiovascular pathology, including endothelial dysfunction. Studies are underway between NASA and physicians at the Mayo Clinic in Minnesota to evaluate this further. If validated, women with PCOS might represent an analog population for astronaut ocular issues, and research to counteract this could benefit both populations 87 . This research may lead to the identification of one-carbon pathway genetic influences on cardiovascular function in astronauts (and women with PCOS). This information will not be used in any sort of selection process, for several reasons, but as a means to identify countermeasures. Given the effects are intertwined with vitamin status, and likely represent higher individual vitamin requirements, targeted B-vitamin supplementation is the most obvious, and lowest risk, countermeasure that needs to be tested. There is tremendous potential for nutrition research to solve one of the key risks to human health on space exploration missions.

To summarize, nutrition is a cross-cutting field that has influence on virtually every system in the body. While we need to understand nutrition to avoid frank deficiencies, we need to understand how optimizing nutrition might also help mitigate other spaceflight-induced human health risks. Examples of this are myriad, ranging from effects of dietary intake on cognition, performance, and morale, inadequate intake on cardiovascular performance, excess nutrient intakes, leading to excess storage and increased oxidative stress, nutrient insufficiencies, leading to bone loss, insufficient fruit and vegetable intake on bone health, radiation protection, and cardiovascular health, to name a just few. Throughout history, nutrition has served, or failed, many a journey to explore. We need to dare to use and expand our twenty-first century knowledge of nutrition, uniting medical and scientific teams, to enable future exploration beyond LEO, while simultaneously benefitting humanity.

The NASA Human Research Program is focused on developing the tools and technologies needed to control the high priority “red” risks to an acceptable level—a great challenge as the risks do not exist in the vacuum of space as standalone entities. They are inherently interconnected and represent the intersection points where the five hazards of spaceflight overlap, and nature meets nurture. This is the space exposome: the total sum of spaceflight and lifetime exposures and how they relate to individual genetics and determine the whole-body outcome. The space exposome will be an important unifying concept as the hazards and risks of spaceflight are evaluated in a systems biology framework to fully uncover the emergent effects of the extraterrestrial experience on the human body. This framework will provide a path forward for mitigating detrimental health and performance outcomes that may stand in the way of successful, long-duration space travel, especially as NASA plans for a return to the Moon, to stay, and beyond to Mars.

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This review was supported in part by a grant to Dr. Patel from the Translational Research Institute for Space Health (TRISH) from the Baylor College of Medicine (The Red Risk School). It was also supported by funding through NASA Human Health and Performance Contract #NNJ15HK11B (Z.S.P., S.R.Z., J.L.H.) and NASA directly (T.J.B., W.J.T., A.M.W., S.M.S., J.L.H.).

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Zarana S. Patel, William J. Tarver, Alexandra M. Whitmire, Sara R. Zwart & Scott M. Smith

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Drs. Z.S.P. and J.L.H. compiled and edited the overall manuscript and drafted the radiation risk overviews. Drs. T.J.B. and W.J.T. drafted the SANS risk overview, Dr. A.M.W. drafted the behavioral health risks overview, and Drs. S.R.Z. and S.M.S. drafted the nutrition risk overview. Data are available upon request.

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Patel, Z.S., Brunstetter, T.J., Tarver, W.J. et al. Red risks for a journey to the red planet: The highest priority human health risks for a mission to Mars. npj Microgravity 6 , 33 (2020). https://doi.org/10.1038/s41526-020-00124-6

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Astronaut : someone who has been trained to be a crew member of a spacecraft. ... more

Capsule : the part of a spacecraft that has been designed to hold the crew and survive re-entry into the Earth's atmosphere. ... more

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Challenges to Human Spaceflight

You wake up on day 180 of your mission, working on the International Space Station. For the last 6 months, you’ve been growing lettuce plants in the low-gravity environment of space. You did very well, and the next group of astronauts will continue the studies of your lettuce plants. But for now, it is time for you to return to Earth.

Astronaut being carried to the medical tent after landing

Our bodies are not made for low-gravity environments. Our muscles, bones, hearts, and other systems function differently in space than they do here on Earth. For example, because our bodies are adapted to the pull of gravity on Earth’s surface, we have body systems that move fluid from our legs (where it can pool due to gravity) to our chests and heads.

An astronaut exercising on a treadmill (chained down) in space

Long-term Effects of Space Travel

Longer term effects of low gravity include bone and muscle loss. Our bodies rely on gravity to tell which way to build our bones. In space, with no gravity to supply this direction, these bone growth systems break down. Also, astronauts typically don’t use certain muscle groups to support their bodies in microgravity. This causes a large amount of muscle to be lost in a short time. Astronauts can lose up 1% of their bone mass per month and as much as 20% of their muscle mass in about a week! These effects can be offset a little by rigorous daily exercise in space. Future human activity in space will require us to create new forms of exercise, including low gravity sports. Can you think of any sports that might be fun in low gravity?

Very long missions can also introduce health risks from cosmic radiation. This hazard has not been well-studied because it only affects humans after a very long time outside of the Earth’s magnetic field. The main danger from this type of radiation is the breaking of DNA in human cells.  We may be able to reduce this risk in the future by adding more shielding to spacecraft.

An astronaut sleeping in space (strapped in)

Additional images via Wikimedia Commons. Image of Anousheh Ansari by NASA.

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Space Travel’s Hidden Impact: Long Missions Trigger Significant Changes in Astronauts’ Brains

By University of Florida July 6, 2023

MRI Brain Image

New research examined how the human brain reacts to space travel, indicating that astronauts should wait approximately three years after lengthy space missions for their brains to fully recover. The research analyzed brain scans from 30 astronauts and revealed significant ventricle expansion in those who underwent missions of six months or longer. Ventricles, which are brain cavities filled with cerebrospinal fluid, expand due to fluid shifts caused by the absence of gravity.

A study has found that long-duration space travel leads to significant expansion of brain ventricles, requiring approximately three years for full recovery. This discovery could impact future mission planning and the timing of repeated space travel.

As we enter a new era in space travel, a study looking at how the human brain reacts to traveling outside Earth’s gravity suggests frequent flyers should wait three years after longer missions to allow the physiological changes in their brains to reset.

Researchers studied brain scans of 30 astronauts from before and after space travel. Their findings, reported today in  Scientific Reports , reveal that the brain’s ventricles expand significantly in those who completed longer missions of at least six months, and that less than three years may not provide enough time for the ventricles to fully recover.

Ventricles are cavities in the brain filled with cerebrospinal fluid, which provides protection, nourishment and waste removal to the brain. Mechanisms in the human body effectively distribute fluids throughout the body, but in the absence of gravity, the fluid shifts upward, pushing the brain higher within the skull and causing the ventricles to expand.

“We found that the more time people spent in space, the larger their ventricles became,” said Rachael Seidler, a professor of applied physiology and kinesiology at the University of Florida and an author of the study. “Many astronauts travel to space more than one time, and our study shows it takes about three years between flights for the ventricles to fully recover.”

Seidler, a member of the Norman Fixel Institute for Neurological Diseases at UF Health, said based on studies so far, ventricular expansion is the most enduring change seen in the brain resulting from spaceflight.

“We don’t yet know for sure what the long-term consequences of this is on the health and behavioral health of space travelers,” she said, “so allowing the brain time to recover seems like a good idea.”

Of the 30 astronauts studied, eight traveled on two-week missions, 18 were on six-month missions, and four were in space for approximately one year. The ventricular enlargement tapered off after six months, the study’s authors reported.

“The biggest jump comes when you go from two weeks to six months in space,” Seidler said. “There is no measurable change in the ventricles’ volume after only two weeks.”

With increased interest in space tourism in recent years, this is good news, as shorter space junkets appear to cause little physiological changes to the brain, she said.

While researchers cannot yet study astronauts who have been in space much longer than a year, Seidler said it’s also good news that the expansion of the brain’s ventricles levels off after about six months.

“We were happy to see that the changes don’t increase exponentially, considering we will eventually have people in space for longer periods,” she said.

The results of the study, which was funded by NASA , could impact future decision-making regarding crew travel and mission planning, Seidler said.

For more on this research, see The Dark Side of Multiple Spaceflights on Human Brain Structure .

Reference: “Impacts of Spaceflight Experience on Human Brain Structure” by Heather R. McGregor, Kathleen E. Hupfeld, Ofer Pasternak, Nichole E. Beltran, Yiri E. De Dios, Jacob J. Bloomberg, Scott J. Wood, Ajitkumar P. Mulavara, Roy F. Riascos, Patricia A. Reuter-Lorenz and Rachael D. Seidler, 8 June 2023, Scientific Reports . DOI: 10.1038/s41598-023-33331-8

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1 comment on "space travel’s hidden impact: long missions trigger significant changes in astronauts’ brains".

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This is a nothing story. Why even blab about it if there isn’t more substance than repeatedly stating, “LONG MISSIONS TRIGGER SIGNIFICANT CHANGES IN ASTRONAUTS’ BRAINS”? Thanks for leading me to a bunch of advertisements.

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The Barriers To Achieving Interstellar Travel And How To Overcome Them

  • Last updated May 12, 2024
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Paolo Barresi

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what is keeping us from interstellar travel

For centuries, humanity has looked up at the stars and wondered what lies beyond our solar system. The possibility of interstellar travel, of exploring new worlds and encountering alien civilizations, has captured the imaginations of scientists, writers, and dreamers alike. However, despite our technological advancements, there are several formidable barriers standing in the way of achieving interstellar travel. From the mind-boggling distances that separate us from other star systems to the immense energy requirements necessary to reach relativistic speeds, the obstacles seem insurmountable. Yet, as we push the boundaries of our understanding and harness the power of innovation, there is hope that we can overcome these challenges and venture into the great unknown. In this article, we will explore some of the key barriers to achieving interstellar travel and discuss potential solutions that could one day make this long-held dream a reality.

What You'll Learn

Technological limitations in interstellar travel, astronomical distances and time constraints, biological challenges for human space travel, economic and political barriers to interstellar exploration.


Interstellar travel has long captivated the imagination of humanity, with visions of exploring distant stars and unknown civilizations. However, the vast distances and harsh conditions of space pose significant technological challenges that currently hinder our ability to embark on interstellar journeys. In this blog post, we will explore some of the key technological limitations in interstellar travel and the current scientific understanding of these limitations.

Propulsion Systems:

One of the primary technological hurdles in interstellar travel is the development of propulsion systems that can propel our spacecraft to speeds necessary for interstellar voyages. The current state-of-the-art propulsion systems, such as chemical rockets and ion thrusters, cannot achieve the speeds required to traverse interstellar distances within a reasonable time frame. Future propulsion technologies, including plasma, antimatter, or nuclear propulsion, show potential, but significant advancements and breakthroughs are necessary before they can be considered viable for interstellar travel.

Energy Requirements:

Interstellar travel demands a tremendous amount of energy to overcome the gravitational forces of stars and travel at near-light speeds. Unfortunately, our current energy sources, such as chemical combustion or nuclear fission, are insufficient for powering interstellar spacecraft on such long voyages. The development of advanced energy sources, such as fusion or antimatter reactions, offers promising options, but these technologies are still in their infancy and face numerous technical and feasibility challenges.

Life Support Systems:

Another critical aspect of interstellar travel is the ability to sustain human life during the journey. Long-duration space missions require sophisticated life support systems capable of providing food, water, breathable air, and shielding from radiation and microgravity effects. The current life support technologies used on the International Space Station (ISS) are not designed for interstellar travel and would need to be significantly enhanced to support prolonged journeys spanning decades or even centuries. Developing efficient closed-loop life support systems is a crucial area of research for enabling interstellar travel.

Navigation and Guidance:

Navigating through the vastness of space presents a unique challenge due to the absence of fixed landmarks and the enormous distances involved. Accurate navigation and guidance systems are essential to ensure that interstellar spacecraft can reach their intended destinations without veering off course or encountering hazardous obstacles. Traditional navigation methods, such as celestial navigation, may be inadequate for interstellar travel, requiring the development of advanced technologies like pulsar-based navigation or autonomous robotic navigation systems.

Interstellar Hazards:

Interstellar space is not empty; it contains various hazards and challenges that spacecraft must overcome. They include micrometeoroids, cosmic radiation, gravitational effects, and the potential presence of unknown interstellar objects. Shielding against these hazards and developing techniques to mitigate their effects on the spacecraft and crew will be crucial for safe interstellar travel. Additionally, understanding the potential dangers associated with traversing different interstellar environments, such as nebulae or stellar remnants, is yet another technological challenge that needs to be addressed.

While interstellar travel remains a tantalizing prospect, it is essential to acknowledge the technological limitations that currently prevent us from embarking on such voyages. The development of advanced propulsion systems, energy sources, life support systems, navigation and guidance technologies, and effective hazard mitigation strategies are key areas of research that require significant breakthroughs. As scientists and engineers continue to push the boundaries of what is possible, it is through their tireless efforts that we may one day overcome these limitations and embark on a journey to the stars.

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Interstellar travel has long been a dream of humanity, but the vast distances involved pose significant challenges. In order to truly explore the stars, we must first understand the immense scale of the universe and how it affects our ability to travel between star systems.

The distances between stars are staggering. Even the closest star to our own solar system, Proxima Centauri, is about 4.24 light-years away. To put this into perspective, one light-year is roughly 5.88 trillion miles. So, traveling to Proxima Centauri would require covering a distance of over 24 trillion miles!

This enormous distance immediately raises the question of how long it would take to travel to another star. Unfortunately, our current technology is far from being able to achieve meaningful interstellar travel. The fastest human-made spacecraft to date, the Parker Solar Probe, can travel at speeds of up to 430,000 miles per hour. At this rate, it would take over 6,000 years to reach Proxima Centauri.

Even if we were able to develop spacecraft that could travel at much higher speeds, such as a significant fraction of the speed of light, we would still face significant time constraints. This is due to the effects of time dilation, a consequence of Einstein's theory of general relativity.

According to time dilation, as an object approaches the speed of light, time for that object slows down relative to a stationary observer. This means that, from the perspective of an astronaut traveling at near-light speeds, time would pass much slower than it does for people on Earth.

While this may sound like a way to cheat the system and travel to distant stars within a human lifetime, there's a catch. Time dilation is a two-way street. From the perspective of the astronaut, time on Earth would appear to be passing much faster. So even if the astronaut were able to travel to a star system and back within their own lifetime, millennia would have passed for the people they left behind on Earth.

To truly achieve interstellar travel, we would need to develop technologies that allow us to overcome the vast distances involved and the time constraints imposed by relativity. This could potentially involve harnessing exotic forms of energy, such as antimatter or black holes, to generate propulsion systems capable of near-light speeds.

Alternatively, we could explore possibilities such as wormholes or warp drives, concepts that have been popularized by science fiction but are not currently within the realm of scientific possibility. These ideas involve bending or warping spacetime itself to create shortcuts or effectively "teleport" between locations.

In conclusion, while the dream of interstellar travel captivates the imagination, the reality of the astronomical distances and time constraints involved present significant challenges. Our current understanding of physics and technology is not yet sufficient to overcome these barriers, but ongoing scientific research and technological advancements may one day bring humanity closer to the stars.

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Interstellar travel, the ability to travel between stars, has long been a dream of scientists and science fiction fans alike. However, there are several biological challenges that must be overcome before this dream can become a reality. In this article, we will explore some of the key biological challenges for human space travel and what is currently being done to address them.

Microgravity Effects on the Human Body:

One of the most significant challenges faced by astronauts during long-duration space missions is the effect of microgravity on the human body. In microgravity, the lack of gravitational forces acting on the body leads to a number of physiological changes, such as muscle and bone loss, cardiovascular deconditioning, and impaired immune function. These changes can have severe implications for the health and well-being of space travelers, especially during interstellar journeys that may last for several decades.

To counter the negative effects of microgravity, researchers are exploring various countermeasures. Regular exercise programs have been implemented on the International Space Station (ISS) to help maintain muscle and bone mass. Specialized resistance training and high-intensity exercise equipment are also being developed to simulate gravity-like effects on the body. Additionally, nutritional strategies are being studied to ensure astronauts receive adequate nutrients to support their physiological functions.

Radiation Exposure in Space:

Another major challenge of long-duration space travel is the high levels of radiation encountered beyond the protective shield of Earth's atmosphere. Cosmic rays and solar flares present a significant health hazard, as they can damage DNA, increase the risk of cancer, and impair cognitive function.

To mitigate the effects of radiation exposure, scientists are investigating various shielding strategies. This includes developing advanced materials that can better protect astronauts from harmful radiation. Additionally, research is being conducted to understand the long-term effects of radiation exposure on the human body and to develop medical countermeasures.

Psychological and Behavioral Factors:

The isolation and confinement experienced by astronauts during interstellar travel can have significant psychological and behavioral impacts. Extended periods of isolation can lead to feelings of loneliness, depression, and decreased cognitive function. Moreover, the confined living space of a spacecraft can also result in reduced privacy and increased stress levels.

To address these challenges, researchers are exploring various psychological support systems. These include virtual reality technologies to simulate natural environments, telemedicine for remote psychological counseling, and crew selection and training programs to ensure individuals are psychologically suited for long-duration space missions.

Reproduction and Development:

For interstellar travel to be sustainable in the long term, it is vital to consider the challenges associated with reproduction and development in space. Currently, the effects of microgravity on human reproduction and fetal development are not well understood. It is unclear how microgravity might affect fertility, embryo development, and the health of offspring.

To investigate these challenges, scientists are conducting studies using animal models and human cells in microgravity environments. This research aims to uncover the mechanisms underlying reproduction and development in space and to develop potential interventions to ensure the reproductive health of space travelers.

In conclusion, interstellar travel holds great promise for the future of humanity. However, before we can embark on such long-duration journeys, several biological challenges need to be addressed. The effects of microgravity on the human body, radiation exposure in space, psychological and behavioral factors, and reproduction and development are all significant areas of research. By understanding and finding solutions to these challenges, we can pave the way for safe and sustainable human interstellar travel in the future.

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Interstellar travel has been a topic of fascination and scientific exploration for many years. The idea of humans traveling beyond our solar system to explore new worlds and potentially colonize other planets has captivated the imaginations of people across the globe. However, despite our technological advancements, there are still significant economic and political barriers that prevent us from achieving this goal.

One of the primary economic barriers to interstellar exploration is the immense cost associated with such an endeavor. The resources required to build a spacecraft capable of traversing the vast distances between star systems are astronomical, both literally and figuratively. The research, development, and manufacturing costs alone would be staggering, not to mention the ongoing maintenance and operational expenses. In addition, the fuel required to power such a spacecraft would be immense, necessitating the development of new and advanced propulsion systems that are currently beyond our capabilities.

Another economic barrier is the lack of tangible immediate returns on investment. Interstellar exploration is a long-term venture, with potential benefits and discoveries that may not be realized for generations. Funding such an endeavor would require governments or private entities to commit significant financial resources without the assurance of a quick return. In an era of tight budgets and competing priorities, it can be challenging to justify investing in interstellar exploration when there are more immediate and pressing needs here on Earth.

The political barriers to interstellar exploration are numerous and complex. Firstly, there are geopolitical considerations. The exploration and colonization of new worlds would require a coordinated effort among nations to avoid conflicts and ensure the fair distribution of resources and opportunities. Historically, geopolitical rivalries and territorial disputes have hindered international cooperation on large-scale projects, and interstellar exploration would likely be no exception.

Furthermore, there is the issue of governance. If humans were to colonize other planets, questions would arise about jurisdiction, laws, and governance structures. How would a new interstellar colony be governed? Who would have authority over the colonists? These questions would require careful consideration and international agreement to avoid potential conflicts in the future.

Lastly, there are ethical concerns. Interstellar exploration raises ethical questions about the potential impact on indigenous life forms or ecosystems that may exist on other planets. The colonization of new worlds would require the careful management of resources to ensure the preservation of these unique environments. Additionally, the potential exploitation of resources on other planets could raise questions about the equitable distribution of wealth and the prevention of environmental degradation.

In conclusion, while the idea of interstellar travel is exciting, there are significant economic and political barriers that currently prevent us from achieving this goal. The immense cost, lack of immediate returns on investment, geopolitical considerations, governance issues, and ethical concerns all pose significant challenges. Overcoming these barriers will require international cooperation, significant financial investment, and careful ethical considerations. However, with continued technological advancements and a collective commitment to exploration, it is possible that one day humanity will overcome these barriers and embark on interstellar adventures.

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Interstellar astronauts would face years-long communication delays due to time dilation

The laws of physics mean that communication with near-light-speed spacecraft would be very challenging.

The supermassive black hole Gargantua plays a major role in the 2014 sci-fi blockbuster Interstellar

Due to the mind-blowing distances and speeds required, interstellar travel would be extraordinarily difficult, if not impossible, for humanity to achieve. But new research highlights yet another challenge: communication blackouts.

The next-closest star system to our own, Alpha Centauri , is over 4 light-years away, so barring any fancy sci-fi technological revolution in the next few centuries, if we want to spread among the stars, we'll have to do it the "slow" way. 

That means we'd need some sort of propulsion method that could get us close to, but not exceed, the speed of light . But even if we were to achieve this ambitious goal, this futuristic mode of transportation would present all sorts of communication challenges, scientists explain in a paper recently uploaded to the preprint database arXiv .

The first problem is that light itself can only travel at a finite speed. While this doesn't severely hinder communication near Earth, engineers already have to deal with this challenge when communicating with probes sent across the solar system . For example, messages take minutes to arrive at Mars and hours to reach the outer planets. For even longer-distance communication — like an imagined scenario of a spacecraft sent to some star system many light-years away — it would mean any message would take years to reach the craft.

Related: Is interstellar travel really possible?

But that's not the only hurdle. Special relativity teaches us that clocks are not synchronized across the universe. Travelers on board the spacecraft would experience time dilation, in which time would flow more slowly than it would for people on Earth. This effect is already measurable; for example, it needs to be taken into account for synchronizing signals from GPS satellites. 

But in our imagined scenario, our travelers are moving as close to the speed of light as possible. This is absolutely essential for propagation out into the galaxy . Because of time dilation, the passengers would not experience the years and decades of travel; for them, depending on how fast they moved, only weeks or months might pass.

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This time dilation would introduce serious issues for coordinating messages, which requires a significant amount of math. While annoying, that wouldn't be the hardest part of interstellar travel. Instead, it's that spacecraft traveling at near light speed would suffer severe communication blackout periods.

In their paper, the researchers investigated two hypothetical interstellar-travel scenarios. In the first, travelers would continue to accelerate their spacecraft at a constant 1 g of acceleration — the same acceleration provided naturally by Earth's gravity . This would send their spacecraft ever closer to the speed of light.

Curiously, this kind of constant acceleration would introduce an event horizon . If the people of Earth sent a message to the spacecraft, that message would be limited to the speed of light. It would race ahead toward the spaceship, but in the meantime, the ship also would move away from the signal. If the message were sent soon enough, it would eventually reach the ship after a significant time delay. But if they were to wait too long, the message would never arrive; the spacecraft would always be one step ahead of the message, and from their perspective, signals from Earth would eventually go dark.

The second scenario offers different challenges. The researchers considered the case of a spacecraft sent to a distant destination. At first the spacecraft would constantly accelerate, but midway through its journey, it would flip itself around and decelerate so that it didn't just fly by its target. This scenario would introduce its own set of communication challenges.

First, the spacecraft would stop receiving messages from Earth after a certain amount of time. These messages would eventually reach the spacecraft, but only after it had reached its destination and stopped moving.

On the other hand, the spacecraft would be able to send signals to Earth, and those signals would always reach their targets. Also, signals sent from the destination (say, a colony already set up on the distant planet) would always reach the spacecraft while it was cruising in that direction.

— Interstellar space travel will have language complications for astronauts

— Interstellar travel requires a long-term approach (and humans are too impatient)

— Researchers unlock the keys to designing an interstellar sail  

But signals sent from the spacecraft to the destination would not arrive until shortly before the craft itself got there, at which time all of the sent messages would pile up on each other, announcing the arrival of the craft.

These realities mean that communication with near-light-speed spacecraft would be very challenging. All interstellar vehicles must operate independently, because after a certain amount of time, they will be cut off from Earth. If a problem arises, they will be able to tell people on Earth about it, but they won't be able to hear a response.

Also, distant colonies wouldn't know about the launch of a spacecraft in their direction until shortly before the craft arrived there. 

No matter what, interstellar travel would be a lonely journey.

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Paul Sutter

Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute in New York City. Paul received his PhD in Physics from the University of Illinois at Urbana-Champaign in 2011, and spent three years at the Paris Institute of Astrophysics, followed by a research fellowship in Trieste, Italy, His research focuses on many diverse topics, from the emptiest regions of the universe to the earliest moments of the Big Bang to the hunt for the first stars. As an "Agent to the Stars," Paul has passionately engaged the public in science outreach for several years. He is the host of the popular "Ask a Spaceman!" podcast, author of "Your Place in the Universe" and "How to Die in Space" and he frequently appears on TV — including on The Weather Channel, for which he serves as Official Space Specialist.

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  • VVet1968 The very first science fiction book that I read in the 1950s was Heinlein's "Time for the Stars" which focused on this same problem. Nearly seventy years later I still remember being blown away by the concepts as it was my first introduction to relativity. Sure, it's not an "adult" scifi, but if you've never read it, pick up a copy. Reply
  • Helio Interstellar travel is perhaps the best case for using particle entanglement for instant communication since one set of them must be carried to the distant locations. Of course, there has been no means found for a working model, but scientists are very clever, so I bet they’ll get ‘er done…someday, Reply
  • billslugg Entanglement cannot be used for communication for two basic reasons. The sender can only send a random message. The timing at the receiving end is either fixed by a predetermined schedule or is indeterminate. Neither a message nor a time can be communicated. Reply
billslugg said: Entanglement cannot be used for communication for two basic reasons. The sender can only send a random message. The timing at the receiving end is either fixed by a predetermined schedule or is indeterminate. Neither a message nor a time can be communicated.
T hese realities mean that communication with near-light-speed spacecraft would be very challenging. All interstellar vehicles must operate independently, because after a certain amount of time, they will be cut off from Earth. If a problem arises, they will be able to tell people on Earth about it, but they won't be able to hear a response .
Also, distant colonies wouldn't know about the launch of a spacecraft in their direction until shortly before the craft arrived there.
  • unclefishbits I have a silly saying: "No internet is fine. Having internet is great" Bad internet is a frustrating nightmare". Now put this at an existential level of loneliness and hope, and you've ramped up that situation to horrific levels. I'd just right off that comms won't work, and make sure you plan ahead to never exist in context of the earth or human race, ever again. Seeding the galaxy is going to be quite lonely. And we thought Tom Hanks in Castaway seemed crazed with lack of connections. Time to rewatch Aniara. Reply
Helio said: In a pair of entangled particles, my limited understanding is that whenever one particle collapses, say "heads", the other will instantly collapse to say "tails", regardless of distance. This would allow for excellent and instant communication. BUT, of course, the problem is getting the "first" one to say "heads" in a way that doesn't break their entanglement. My hope is that someone will find a way to do it.
Helio said: Also there are some interesting points raised in this article: " But signals sent from the spacecraft to the destination would not arrive until shortly before the craft itself got there, at which time all of the sent messages would pile up on each other, announcing the arrival of the craft." This is interesting because the travelers who, say reach alpha Centauri in one year, will argue their travel transmissions were never more than 1 year from Earth. But those on Earth would, I think, disagree and say that, for instance, when the travelers were 6 months out (half way) then Earth would see the transmission from 2.15 lyrs distance, so it would take 2.15 years to get the transmission, which is 1.15 years after the ship arrived. So, WAIM ( What Am I Missing?). Under SR, I don't see how this is true, eventually any signal sent will take one year for every lightyear distance the ship is from Earth when the transmission was sent. WAIM? This is puzzling as well. Assuming SR effects reduce their travel time (or distance) then any Earth broadcast will come after their arrival, not before. If the spacecraft sends a transmission to the destination after it has slowed enough, then this would make sense, admittedly.
dgmesser said: This would violate Einstein's principle that information cannot travel faster in an inertial frame than the speed of light. If that is violated, it would be a very big deal.
Helio said: "Spooky action at a distance", as Einstein called it. "Instantaneous" action has been demonstrated to be correct, thanks greatly to John Bell.
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difficulties with space travel

Blue Marble Space Institute of Science

Blue Marble Space Institute of Science

Space exploration begins at home

The limits of human exploration: Problems and solutions to cosmic space travel

by Oliver Kimmance


The concept of traveling between stars throughout the universe has been envisioned by humanity for thousands of years. Until recently, this has purely been through our imagination, but with recent leaps in technological development in the last 50 years, this notion has turned from fiction into a real and exciting possibility. Not only this, interstellar space travel is becoming increasingly necessary as our pursuit to find and understand extraterrestrial life grows exponentially, and as we uncover more information about the finite resources and environmental problems we face here on Earth. 

In the unlikely absence of any major natural disasters, it has been theorised that Earth can continue to support life for another 1.75 billion years (Parry, 2013). This figure arises from the fact that around that time our Sun will have exhausted a considerable amount of its hydrogen fuel in a process called nuclear fusion, reducing its mass but releasing more and more energy in the form of solar radiation. This will dramatically alter the Goldilocks Zone of our solar system (the region in which liquid water can stand at the surface of a planet based solely on orbital characteristics), with Earth being withdrawn from this zone and becoming increasingly too hot to support life as we know it after approximately 1.75 billion years. You may think this is a huge range of time for humanity to leave Earth and settle in other solar systems, and you would be correct, however it also assumes that Earth is not made uninhabitable by other factors such as a large planetary impact or runaway climate change within that time frame. 

Either way our time on planet Earth is finite, making it inevitable that in the near or distant future we must settle on other planets and even other star systems in the Milky Way. This of course requires transporting large numbers of people across vast interstellar distances (at least assuming we maintain our current physical form), a task unsurprisingly met by numerous problems.

Problems with interstellar travel: Current technologies

Interstellar distances are of course immense, but it can be difficult to fathom quite how immense. Travelling to our closest other star, Proxima Centauri, would be equivalent to travelling to and back from Pluto at its furthest distance from us in its orbit 2667 times. Even light, the fastest travelling entity in the universe, takes 4.24 years to reach us here on Earth from Proxima Centauri.

difficulties with space travel

Unfortunately, current spacecraft technologies simply cannot travel such astronomical distances in feasible time frames, especially not when carrying the weight of hundreds of people to settle in other solar systems. Even using the fastest ever crewed spacecraft, Apollo 10, which travelled at approximately 40,000 km/h , it would take over 115,000 years to complete the journey to Proxima Centauri. To put this into perspective, the Homo sapiens species only developed speech as a cognitive function around 50,000-150,000 years ago, so the time it would take the generations of humans on the spacecraft to reach their destination would be equivalent to the time taken for the entirety of human verbal communicative history to play out. 

In the confinements of a spacecraft with little to no selective pressures (except perhaps the ability to cope in such an environment), and with the ever increasing time taken to send and receive transmissions with humans back on Earth, the rate of adaptation and evolution of people on such a mission would dramatically decrease. Provided humanity back on Earth avoided any major disasters and continued to learn and develop new technologies, hypothetically, by the time such a crew reached Proxima Centauri they may be as underdeveloped compared to humans back on Earth as cavemen are to us right now! This is a disconcerting thought, but it highlights the necessity for new ideas and faster space travel if humanity is to leave Earth and venture into other solar systems.

All rockets used to date have been powered by chemical-based fuel which is heavy to carry and extremely inefficient. Chemical fuels exploit the energy released from the rearrangement of bonds during chemical reactions to generate thrust and accelerate the spacecraft. However these types of reactions only rearrange the electrons in atoms which for hydrogen, the predominantly used fuel source, makes up a mere 0.05% of the atom’s total mass. As explained by the equation for mass-energy equivalence from Einstein’s General Relativity: E = mc 2 , the smaller the mass involved with an interaction, the less energy is released. This can be used to calculate the efficiency of chemical fuel, coming in at a measly 0.0001% (Siegel, 2020). 

Evidently we require the development of more efficient sources of power that can accelerate spacecraft to much faster speeds than is currently possible, in order to reduce the travel time of interstellar missions to a practical length. Fortunately, there are multiple exciting new technologies under development, and many more theoretical solutions that with increased scientific information may become feasible in the near future.

Potential new technologies

One exciting new concept is nuclear fusion fuel. Nuclear fusion has been recognised for some time, it is the process that occurs in the core of the Sun and other stars as the incredibly high temperatures achieved as a result of immense gravitational pressure causes small atoms such as hydrogen to fuse together and form helium and larger atoms, releasing energy in the process. Using nuclear fusion fuel to power spacecraft attempts to replicate this process, but the temperatures required to initiate these reactions are extremely hard to reach without the immense pressures achieved in the centres of stars due to their gravity. Consequently, new methods are required to stably heat hydrogen to the temperatures required for nuclear fusion reactions to take place. 

difficulties with space travel

One such method is the ‘ITER magnetic confinement reactor’ currently under development in France. A circle of enormous electromagnetic superconductors generate enough pressure to squeeze a central ring of hydrogen gas into a plasma, allowing their nuclei to fuse into helium and generate energy. However, to prevent the superconductors vaporising themselves, a network of tubes containing liquid helium cooled to -270°C  must surround the superconductors to stabilise the system. This requires huge amounts of energy to maintain and currently the energy input to carry out the reaction is more than is obtained from nuclear fusion. 

Nevertheless, continued improvements to this technology could approve nuclear fusion as a more efficient and powerful means of powering the next generation of spacecraft. Compared to chemical-based fuel which utilises only 0.05% of the reagents total mass, nuclear fusion reactions rearrange the proton and neutron nuclei of atoms, entities with significantly more mass than elections and subsequently producing around 10,000 times more energy per unit mass of fuel. Spacecraft equipped with nuclear-based fuel could therefore be accelerated for much longer periods of time and achieve much faster speeds than current spacecraft. Furthermore, the only reagents necessary for hydrogen nuclear fusion are hydrogen isotopes, the most abundant elements in the universe. Theoretically, future spacecraft using this technology could gather cosmic hydrogen as they travel, feed the hydrogen into the reactor, and continue generating thrust. This would also reduce the amount of initial hydrogen required to power the spacecraft, greatly reducing its mass and therefore allowing it to be accelerated to greater speeds.

Another proposed method for dramatically increasing the speed of spacecraft is laser-powered propulsion, well known for being used in the ‘Breakthrough Starshot’ program. This revolutionary technology removes the need for on board fuel since the energy is provided from a ground based source. This dramatically reduces the weight of the spacecraft allowing it to be accelerated to much faster speeds than previously possible. A ground-based array of high power lasers all fire a beam into space that converges on a huge reflective light-sail attached to the spacecraft in orbit to accelerate it. To be effective, the mass of the entire spacecraft must be in the order of grams, whilst the combined energy output of the laser array must be in the order of hundreds of Megawatts. 

Nevertheless, the Breakthrough Starshot program predicts that their ~1 gram probes can be accelerated to around 20% the speed of light, an incredible achievement that would reduce the travel time to Proxima Centauri down to only 22 years. This is more than a 5000-fold decrease in travel time compared to the fastest crewed spacecraft to date, and would allow the mission to be completed and data to be returned to Earth well within a human’s lifetime. The success of a mission like Breakthrough Starshot would be invaluable for gaining information on other solar systems and potentially habitable exoplanets, and would be an immense achievement, however a 1 gram spacecraft is clearly insufficient for transporting people across interstellar space in the eventuality of settling on worlds around other stars. 

difficulties with space travel

The obvious problem of scaling up the technology to power a spacecraft capable of accommodating passengers on its journey is a massive engineering dilemma, and perhaps is not possible. This is coupled with the fact that there is also no current solution as to how human passengers would survive the force experienced when traveling at such speeds. One problem will be the construction of a light-sail large enough to reflect enough energy to power a spacecraft many orders of magnitude heavier than those currently under development, but the most difficult problem will be providing the amount of energy required in the first place. Let’s say we wish to accelerate a spacecraft to 20% the speed of light with the mass of Apollo 10 (28,834 kg), bearing in mind this had only 3 crew members. The energy required to accelerate the Breakthrough Starshot probe to this speed is around 100 MW so the energy required to accelerate this manned spacecraft to the same speed would be 2.9 million GW, almost double the entire energy consumption of India in a one hour period . Evidently this amount of energy usage is far from possible, however there are theories for future energy harnessing methods that may bring this into the realms of possibility. Such proposals include construction of a Dyson sphere or harvesting cosmic hydrogen for fuel cells, but are beyond the scope of this article. 

Further problems with laser-beam propulsion include collision of the spacecraft with interstellar dust particles, which would completely destroy the lightweight light-sail at such high speeds. Breakthrough Starshot will attempt to evade this by propelling thousands of probes in the hope that at least a few reach Proxima Centauri undamaged. 

Limitations of intergalactic space travel

The prolonged future survival of humanity noticeably depends on the settlement of other planets and eventually other star systems in our galaxy. Accomplishing this would be a tremendous achievement and would allow humanity to expand and develop exponentially as the raw materials from more planets and the energy from more stars could be harnessed. However, unfortunately there seems to be a physical limit to the extent of human expansion that is built into the physics of space and time.

At the instant of the Big Bang 13.8 billion years ago the universe was a tiny pocket of energy, with high density and low density regions due to quantum fluctuations. Moments after the big bang, cosmic inflation stretched these sub-atomic density differences into enormous differences, millions of light years across in distance. After this event, gravity began to combine the more dense regions back together, forming clusters of galaxies, whilst the less dense regions had insufficient mass for gravity to prevail and so continued expanding. The result was the formation of millions of clusters of galaxies, all separated by expanding space. This also means that we are only gravitationally bound to our ‘local group’, a cluster of galaxies including our Milky Way, the Andromeda Galaxy, and around 50 dwarf galaxies, whilst all other clusters of galaxies are moving away from us as a consequence of the expansion of the Universe. Well, technically these superclusters are not moving away from us themselves, rather the space in between us and them is expanding and so the relative distance is increasing. Regrettably, this means that even if we could travel near the speed of light, the local group is likely to be the only area of the Universe humanity will be able to explore. 

Moreover, as a result of dark energy, a phenomenon not officially discovered but whose effects can be observed, the expansion of the universe is accelerating. The further and further away we look into space, the faster and faster those galaxies and superclusters are moving away from us. This provides us with a ‘reachable zone’, much like a black hole event horizon where anything that passes beyond this horizon is traveling faster relative to us than the speed of light and is therefore unreachable and impossible to interact with. Earth’s ‘reachable zone’ is approximately 18 million light years in radius and makes up only 6% of the observable universe, comprising our local group and many other clusters that are accelerating away from us but at a slower rate relative to us than the speed of light. This can be a discouraging statistic, but the reachable zone still contains billions and billions of stars and probably even more planets to be explored. 

Theoretical solutions to intergalactic space travel

There are even hypothetical means permitted by our current laws of physics that may make it possible to travel faster than light, making the unreachable universe theoretically reachable. Two dominating and exciting theories are wormholes and warp drives. 

Wormholes are hypothetical phenomena that form when two extremely dense entities such as two black holes distort and bend the fabric of spacetime so much that a tunnel between the two is formed. This may be between two points in space or even between two points in parallel Universes. There is currently no evidence of wormholes existing, but Einstein’s theory of general relativity allows for their existence. Unfortunately even if wormholes were discovered, it is also theorised that any matter passing through the wormhole, even a single particle, would trigger catastrophe and cause the wormhole to break down. One theoretical solution to this is to feed antimatter into the wormhole along with whatever matter is attempting to pass through, which would potentially stabilise the effects of regular matter passing through.

difficulties with space travel

The concept of warp drives similarly plays on our understanding of space-time given by general relativity. This is a theoretical technology that would be attached to future spacecraft and distort space-time in front of the craft, massively reducing the distance between the spacecraft and its destination. By cleverly and carefully exploiting large quantities of matter and antimatter, space-time could be shortened in front of the spacecraft and lengthened behind it, creating a stable bubble of space surrounding the spacecraft and allowing it to travel to a particular destination far quicker than the speed of light.

Evidently, these two potential means of traveling faster than light are far from plausible at the present date, but the fact that they are both possible under our current laws of physics is extremely exciting. Perhaps with future discoveries and information gained on dark energy, dark matter, antimatter, and black holes, these models will become real-world technologies and allow humanity to freely explore the universe!

Oliver Kimmance is a Research Associate with the Blue Marble Space Institute of Science and is an undergraduate student in Biochemistry at the University of Bristol, UK.

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Boeing's troubled Starliner spacecraft launch is delayed again

John Helton

difficulties with space travel

Boeing's Starliner capsule atop an Atlas V rocket is seen at Space Launch Complex 41 at the Cape Canaveral Space Force Station on May 7, a day after its mission to the International Space Station was scrubbed because of an issue with a pressure regulation valve. John Raoux/AP hide caption

Boeing's Starliner capsule atop an Atlas V rocket is seen at Space Launch Complex 41 at the Cape Canaveral Space Force Station on May 7, a day after its mission to the International Space Station was scrubbed because of an issue with a pressure regulation valve.

The first crewed launch of Boeing's troubled Starliner spacecraft has been delayed again, to May 25, this time because of a helium leak in the service module.

NASA had set the liftoff for May 21 after scrubbing a May 6 launch but the helium leak was discovered on Wednesday. While the agency said the leak in the craft's thruster system was stable and wouldn't pose a risk during the flight, "Boeing teams are working to develop operational procedures to ensure the system retains sufficient performance capability and appropriate redundancy during the flight."

While that work is going on, NASA said its Commercial Crew Program (CCP) and the International Space Station Program will review data and procedures before making a final determination whether to proceed with a countdown.

The delay is the latest for the Starliner's first crewed mission, which will carry NASA astronauts Barry "Butch" Wilmore and Sunita "Suni" Williams to the International Space Station. The astronauts are to spend about a week aboard the space station before making a parachute and airbag-assisted landing in the southwestern U.S.

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If that mission is successful, NASA will begin the final process to certify Starliner for crewed rotation missions to the space station.

The delay comes roughly a decade after NASA awarded Boeing a more than $4 billion contract as part of the agency's Commercial Crew Program, which pays private companies to ferry astronauts to and from the space station after the space shuttle was retired in 2011.

SpaceX, which was also awarded a $2 billion contract under the CCP initiative, has flown eight crewed missions for NASA and another four private, crewed spaceflights since 2020.

A history of delays and design problems

But the Starliner program has been plagued with delays and design problems for several years.

It failed to reach the space station during its first mission in 2019 after its onboard clock, which was set incorrectly , caused a computer to fire the capsule's engines too early. The spacecraft successfully docked with the space station during its second test flight in 2022, despite the failure of some thrusters during the launch.

Boeing then scrapped the planned launch of the Starliner's first crewed flight last year, after company officials realized that adhesive tape used on the craft to wrap hundreds of yards of wiring was flammable, and lines connecting the capsule to its three parachutes appeared to be weaker than expected. The launch was delayed indefinitely.

After 6 months in space and a fiery return over the U.S., NASA's Crew-7 is back home

After 6 months in space and a fiery return over the U.S., NASA's Crew-7 is back home

The May 6 launch was scrubbed because of a faulty oxygen relief valve, NASA said.

Wilmore and Williams remain quarantined in Houston and will fly back to NASA's Kennedy Space Center in Florida closer to the new launch date, NASA said. The Starliner, which sits atop a United Launch Alliance Atlas V rocket, remains in the Vertical Integration Facility at Space Launch Complex 41 on Cape Canaveral Space Force Station in Florida.

Boeing has faced intense scrutiny this year on the commercial aviation side of its business after a rear door plug blew out of an Alaska Airlines flight shortly after takeoff in January.

Whistleblowers have since come forward to detail alleged quality control lapses at the storied company, and the Federal Aviation Administration said it was auditing Boeing's production. The Justice Department also announced it would open a criminal investigation into the Alaska Airlines incident.

NPR's Joe Hernandez and Geoff Brumfiel contributed reporting.

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Human Health during Space Travel: State-of-the-Art Review

Chayakrit krittanawong.

1 Department of Medicine and Center for Space Medicine, Section of Cardiology, Baylor College of Medicine, Houston, TX 77030, USA

2 Translational Research Institute for Space Health, Houston, TX 77030, USA

3 Department of Cardiovascular Diseases, New York University School of Medicine, New York, NY 10016, USA

Nitin Kumar Singh

4 Biotechnology and Planetary Protection Group, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA

Richard A. Scheuring

5 Flight Medicine, NASA Johnson Space Center, Houston, TX 77058, USA

Emmanuel Urquieta

6 Department of Emergency Medicine and Center for Space Medicine, Baylor College of Medicine, Houston, TX 77030, USA

Eric M. Bershad

7 Department of Neurology, Center for Space Medicine, Baylor College of Medicine, Houston, TX 77030, USA

Timothy R. Macaulay

8 KBR, Houston, TX 77002, USA

Scott Kaplin

9 Department of Dermatology, Baylor College of Medicine, Houston, TX 77030, USA

Stephen F. Kry

10 Department of Radiation Physics, University of Texas MD Anderson Cancer Center, Houston, TX 77030, USA

Thais Russomano

11 InnovaSpace, London SE28 0LZ, UK

Marc Shepanek

12 Office of the Chief Health and Medical Officer, NASA, Washington, DC 20546, USA

Raymond P. Stowe

13 Microgen Laboratories, La Marque, TX 77568, USA

Andrew W. Kirkpatrick

14 Department of Surgery and Critical Care Medicine, University of Calgary, Calgary, AB T2N 1N4, Canada

Timothy J. Broderick

15 Florida Institute for Human and Machine Cognition, Pensacola, FL 32502, USA

Jean D. Sibonga

16 Division of Biomedical Research and Environmental Sciences, NASA Lyndon B. Johnson Space Center, Houston, TX 77058, USA

Andrew G. Lee

17 Department of Ophthalmology, University of Texas Medical Branch School of Medicine, Galveston, TX 77555, USA

18 Department of Ophthalmology, Blanton Eye Institute, Houston Methodist Hospital, Houston, TX 77030, USA

19 Department of Ophthalmology, University of Texas MD Anderson Cancer Center, Houston, TX 77030, USA

20 Department of Ophthalmology, Texas A and M College of Medicine, College Station, TX 77807, USA

21 Department of Ophthalmology, University of Iowa Hospitals and Clinics, Iowa City, IA 52242, USA

22 Departments of Ophthalmology, Neurology, and Neurosurgery, Weill Cornell Medicine, New York, NY 10021, USA

Brian E. Crucian

23 National Aeronautics and Space Administration (NASA) Johnson Space Center, Human Health and Performance Directorate, Houston, TX 77058, USA

Associated Data

The field of human space travel is in the midst of a dramatic revolution. Upcoming missions are looking to push the boundaries of space travel, with plans to travel for longer distances and durations than ever before. Both the National Aeronautics and Space Administration (NASA) and several commercial space companies (e.g., Blue Origin, SpaceX, Virgin Galactic) have already started the process of preparing for long-distance, long-duration space exploration and currently plan to explore inner solar planets (e.g., Mars) by the 2030s. With the emergence of space tourism, space travel has materialized as a potential new, exciting frontier of business, hospitality, medicine, and technology in the coming years. However, current evidence regarding human health in space is very limited, particularly pertaining to short-term and long-term space travel. This review synthesizes developments across the continuum of space health including prior studies and unpublished data from NASA related to each individual organ system, and medical screening prior to space travel. We categorized the extraterrestrial environment into exogenous (e.g., space radiation and microgravity) and endogenous processes (e.g., alteration of humans’ natural circadian rhythm and mental health due to confinement, isolation, immobilization, and lack of social interaction) and their various effects on human health. The aim of this review is to explore the potential health challenges associated with space travel and how they may be overcome in order to enable new paradigms for space health, as well as the use of emerging Artificial Intelligence based (AI) technology to propel future space health research.

1. Introduction

Until now space missions have generally been either of short distance (Low Earth Orbit—LEO) or short duration (Apollo Lunar Missions). However, upcoming missions are looking to push the boundaries of space travel, with plans to travel for longer distances and durations than ever before. Both the National Aeronautics and Space Administration (NASA) and several commercial space companies (e.g., Blue Origin, SpaceX) have already started the process of preparing for long distance, long-duration space exploration and currently plan to explore inner solar planets (e.g., Mars) by the 2030s [ 1 ].

Within the extraterrestrial environment, a multitude of exogenous and endogenous processes could potentially impact human health in several ways. Examples of exogenous processes include exposure to space radiation and microgravity while in orbit. Space radiation poses a risk to human health via a number of potential mechanisms (e.g., alterations of gut microbiome biosynthesis, accelerated atherosclerosis, bone remodeling, and hematopoietic effects) and prolonged microgravity exposure presents additional potential health risks (e.g., viral reactivation, space motion sickness, muscle/bone atrophy, and orthostatic intolerance) [ 2 , 3 , 4 , 5 , 6 , 7 ]. Examples of endogenous processes potentially impacted by space travel include alteration of humans’ natural circadian rhythm (e.g., sleep disturbances) and mental health disturbances (e.g., depression, anxiety) due to confinement, isolation, immobilization, and lack of social interaction [ 8 , 9 , 10 ]. Finally, the risk of unknown exposures, such as yet undiscovered pathogens, remain persistent threats to consider. Thus, prior to the emergence of long distance, long duration space travel it is critical to anticipate the impact of these varied environmental factors and identify potential mitigating strategies. Here, we review the available medical literature on human experiments conducted during space travel and summarize our current knowledge on the effects of living in space for both short and long durations of time. We also discuss the potential countermeasures currently employed during interstellar travel, as well as future directions for medical research in space.

1.1. Medical Screening and Certification Prior to Space Travel

When considering preflight medical screening and certification, the requirements and recommendations vary based on the duration of space travel. Suborbital spaceflight, part of the new era of space travel, has participants launching to the edge of space (defined as the Karman line, 100 Km above mean sea level) for brief 3–5 min microgravity exposures. Orbital spaceflight, defined as microgravity exposure for up to 30 days, involves healthy individuals with preflight medical screening. In addition to a physical examination and metabolic screening, preflight medical screening assessing aerobic capacity (VO 2max ), and muscle strength and function may be sufficient to ensure proper conditioning prior to mission launch [ 11 , 12 , 13 , 14 ]. Age-appropriate health screening tests (e.g., colonoscopy, serum prostate specific antigen in men, and mammography in women) are generally recommended for astronauts in the same fashion as their counterparts on Earth. In individuals with cardiovascular risk factors or with specific medical conditions, additional screening may be required [ 15 ]. The goal of these preflight screening measures is to ensure that medical conditions that may result in sudden incapacitation are identified and either disqualified or treated before the mission begins. In addition to the medical screening described above, short-duration space travelers are also required to undergo acceleration training, hypobaric and hypoxia exposure training, and hypercapnia awareness procedures as part of the preflight training phase.

In preparation for long-duration space travel, astronauts generally undergo a general physical examination, as well as imaging and laboratory studies at the time of initial selection. These screening tests would then be repeated annually, as well as upon assignment to an International Space Station (ISS) mission. ISS crew members are medically certified for long-duration spaceflight missions through individual agency medical boards (e.g., NASA Aerospace Medical Board) and international medical review boards (e.g., Multilateral Space Medicine Board) [ 16 , 17 ]. In order for an individual to become certified for long-duration space travel, an individual must be at the lowest possible risk for the occurrence of medical events during the preflight, infight, and postflight periods. Following spaceflight, it is recommended that returning astronauts undergo occupational surveillance for the remainder of their lifetime for the detection of health issues related to space travel (e.g., NASA’s Lifetime Surveillance of Astronaut Health program) [ 18 ]. Table 1 summarizes the preflight, inflight, and postflight screening recommendations for each organ system. Further research utilizing data from either long-term space missions or simulated environments is required in order to develop an adequate preflight scoring system capable of predicting inflight and postflight health outcomes in space travelers based on various risk factors.

Summarizes the pre-flight, in-flight and post-flight screening in each system.

Below we discuss potential Space Hazards for each organ system along with possible countermeasures ( Table 2 ). Table 3 lists prospective opportunities for artificial intelligence (AI) implementation.

Summary of Space Hazards to each organ system and potential countermeasures.

Potential AI applications in space health.

1.2. Effects on the Cardiovascular System

During short-duration spaceflight, microgravity alters cardiovascular physiology by reducing circulatory blood volume, diastolic blood pressure, left ventricular mass, and cardiac contractility [ 42 , 123 ]. Several studies have demonstrated that peak exercise performance is reduced both inflight and immediately after short-duration spaceflight due primarily to a reduction in maximal cardiac output and O 2 delivery [ 124 , 125 ]. Prolonged exposure to microgravity does cause unloading of the cardiovascular system (e.g., removal of expected loading effects from Earth’s gravity when upright during the day), resulting in cardiac atrophy. These changes may be an example of adaptive physiologic changes (“physiologic atrophy”) that returns to baseline after returning from spaceflight. This process may be similar to the adaptive physiologic changes to the cardiovascular system seen during athletic training (“physiologic hypertrophy”). Thus far, there is no evidence that the observed short-term cardiac atrophy could permanently impair systolic function. However, this physiologic adaptation to microgravity in space could lead to orthostatic hypotension/intolerance upon returning to Earth’s gravity due to changes in the comparative position of peripheral resistance and sympathetic nerve activity [ 41 , 126 , 127 ]. Figure 1 demonstrates potential effects of the space environment on each organ system.

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Potential effects of the space environment on each organ system.

Another potential effect of microgravity exposure is that an alteration of hydrostatic forces in the vertical gravitational (Gz) axis could lead to the formation of internal jugular vein thromboses [ 28 , 29 ]. Anticoagulation would not be an ideal choice for prevention as astronauts have an increased risk of suffering traumatic injury during spaceflight, thus potentially inflating the risk of developing an intracerebral hemorrhage or subdural hematoma. In addition, if a traumatic accident were to occur during spaceflight, the previously discussed cardiovascular adaptations could impair the body’s ability to tolerate blood loss and shock [ 45 , 46 , 47 ].

During long-duration spaceflight, one recent study demonstrated that astronauts did not experience orthostatic hypotension/intolerance during routine activities or after landing following 6 months in space [ 128 ]. It is worth noting that all of these astronauts performed aggressive exercise countermeasures while in flight [ 128 ]. Another study of healthy astronauts after 6 months of space travel showed that the space environment caused transient changes in left atrial structure/electrophysiology, increasing the risk of developing atrial fibrillation (AF) [ 129 ]. However, there was no definitive evidence of increased incidence of supraventricular arrhythmias and no identified episodes of AF [ 129 ]. Evaluation with echocardiography or cardiac MRI may be considered following long-duration spaceflight in certain cases.

Prior human studies with supplemental data obtained from animal studies, have shown that healthy individuals with prolonged exposure to ionizing radiation may be at increased risk for the development of accelerated atherosclerosis secondary to radiation-induced endothelial damage and a subsequent pro-inflammatory response [ 3 , 4 , 57 , 58 , 59 , 60 , 123 ]. One study utilizing human 3D micro-vessel models showed that ionizing radiation inhibits angiogenesis via mechanisms dependent on the linear energy transfer (LET) of charged particles [ 130 ], which could eventually lead to cardiac dysfunction [ 131 , 132 ]. In fact, specific characteristics of the radiation encountered in space may be an important factor to understanding its effects. For example, studies of pediatric patients undergoing radiotherapy have shown an increase in cardiac-related morbidity/mortality due to radiation exposure, but not until radiation doses exceeded 10 Gy [ 133 ]. At lower dose levels the risk is less clear: while a study of atomic bomb survivors with more than 50 years of followup demonstrated elevated cardiovascular risks at doses < 2 Gy [ 134 ]. A recent randomized clinical trial with a 20-year follow-up showed no increase in cardiac mortality in irradiated breast cancer patients with a median dose of 3.0 Gy (1.1–8.1 Gy) [ 135 ]. The uncertainty in cardiovascular effects of ionizing radiation, are accentuated in a space environment as the type and quality of radiation likely play an important role as well.

Further research is required to understand the radiation dosage, duration, and quality necessary for cardiovascular effects to manifest, as well as develop preventive strategies for AF and internal jugular vein thrombosis during space travel.

1.3. Effects on the Gastrointestinal System

During short-duration spaceflight, the presence of gastrointestinal symptoms (e.g., diarrhea, vomiting, and inflammation of the gastrointestinal tract) are common due to microgravity exposure [ 35 , 136 , 137 ]. Still unknown however is whether acute, surgical conditions such as cholecystitis and appendicitis occur more frequently due to microgravity-induced stone formation or alterations in human physiology/anatomy, and immunosuppression [ 40 ]. Controlling for traditional risk factors associated with the development of these conditions (e.g., adequate hydration, maintenance of a normal BMI, dietary fat avoidance, etc.) may help mitigate the risk.

During long-duration spaceflight, it is possible that prolonged radiation exposure could lead to radiation-induced gastrointestinal cancer. Gamma radiation exposure is a known risk factor for colorectal cancer via an absence of DNA methylation [ 138 ]. NASA has recently developed a space radiation simulator, named the “GCR Simulator”, which allows for the more accurate radiobiologic research into the development and mitigation of radiation-induced malignancies [ 139 ]. Preflight colorectal cancer screening via colonoscopy or inflight screening via gut microbiome monitoring may be beneficial, but further research is required to demonstrate their clinical utility. Several studies, including the NASA Twins study have shown that microgravity could lead to alterations in an individual’s gut microbial community (i.e., gut dysbiosis) [ 2 , 140 , 141 , 142 ]. While changes to an individual’s gut microbiome can cause inflammation of the gastrointestinal tract [ 143 , 144 ], it remains unclear whether the specific alterations observed during spaceflight pose a risk to astronaut health. In fact, increased gut colonization by certain bacterial species is even associated with a beneficial effect on the gastrointestinal tract [ 2 , 140 ]. ( Table S1 ) Certain limitations of these studies, such as variations in genomic profile, diet, and a lack of adjusted confounders (e.g., the microbial content of samples) should be considered. Another potential consequence of prolonged microgravity exposure is the possibility of increased fatty-acid processing [ 145 ], leading to the development of non-alcoholic fatty liver disease (NAFLD) and hepatic fibrosis [ 146 , 147 ].

Further research is required to better understand gut microbial dynamics during space travel, as well as spaceflight-associated risk factors for the development of NAFLD, cholecystitis, and appendicitis.

1.4. Effects on the Immune System

During spaceflight, exposure to microgravity could potentially induce modifications in the cellular function of the human immune system. For example, it has been hypothesized that microgravity exposure could lead to an increase in the production of inflammatory cytokines [ 148 ] and stress hormones [ 149 , 150 ], alterations in the function of certain cell lines (NK cells [ 151 , 152 ], B cells [ 153 ], monocytes [ 154 ], neutrophils [ 154 ], T cells [ 5 , 155 ]), and impairments of leukocyte distribution [ 156 ] and proliferation [ 155 , 157 , 158 ]. The resultant immune system dysfunction could lead to the reactivation of latent viruses such as Epstein-Bar Virus (EBV), Varicella-Zoster Virus (VZV), and Cytomegalovirus (CMV) [ 31 , 32 ]. Persistent low-grade pro-inflammatory responses microgravity could lead to space fever. [ 159 ] Studies are currently underway to evaluate countermeasures to improve immune function and reduce reactivation of latent herpesviruses [ 33 , 160 , 161 , 162 ]. Microgravity exposure could also lead to the development of autoantibodies, predisposing astronauts to various autoimmune conditions [ 136 , 163 ]. ( Table S2 ) Most importantly, studies have shown that bacteria encountered within the space environment appear to be more resistant to antibiotics and more harmful in general compared to bacteria encountered on Earth [ 164 , 165 ]. This is in addition to the threat of novel bacteria species (e.g., Methylobacterium ajmalii sp. Nov. [ 76 ]) that we have not yet discovered.

Upon returning from the space environment astronauts remain in an immunocompromised state, which has been particularly problematic in the era of the COVID-19 pandemic. Recently, NASA has recommended postflight quarantine and immune status monitoring (i.e., immune-boosting protocol) to mitigate the risk of infection [ 77 ]. This is similar to the Apollo and NASA Health Stabilization Programs that helped establish the preflight protocol (pre-mission quarantine) currently used for this purpose.

Further research is required to understand the mechanisms of antibiotic resistance and the modifications in inflammatory cytokine dynamics, in order to develop immune boosters and surrogate immune biomarkers.

1.5. Effects on the Hematologic System

During short-duration spaceflight, the plasma volume and total blood volume de-crease within the first hours and remain reduced throughout the inflight period, a finding previously identified as space anemia [ 166 ]. Space anemia during spaceflight is perhaps due to a normal physiologic adaptation of newly released blood cells and iron metabolism to microgravity [ 167 ].

During long-duration spaceflight, microgravity exposure could potentially induce hemoglobin degradation, leading to hemolytic anemia. In a recent study of 14 astronauts who were on 6-month missions onboard the ISS, a 54% increase in hemolysis was ob-served after landing one year later [ 50 ]. In another small study, nearly half of astronauts (48%) landing after long duration missions were anemic and hemoglobin levels were characterized as having a dose–response relationship with microgravity exposure [ 51 ]. An additional study collected whole blood sample from astronauts during and after up to 6 months of orbital spaceflight [ 168 ]. Upon analysis, once the astronauts returned to Earth RBC and hemoglobin levels were significantly elevated. It is worth noting that these studies analyzed blood samples from astronauts collected after spaceflight, which may be influenced by various factors (e.g., the stress of landing and re-adaptation to conditions on Earth). In addition, these studies may be confounded by other extraterrestrial environmental factors such as fluid shifts, dehydration, and alteration of the circadian cycle.

Further research is urgently needed to understand plasma volume physiology dur-ing spaceflight and delineate the etiology and degree of hemolysis with longer space exposure, such as 1-year ISS or Mars exploration missions.

1.6. Oncologic Effects

Even during short-duration spaceflight, the stochastic nature of cancer development makes it possible that space radiation exposure could cause cancer via epigenomic modifications [ 63 ]. Currently, our epidemiological understanding of radiation-induced cancer risk is based primarily on atomic bomb survivors and accidental radiation exposures, which both show a clear association between radiation exposure and cancer risk [ 169 , 170 ]. However, these studies are hard to generalize to spaceflight as the patient populations vary significantly (generally healthy astronauts vs. atomic bomb survivors [NCRP 126]) [ 171 ]. Moreover, the radiation encountered in space is notably different than that associated with atomic bomb exposure. Most terrestrial exposures are based on low LET radiation (e.g., atomic bomb survivors received <1% dose from high LET neutrons) [ 172 ], whereas space radiation is comprised of higher LET ions (solar energetic particles and galactic cosmic rays) [ 173 , 174 ].

During long-duration spaceflight, our current understanding of cancer risk is also largely unknown. Our current epidemiologic understanding of long-duration radiation exposure and cancer risk is primarily based on the study of chronic occupational exposures and medically exposed individuals, supplemented with data obtained from animal studies, which are again based overwhelmingly on low LET radiation [ 169 , 170 , 175 , 176 ]. In animal studies, exposure to ionizing radiation (up to 13.5 months) has been associated with an increased risk of developing a variety of cancers [ 162 , 177 , 178 , 179 , 180 ]. Ionizing radiation exposure may cause DNA methylation patterns similar to the specific patterns observed in human adenocarcinomas and squamous cell carcinomas [ 63 ]; however, this response is not yet certain [ 181 , 182 ].

For the purposes of risk prediction, the elevated biological potency of heavy ions is modeled through concepts such as the radiation weighting factor, with NASA recently releasing unique quality factors ( Q NASA ) focused on high density tracks [ 183 ]. Although these predictive models can only estimate the impact of radiation exposure, extrapolation of current terrestrial-based data suggest that this risk could be at least substantial for astronauts. NASA, for example, has updated crew permissible career exposure limits to 0.6Sv, independent of age and sex. This degree of exposure results in a 2–3% mean increased risk of death from radiation carcinogenesis (NCRP 2021) [ 184 ]. This limit would be reached between 200 and 400 days of space travel (depending on degree of radiation shielding) [ 48 ].

Further research is urgently needed to understand the true risk of space radiation exposure. This is especially important for individuals with certain genotype-phenotype profiles (e.g., BRCA1 or DNA methylation signatures) who may be more sensitive to the effects of radiation exposure. Most importantly, the utilization of genotype-phenotype profiles of astronauts or space travelers is valuable not just for pre-flight screening, but also during in-flight travel, especially for long-duration flights to deeper space. An individual’s genetic makeup will in-variably change during spaceflight due to the shifting epigenetic microenvironment. Future crewed-missions to deep space will have to adapt to these anticipated changes, be-come aware of impending red-flag situations, and determine whether any meaningful shift or change to ones’ genetic makeup is possible. For example, personalized radiation shields could potentially be tailored to an individuals’ genotype-phenotype profile, individualized pulmonary capillary wedge pressure under microgravity may be different due to transient changes in left atrial structure, or preflight analysis of the globin gene for the prediction of space anemia [ 50 , 129 , 185 ]. This research should be designed to identify the radiation type, dose, quality, frequency, and duration of exposure required for cancer development.

1.7. Effects on the Neurologic System

During the initial days of spaceflight, space motion sickness (SMS) is the most commonly encountered neurologic condition. Microgravity exposure during spaceflight commonly leads to alterations in spatial orientation and gaze stabilization (e.g., shape recognition [ 186 ], depth perception and distance [ 187 , 188 ]). Postflight, impairments in object localization during pitch and roll head movements [ 189 , 190 ] and fine motor control (e.g., force modulation [ 191 ], keyed pegboard completion time [ 192 ], and bimanual coordination [ 193 ]) are common. Anecdotally, astronauts also reported alterations in smell and taste sensations during their missions [ 27 , 194 , 195 ]. The observed impairment in olfactory function is perhaps due to elevated intracranial pressure (ICP) with increased cerebrospinal fluid outflow along the cribriform plate pathways [ 196 ]. However, to date, there have been no studies directly measuring ICP during spaceflight.

Upon returning from spaceflight, studies have observed that astronauts experience decrements in postural and locomotor control that can increase fall risk [ 197 ]. These decrements have been observed in both standard sensorimotor testing and functional tasks. While recovery of sensorimotor function occurs rapidly following short-duration spaceflight (within the first several days after return) [ 192 , 198 ], recovery after long-duration spaceflight often takes several weeks. Similar to SMS, post-flight motion sickness (PFMS) is very common and occurs soon after g-transition [ 30 ]. Deficits in dexterity, dual-tasking, and vehicle operation [ 199 ] are also commonly observed immediately after spaceflight. Therefore, short-duration astronauts are recommended to not drive automobiles for several days, and only after a sensorimotor evaluation (similar to a field sobriety test).

Similarly to the effects seen following short-duration spaceflight, those returning from long-duration spaceflight can also experience deficits in dexterity, dual-tasking, and vehicle operation. Long-duration astronauts are recommended to not drive automobiles for several weeks, and also require a sensorimotor evaluation. While central nervous system (CNS) changes [ 53 ] associated with long-duration spaceflight are commonly observed, the resulting effects of these changes both during and immediately after spaceflight remain unclear [ 199 ]. Observed CNS changes include structural and functional alterations (e.g., upward shift of the brain within the skull [ 54 ], disrupted white matter structural connectivity [ 55 ], increased fluid volumes [ 56 ], and increased cerebral vasoconstriction [ 200 ]), as well as modifications to adaptive plasticity [ 53 ]. Adaptive reorganization is primarily observed in the sensory systems. For example, changes in functional connectivity during plantar stimulation have been observed within sensorimotor, visual, somatosensory, and vestibular networks after spaceflight [ 201 ]. In addition, functional responses to vestibular stimulation were altered after spaceflight―reducing the typical deactivation of somatosensory and visual cortices [ 202 ]. These studies provide evidence for sensory reweighting among visual, vestibular, and somatosensory inputs.

Further research is required to fully understand the observed CNS changes. In addition, integrated countermeasures are needed for the acute effects of g-transitions on sensorimotor and vestibular function.

1.7.1. Effects on the Neuro-Ocular System

Prolonged exposure to ionizing radiation is well known to produce secondary cataracts [ 61 , 62 ]. Most importantly, Spaceflight Associated Neuro-Ocular Syndrome (SANS) is a unique constellation of clinical and imaging findings which occur to astronauts both during and after spaceflight, and is characterized by: hyperopic refractive changes (axial hyperopia), optic disc edema, posterior globe flattening, choroidal folds, and cotton wool spots [ 43 ]. Ophthalmologic screening for SANS, including both clinical and imaging assessments is recommended. ( Figure 2 ) Although the precise etiology and mechanism for SANS remain ill-defined, some proposed risk factors for the development of SANS include microgravity related cephalad fluid shifts [ 203 ], rigorous resistive exercise [ 204 ], increased body weight [ 205 ], and disturbances to one carbon metabolic pathways [ 206 ]. Many scientists believe that the cephalad fluid shift secondary to microgravity exposure is the major pathophysiological driver of SANS [ 203 ]. Although inflight lumbar puncture has not been attempted, several mildly elevated ICPs have been recorded in astronauts with SANS manifestations upon returning to Earth [ 43 ]. Moreover, changes to the pressure gradient between the intraocular pressure (IOP) and ICP (the translaminar gradient) have been proposed as a pathogenic mechanism for SANS [ 207 ]. The translaminar gradient may explain the structural changes seen in the posterior globe such as globe flattening and choroidal folds [ 207 ]. Alternatively, the microgravity induced cephalad fluid shift may impair venous or cerebrospinal drainage from the cranial cavity and/or the eye/orbit (e.g., choroid or optic nerve sheath). Impairment of the glymphatic system has also been proposed as a contributing mechanism to SANS, but this remains unproven [ 208 , 209 ]. Although permanent visual loss has not been observed in astronauts with SANS, some structural changes (e.g., posterior globe flattening) may persist and have been documented to remain for up to 7 years of long-term follow-up [ 210 ]. Further research is required to better understand the mechanism of SANS, and to develop effective countermeasures prior to longer duration space missions.

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Ophthalmologic screening for SANS.

1.7.2. Effects on the Neuro-Behavioral System

The combination of mission-associated stressors with the underlying confinement and social isolation of space travel has the potential to lead to cognitive deficits and the development of psychiatric disorders [ 211 ]. Examples of previously identified cognitive deficits associated with spaceflight include impaired concentration, short-term memory loss, and an inability to multi-task. These findings are most evident during G-transitions, and are likely due to interactions between vestibular and cognitive function [ 212 , 213 ]. Sopite syndrome, a neurologic component of motion sickness, may account for some cognitive slowing. The term “space fog”, has been used to describe the generalized lack of focus, altered perception of time, and cognitive impairments associated with spaceflight, which can occur throughout the mission. This may be related to chronic sleep deprivation as deficiencies (including decreased sleep duration and quality of sleep) are prevalent despite the frequent use of sleep medications [ 71 ]. These results highlight the broad impact of space travel on cognitive and behavioral health, and support the need for integrated countermeasures for long-duration explorative missions.

1.8. Effects on the Musculoskeletal System

During short-duration spaceflight, low back pain and disk herniation are common due to the presence of microgravity. While the pathogenesis of space-related low back pain and disk herniation is complex, the etiology is likely multifactorial in nature (e.g., microgravity induced hydration and swelling of the vertebral disk, muscle atrophy of the neck and lower back) [ 19 , 214 , 215 ]. Additionally, various joint injuries (e.g., space-suited shoulder injuries) can also occur in space due to the presence of microgravity [ 16 , 216 , 217 , 218 ]. Interestingly, one study showed that performing specific exercises could potentially promote automatic and tonic activation of lumbar multifidus and transversus abdominis as well as prevent normal lumbopelvic positioning against gravity following bed rest as a simulation of space flight [ 219 ], and the European Space Agency suggested that exercise program could relieve low back pain during spaceflight [ 220 ]. Further longitudinal studies are required to develop specialized exercise protocols during space travel.

During long-duration spaceflight, the presence of microgravity could cause an alteration in collagen fiber orientation within tendons, reduce articular cartilage and meniscal glycosaminoglycan content, and impair the wound healing process [ 22 , 23 , 24 , 221 ]. These findings seen in animal studies suggest that mechanical loading is required in order for these processes to occur in a physiologic manner. It is theorized that there is a mandatory threshold of skeletal loading necessary to direct balanced bone formation and resorption during healthy bone remodeling [ 222 , 223 ]. Despite the current countermeasure programs, the issue of skeletal integrity is still not solved [ 224 , 225 , 226 ].

Space radiation could also impact bone remodeling, though the net effect differs based on the amount of radiation involved [ 6 ]. In summary, high doses of space radiation lead to bone destruction with increased bone resorption and reduced bone formation, while low doses of space radiation actually have a positive impact with increased mineralization and reduced bone resorption. Most importantly, space radiation, particularly solar particle events in the case of a flare, may induce acute radiation effects, leading to hematopoietic syndrome [ 7 ]. This risk is highest for longer duration missions, but can be substantially minimized with current spacecraft shielding options.

Longitudinal studies are required to develop special exercise protocols and further assess the aforementioned risk of space radiation on the development of musculoskeletal malignancies.

1.9. Effects on the Pulmonary System

During short-duration spaceflight, a host of changes to normal, physiologic pulmonary function have been observed [ 73 , 227 ]. Studies during parabolic flight have shown that the diaphragm and abdomen are displaced cranially due to microgravity, which is accompanied by an increase in the diameter of the lower rib cage with outward movement. Due to the observed changes to the shape of the chest wall, diaphragm, and abdomen, alterations to the pressure-volume curve resulted in a net reduction in lung volumes [ 228 ]. In five subjects who underwent 25 s of microgravity exposure during parabolic flight, functional residual capacity (FRC) and vital capacity (VC) were found to be reduced [ 229 ]. During the Spacelab Life Sciences-1 mission, microgravity exposure resulted in 10%, 15%, 10–20%, and 18% reductions in VC, FRC, expiratory reserve volume (ERV), and residual volume (RV), respectively, compared to values seen in Earth’s gravity [ 227 ]. The observed physiologic change in FRC is primarily due to the cranial shift of the diaphragm and abdominal contents described previously, and secondarily to an increase in intra-thoracic blood volume and more uniform alveolar expansion [ 227 ].

One surrogate measure for the inhomogeneity of pulmonary perfusion can be assessed through changes in cardiogenic oscillations of CO 2 (oscillations in exhaled gas composition due to differential flows from different lung regions with differing gas composition). Following exposure to microgravity, the size of cardiogenic oscillations were significantly reduced to 60% in comparison to the preflight standing values [ 230 , 231 ]. Possible causes of the observed inhomogeneity of ventilation include regional differences in lung compliance, airway resistance, and variations in motion of the chest wall and diaphragm. Access to arterial blood gas analysis would allow for enhanced physiologic evaluations, as well as improved management of clinical emergencies (e.g., pulmonary embolism) occurring during space travel. However, there is currently no suitable method for assessing arterial blood in space. The earlobe arterialized blood technique for collecting blood gas has been proposed, but evidence is limited [ 232 ]. Further research is required in this area to establish an effective means for sampling arterial blood during spaceflight.

In comparison to the changes seen during short-duration spaceflight, studies conducted during long-duration spaceflight showed that the heterogeneity of ventilation/perfusion (V/Q) was largely unchanged, with preserved gas exchange, VC, and respiratory muscle strength [ 73 , 233 , 234 ]. This resulted in overall normal lung function. This is supported by long-duration studies (up to 6 months) in microgravity which demonstrated that the function of the normal human lungs is largely unchanged following the removal of gravity [ 233 , 234 ]. It is worth noting that there were some small changes which were observed (e.g., an increase in ERV in the standing posture) following long-duration spaceflight, which can perhaps be attributed to a reduction in circulating blood volume [ 233 , 234 ]. However, while microgravity can causes temporary changes in lung function, these changes were reversible upon return to Earth’s gravity (even after 6 months of exposure to microgravity). Based on the currently available data, the overall effect of acute and sustained exposure to microgravity does not appear to cause any deleterious effects to gas exchange in the lungs. However, the biggest challenge for long-duration spaceflight is perhaps extraterrestrial dust exposure. Further research is required to identify the long term consequences of extraterrestrial dust exposure and develop potential countermeasures (e.g., specialized face masks) [ 73 ].

1.10. Effects on the Dermatologic System

During short-duration space travel, skin conditions such as contact dermatitis, skin sensitivity, biosensor electrolyte paste reactions, and thinning skin are common [ 44 , 235 ]. However, these conditions are generally mild and unlikely to significantly impact astronaut safety or prevent completion of space missions [ 44 ].

The greatest dermatologic concern for long-duration space travelers is the theoretical increased risk of developing skin cancer due to space radiation exposure. This hypothesis is supported by one study which found the rate of basal cell carcinoma, melanoma, and squamous cell carcinoma of the skin to be higher among astronauts compared to a matched cohort [ 236 ]. While the three-fold increase in prevalence was significant, there were a number of confounders (e.g., the duration of prolonged UV exposure on Earth for training or recreation, prior use of sunscreen protection, genetic predisposition, and variations in immune system function) that must also be taken into account. A potential management strategy for dealing with various skin cancers during space travel involves telediagnostic and telesurgical procedures. Further research is needed to improve the telediagnosis and management of dermatological conditions (e.g., adjustment for a lag in communication time) during spaceflight.

1.11. Diagnostic Imaging Modalities in Space

In addition to routine physical examination, various medical imaging modalities may be required to monitor and diagnose medical conditions during long-duration space travel. To date, ultrasound imaging acquired on space stations has proven to be helpful in diagnosing a wide array of medical conditions, including venous thrombosis, renal and biliary stones, and decompression sickness [ 29 , 237 , 238 , 239 , 240 , 241 , 242 ]. Moreover, the Focused Assessment with Sonography for Trauma (FAST), utilized by physicians to rapidly evaluate trauma patients, may be employed during space missions to rule out life-threatening intra-abdominal, intra-thoracic, or intra-ocular pathology [ 243 ]. Remote telementored ultrasound (aka tele-ultrasound) has been previously investigated during the NASA Extreme Environment Missions Operations (NEEMO) expeditions [ 244 ]. Today, the Butterfly iQ portable ultrasound probe can be linked directly to a smartphone through cloud computing, allowing physicians/specialists to promptly analyze remote ultrasound images [ 245 ].

Currently, alternative imaging modalities such as X-ray, CT, PET and MRI scan are unable to be used in space due to substantial limitations (e.g., limited space for large imaging structures, difficulties in interpretation due to microgravity). However, it is possible that the future development of a photocathode-based X-ray source may one day make this a possibility [ 101 , 246 ]. If X-ray imaging was possible, certain caveats would need to be taken into account for accurate interpretation. For example, pleural effusions, air-fluid levels, and pulmonary cephalization commonly seen on terrestrial imaging, would need to be interpreted in an entirely different way due to the effect of microgravity [ 247 ]. While this adjustment might be challenging, the altered principles of weightless physiology may provide some advantages as well. For example, one study found that intra-abdominal fluid was better able to be detected in space than in the terrestrial environment due to gravitational alterations in fluid dynamics [ 248 ]. Further research is required to identify and optimize inflight imaging modalities for the detection and treatment of various medical conditions.

1.12. Medical and Surgical Procedures in Space

Despite the presence of microgravity, both basic life support and advanced cardiac life support are feasible during space travel with some modifications [ 249 , 250 ]. For example, the recent guidelines for CPR in microgravity recommend specialized techniques for delivering chest compressions [ 251 ]. The use of mechanical ventilators, and moderate sedation or general anesthesia in microgravity are also possible but the evidence is extremely limited [ 252 , 253 ]. In addition, there are several procedures such as endotracheal intubation, percutaneous tracheostomy, diagnostic peritoneal lavage, chest tube insertion, and advanced vascular access which have only been studied through artificial stimulation [ 254 , 255 ].

Once traditionally “surgical” conditions are appropriately diagnosed, the next step is to determine whether these conditions should be managed medically, percutaneously, or surgically (laparoscopic vs. open procedures) [ 47 , 256 ]. For example, acute appendicitis or cholecystitis that would historically be managed surgically in terrestrial hospitals, could instead be managed with antibiotics rather than surgery. While the use of antibiotics for these conditions is usually effective on Earth, there remain concerns due to space-induced immune alterations, increased pathogenicity and virulence of microorganisms, and limited resources to “rescue” cases of antibiotic failure [ 39 ]. In cases of antibiotic failure, one potential minimally invasive option could be ultrasound-guided percutaneous drainage, which has previously been demonstrated to be possible and effective in microgravity [ 257 ]. Another potential approach is to focus on the early diagnosis and minimally invasive treatment of appropriate conditions, rather than treating late stage disease. In addition to expediting the patient’s post-operative recovery, minimally invasive surgery in space has the added benefit of protecting the cabin environment and the remainder of the crew [ 258 , 259 ].

As in all aspects of healthcare delivery in space, the presence of microgravity can complicate even the most basic of procedures. However, based on collective experience to date, if the patient, operators, and all required equipment are restrained, the flow of surgical procedures remains relatively unchanged compared to the traditional, terrestrial experience [ 260 ]. A recent animal study confirmed that it was possible to perform minor surgical procedures (e.g., vessel and wound closures) in microgravity [ 261 ]. Similar study during parabolic flight has further confirmed that emergent surgery for the purpose of “damage control” in catastrophic scenarios can be conducted in microgravity [ 262 ]. As discussed previously, telesurgery may be feasible if the surgery can be performed with an acceptably brief time lag (<200 ms) and if the patient is within a low Earth orbit [ 263 , 264 ]. However, further research and technological advancements are required for this to come to fruition.

1.13. Lifestyle Management in Space

Based on microgravity simulation studies, NASA has proposed several potential biomedical countermeasures in space [ 33 , 160 , 161 ]. Mandatory exercise protocols in space are crucial and can be used to maintain physical fitness and counteract the effects of microgravity. While these protocols may be beneficial, exercise alone may not be enough to prevent certain effects of microgravity (e.g., an increase in arterial thickness/stiffness) [ 20 , 265 , 266 , 267 ]. For example, a recent study found that resistive exercise alone could not suppress the increase in bone resorption that occurs in space [ 20 ]. Hence, a combination of resistance training and an antiresorptive medication (e.g., bisphosphonate) appears to be optimal for promotion of bone health [ 20 , 21 ]. Further research is needed to identify the optimal exercise regimen including recommended exercises, duration, and frequency.

In addition to exercise, dietary modification may be another potential area for optimization. The use of a diet based on caloric restriction (CR) in space remains up for debate. Based on data from terrestrial studies, caloric restriction may be useful for improving vascular health; however, this benefit may be offset by the associated muscle atrophy and osteoporosis [ 268 , 269 ]. Given that NASA encourages astronauts to consume adequate energy to maintain body mass, there has been an attempt to mimic the positive effects of CR on vascular health while providing appropriate nutrition. Further research is needed this area to identify the ideal space diet.

Based on current guidelines, only vitamin D supplementation during space travel is recommended. Supplementation of A, B6, B12, C, E, K, Biotin, folic acid are not generally recommended at this time due to insufficient evidence [ 64 ] ( Table S3 ). The use of traditional prescription medications may not function as intended on Earth. Therefore, alternative methods such as synthetic biologic agents or probiotics may be considered [ 35 , 38 ]. However, evidence in this area is extremely limited, and it is possible that the synthetic agents or probiotics may themselves be altered due to microgravity and radiation exposure. Further research is needed to investigate the relationship between these supplements and potential health benefits in space.

Currently, most countermeasures are directed towards cardiovascular system and musculoskeletal pathologies but there is little data against issues like immune and sleep deprivation, SANS, skin, etc. Artificial Gravity (AG) has been postulated as adequate multi-system countermeasure especially the chronic exposure in a large radius systems. Previously, the main barrier is the huge increase in costs [ 270 , 271 , 272 ]. However, there are various studies that show the opposite and also the recent decrease in launch cost makes the budget issue nearly irrelevant especially when a huge effort is paid to counteract the lack of gravity. The use of AG especially long-radius chronic AG is feasible. Further studies are needed to determine the utilization of AG in long-duration space travel.

1.14. Future Directions for Precision Space Health with AI

In this new era of space travel and exploration, ‘future’ tools and novel applications are needed in order to prepare deep space missions, particularly pertaining to strategies for mitigating extraterrestrial environmental factors, including both exogenous and en-dogenous processes. Such ‘future’ tools could help assist and ensure a safe travel to deep space, and more importantly, help bring space travelers and astronauts back to Earth. These tools and methods may initially be ‘remotely’ controlled, or have its data sent back to Earth for analysis. Primarily efforts should be focused on analyzing data in situ, and on site during the mission itself, both for the purpose of efficiency, and for the progressive purpose of slowly weaning off a dependency on Earth.

AI is an emerging tool in the big data era and AI is considered a critical aspect of ‘fu-ture’ tools within the healthcare and life science fields. A combination of AI and big data can be used for the purposes of decision making, data analysis and outcome prediction. Just recently, there have been encourage in advancements in AI and space technologies. To date, AI has been employed by astronauts for the purpose of space exploration; however, we may just be scratching the surface of AI’s potential. In the area of medical research, AI technology can be leveraged for the enhancement of telehealth delivery, improvement of predictive accuracy and mitigation of health risks, and performance of diagnostic and interventional tasks [ 273 ]. The AI model can then be trained and have its inference leveraged through cloud computing or Edge TPU or NVIDIA Jetson Nano located on space stations. ( Table 3 and Table S4 ) Figure 3 demonstrates potential AI applications in space.

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Object name is cells-12-00040-g003.jpg

Potential AI applications in space.

As described previously, the capability to provide telemedicine beyond LEO is primarily limited by the inability to effectively communicate between space and Earth in real-time [ 274 ]. However, AI integration may be able to bridge the gap and advance communication capabilities within the space environment [ 275 , 276 ]. One study demonstrated a potential mechanism for AI incorporation in which an AI-generated predictive algorithm displayed the projected motion of surgical tools to adjust for excess communication lag-time [ 277 ]. This discovery could potentially enable AI-enhanced robotics to complete repetitive, procedural tasks in space without human inputs (e.g., vascular access) [ 278 ]. Today, procedures performed with robotic assistance are not yet fully autonomous (they still require at least one human expert). It is possible with future iterations that an AI integration could be created with the ability to fully replicate the necessary human steps to make terrestrial procedures (e.g., percutaneous coronary intervention, incision and drainage [ 103 ], telecholecystectomy [ 105 , 106 ], etc.) feasible in space [ 275 , 279 ]. The seventh NEEMO mission previously demonstrated that robotic surgery controlled by a remote physician is feasible within the environment of a submarine, but it remains to be seen whether this can be expanded to the space environment [ 280 ].

On space stations, Edge TPU-accelerated AI inference could be used to generate accurate risk prediction models based on data obtained from simulated environments (e.g., NASA AI Risk Prediction Challenge) [ 281 ]. For example, AI could potentially utilize data (e.g., -omics) obtained from research conducted both on Earth and in simulated environments (e.g., NASA GCR Simulator) to predict an astronaut’s risk of developing cancer due to high-LET radiation exposure (cytogenetic damage, mitochondrial dysregulation, epigenetic alterations, etc.) [ 63 , 78 , 79 , 282 , 283 , 284 ].

Another potential area for AI application is through integration with wearable technology to assist in the monitoring and treatment of a variety of medical conditions. For example, within the field of cardiovascular medicine, wearable sensor technology has the capability to detect numerous biosignals including an individual’s cardiac output, blood pressure, and heart rate [ 285 ]. AI-based interpretation of this data can facilitate prompt diagnosis and treatment of congestive heart failure and arrhythmias [ 285 ]. In addition, several wearable devices in various stages of development are being created for the detection and treatment of a wide array of medical conditions (obstructive sleep apnea, deep vein thrombosis, SMS, etc.) [ 285 , 286 , 287 , 288 ].

As discussed previously, the confinement and social isolation associated with prolonged space travel can have a profound impact on an astronaut’s mental health [ 8 , 10 , 67 ]. AI-enhanced facial and voice recognition technology can be implemented to detect the early signs of depression or anxiety better than standardized screening questionnaires (e.g., PHQ-9, GAD-7) [ 68 , 69 ]. Therefore, telepsychology or telepsychiatry can be used pre-emptively for the diagnosis of mental illness [ 68 , 69 , 289 ].

2. Conclusions

Over the next decade, NASA, Russia, Europe, Canada, Japan, China, and a host of commercial space companies will continue to push the boundaries of space travel. Space exploration carries with it a great deal of risk from both known (e.g., ionizing radiation, microgravity) and unknown risk factors. Thus, there is an urgent need for expanded research to determine the true extent of the current limitations of long-term space travel and to develop potential applications and countermeasures for deep space exploration and colonization. Researchers must leverage emerging technology, such as AI, to advance our diagnostic capability and provide high-quality medical care within the space environment.


The authors would like to thank Tyson Brunstetter, (NASA Johnson Space Center, Houston, TX) for his suggestions and comments on this article as well as providing the update NASA’s SANS Evidence Report, Ajitkumar P Mulavara, (Neurosciences Laboratory, KBRwyle, Houston, TX), Jonathan Clark, (Neurology & Space Medicine, Center for Space Medicine, Houston, TX), Scott M. Smith, (Nutritional Biochemistry, Biomedical Research and Environmental Sciences Division, Human Health and Performance Directorate, NASA Johnson Space Center, Houston, TX) for his suggestions and providing the update NASA’s Nutrition Report, G. Kim Prisk, (Department of Medicine, Division of Physiology, University of California, San Diego, La Jolla, CA), Lisa C. Simonsen (NASA Langley Research Center, Hampton, VA), Siddharth Rajput, (Royal Australasian College of Surgeons, Australia and Aerospace Medical Association and Space Surgery Association, USA), David S. Martin, MS, (KBR, Houston, TX), ‪David W. Kaczka, (Department of Anesthesia, University of Iowa Carver College of Medicine, Iowa City, Iowa), Benjamin D. Levine (Institute for Exercise and Environmental Medicine, Texas Health Presbyterian Hospital, Dallas, University of Texas Southwestern Medical Center), Afshin Beheshti (NASA Ames Research Center), Christopher Wilson (NASA Goddard Space Flight Center), Michael Lowry (NASA Ames Research Center), Graham Mackintosh (NASA Advanced Supercomputing Division), and staff from NASA Goddard Space Flight Center for their suggestions. In addition, the authors would like to thank the anonymous reviewers for their careful reading of our manuscript, constructive criticism, and insightful comments and suggestions.


Supplementary materials.

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/cells12010040/s1 , Table S1: title Summary of gut microbial alteration during spaceflight; Table S2: title Summary of immune/cytokine changes during spaceflight; Table S3: title Summary of diet recommendation during spaceflight; Table S4: title Summary of AI technology and potential applications in space.

Funding Statement

This research received no external funding.

Conflicts of Interest

Krittanawong discloses the following relationships-Member of the American College of Cardiology Solution Set Oversight Committee, the American Heart Association Committee of the Council on Genomic and Precision Medicine, the American College of Cardiology/American Heart Association (ACC/AHA) Joint Committee on Clinical Data Standards (Joint Committee), and the American College of Cardiology/American Heart Association (ACC/AHA) Task Force on Performance Measures, The Lancet Digital Health (Advisory Board), European Heart Journal Digital Health (Editorial board), Journal of the American Heart Association (Editorial board), JACC: Asia (Section Editor), and The Journal of Scientific Innovation in Medicine (Associate Editor). Other authors have no disclosure.

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

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Space Travel Obstacles?

by Editorial Staff | January 3, 2016

Star Wars would have you believe that the greatest challenges to space travel is asteroids, lack of resources like water or fuel, or even the threat of unfriendly, intelligent alien life. But in reality, scientists are finding that the biggest obstacle to today's space travel is dust. Yes, space dust.

While our human bodies have natural defenses (like nose hairs) against dust, some dust particles are small enough to bypass our own "deflector shield" if you will... and settle in lung tissues and airways, causing injury to our lungs. This is similar to the health hazards of particle pollution or even how coal dust can harm the lungs of coal miners .

In zero gravity, dust doesn't just settle to the ground, away from our noses and mouths as it would on Earth, but floats freely, easily getting into lungs and eyes.

As Neil Armstrong discovered, this dust is also a key feature of our very own moon and most likely other planets. As they found, beyond the zero gravity effect, planetary dust sticks to astronauts through static electricity has sharp edges, and follows them back into their spacecraft, making it more likely that the dust will enter their lungs and cause harm. Neil and his team even said they could smell and even taste this moon dust!

Beyond damaging the lung health of astronauts, space dust also wreaks havoc on equipment and ventilation systems, which are also necessary for survival in space.

In March of 2015, NASA conducted an ISS airlock experiment to test how space dust affects the lungs, hoping to identify how lungs perform in space.

Tests like these will be extremely important to the future of space travel and colonization. As you may know, the red hue of Mars is due to the fact that the planet is covered with red dust. According to NASA , this planetary dust is sometimes blown into dust storms, ranging from tiny dust storms that can look like tornadoes to larger ones that can cover the entire planet. Dust may challenge the ability to establish this permanent settlement, and special attention to lung health both on Mars and in space will be key to survival.

Learn more about how particle pollution threatens the health of humans here on our home base—Earth.

  • IMAGE: http://www.nasa.gov/press/2014/may/nasa-simulator-successfully-recreates-space-dust/#.VoQXy_krKUk
  • http://www.esa.int/Our_Activities/Human_Spaceflight/Futura/Testing_astronauts_lungs_in_Space_Station_airlock
  • http://www.ncbi.nlm.nih.gov/pubmed/16140136
  • http://www.nasa.gov/audience/forstudents/k-4/stories/nasa-knows/what-is-mars-k4.html
  • http://www.ncbi.nlm.nih.gov/pubmed/9200637
  • http://www.gizmag.com/iss-airlock-experiment/36473/
  • http://www.ncbi.nlm.nih.gov/pubmed/23590267
  • http://www.mars-one.com/
  • http://www.ccohs.ca/oshanswers/chemicals/lungs_dust.html

Blog last updated: August 30, 2023

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An expert offers insight into how space travel impacts the human body

S pace travel is not for the faint of heart. It is a challenging and risky endeavor that requires rigorous training, preparation, and adaptation. The human body is not designed to survive in the harsh environment of space, where gravity, radiation, and isolation can have detrimental effects on health and well-being.

NASA has been studying the effects of space travel on the human body for more than 50 years, through its Human Research Program (HRP). The program aims to understand and mitigate the risks of human exploration, as NASA plans for extended missions on the Moon and Mars.

One of the most ambitious projects of the HRP was the Twins Study, which involved Scott Kelly and his identical twin brother Mark Kelly, both retired astronauts. Scott spent nearly a year in space onboard the International Space Station (ISS), while Mark stayed on Earth as a control subject. The study compared the physiological and psychological changes that occurred in Scott and Mark during and after the mission, providing valuable data on the effects of long-duration spaceflight.

Scott was not the only American astronaut to spend almost a year in space. Christina Koch also completed a 328-day mission on the ISS, setting a record for the longest single spaceflight by a woman. Both Scott and Christina experienced changes in their bodies, such as alterations in gene expression, immune system, microbiome, metabolism, and cognition.

However, spending a year in space is not the same as spending a year on Earth. Space travelers face a number of challenges that can affect their health and performance. One of the first and most common problems is space sickness, which is caused by the lack of gravity on the inner ear. This affects balance, coordination, and spatial orientation, and can also impair the ability to track moving objects.

Another challenge is the loss of muscle and bone mass, which occurs due to the lack of mechanical stress on the body. Astronauts can lose up to 20% of their muscle mass and 1-2% of their bone density per month in space, which can increase the risk of fractures, injuries, and osteoporosis. To prevent this, astronauts exercise for two hours a day on the ISS, using specially designed equipment such as treadmills, bikes, and resistance devices.

A more recent discovery is the effect of space travel on vision. Some astronauts have reported blurred vision, reduced contrast sensitivity, and changes in eye shape after returning from space. This is thought to be caused by the increased pressure on the brain and the eye, which results from the fluid shift in the body due to microgravity. NASA is investigating the causes and consequences of this phenomenon, as well as possible countermeasures.

In addition to the physical effects, space travel can also have psychological and social impacts. Astronauts are exposed to isolation, confinement, monotony, and distance from Earth, which can affect their mood, motivation, and mental health. They also have to cope with the stress of living and working in a hostile and closed environment, where any mistake can have serious consequences. NASA provides astronauts with psychological support, communication, and entertainment to help them deal with these challenges.

NASA is also researching the risks of space travel for future missions to Mars, which are expected to last for several years. These risks are grouped into five categories, related to the stressors they place on the body. These can be summarized with the acronym “RIDGE,” short for Space Radiation, Isolation and Confinement, Distance from Earth, Gravity fields, and Hostile/Closed Environments.

Space travel is not easy, but it is also not impossible. As we continue to explore the final frontier, we must also continue to learn and adapt, ensuring that our astronauts are as prepared as possible for the journey ahead. Space travel is a fascinating and rewarding endeavor, but it also requires careful planning, innovative thinking, and a commitment to understanding and mitigating the risks involved.

Relevant articles:

– The Human Body in Space – NASA

– The effects of space travel on the human body – BBC

– The Health Risks of Space Tourism: What are They?

Space travel is not for the faint of heart. It is a challenging and risky endeavor that requires rigorous training, preparation, and adaptation. The human body is not designed to survive in the harsh environment of space, where gravity, radiation, and isolation can have detrimental effects on health and well-being. NASA has been studying the […]

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Boeing is on the verge of launching astronauts aboard new capsule, the latest entry to space travel

Boeing's Starliner capsule atop an Atlas V rocket is rolled out to the launch pad at Space Launch Complex 41, Saturday, May 4, 2024, in Cape Canaveral, Fla. NASA astronauts Butch Wilmore and Suni Williams will launch aboard to the International Space Station, scheduled for liftoff on May 6, 2024. (AP Photo/Terry Renna)

Boeing’s Starliner capsule atop an Atlas V rocket is rolled out to the launch pad at Space Launch Complex 41, Saturday, May 4, 2024, in Cape Canaveral, Fla. NASA astronauts Butch Wilmore and Suni Williams will launch aboard to the International Space Station, scheduled for liftoff on May 6, 2024. (AP Photo/Terry Renna)

NASA’s Boeing Crew Flight Test astronauts Suni Williams and Butch Wilmore exit the Neil A. Armstrong Operations and Checkout Building at the agency’s Kennedy Space Center in Florida during a mission dress rehearsal on Friday, April 26, 2024. The first flight of Boeing’s Starliner capsule with a crew on board is scheduled for Monday, May 6, 2024. (Frank Micheaux/NASA via AP)

Boeing Crew Flight Test crew members Suni Williams and Butch Wilmore work in the Boeing Starliner simulator at the Johnson Space Center in Houston on Nov. 3, 2022. The first flight of Boeing’s Starliner capsule with a crew on board is scheduled for Monday, May 6, 2024. (NASA/Robert Markowitz)

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CAPE CANAVERAL, Fla. (AP) — After years of delays and stumbles, Boeing is finally poised to launch astronauts to the International Space Station for NASA.

It’s the first flight of Boeing’s Starliner capsule with a crew on board, a pair of NASA pilots who will check out the spacecraft during the test drive and a weeklong stay at the space station.

NASA turned to U.S. companies for astronaut rides after the space shuttles were retired. Elon Musk’s SpaceX has made nine taxi trips for NASA since 2020, while Boeing has managed only a pair of unoccupied test flights.

Boeing program manager Mark Nappi wishes Starliner was further along. “There’s no doubt about that, but we’re here now.”

The company’s long-awaited astronaut demo is slated for liftoff Monday night.

Provided this tryout goes well, NASA will alternate between Boeing and SpaceX to get astronauts to and from the space station.

A look at the newest ride and its shakedown cruise:

NASA's Boeing Crew Flight Test astronauts Suni Williams and Butch Wilmore exit the Neil A. Armstrong Operations and Checkout Building at the agency's Kennedy Space Center in Florida during a mission dress rehearsal on Friday, April 26, 2024. The first flight of Boeing’s Starliner capsule with a crew on board is scheduled for Monday, May 6, 2024. (Frank Micheaux/NASA via AP)

NASA’s Boeing Crew Flight Test astronauts Suni Williams and Butch Wilmore exit the Neil A. Armstrong Operations and Checkout Building at the agency’s Kennedy Space Center in Florida during a mission dress rehearsal on Friday, April 26, 2024. (Frank Micheaux/NASA via AP)


White with black and blue trim, Boeing’s Starliner capsule is about 10 feet (3 meters) tall and 15 feet (4.5 meters) in diameter. It can fit up to seven people, though NASA crews typically will number four. The company settled on the name Starliner nearly a decade ago, a twist on the name of Boeing’s early Stratoliner and the current Dreamliner.

No one was aboard Boeing’s two previous Starliner test flights. The first, in 2019, was hit with software trouble so severe that its empty capsule couldn’t reach the station until the second try in 2022. Then last summer, weak parachutes and flammable tape cropped up that needed to be fixed or removed.

In this photo released by Xinhua News Agency, workers open up the capsule of the Shenzhou-17 manned spaceship after it lands successfully at the Dongfeng landing site in north China's Inner Mongolia Autonomous Region, Tuesday, April 30, 2024. China's Shenzhou-17 spacecraft returned to Earth Tuesday, carrying three astronauts who have completed a six-month mission aboard the country's orbiting space station. (Lian Zhen/Xinhua via AP)

Veteran NASA astronauts Butch Wilmore and Suni Williams are retired Navy captains who spent months aboard the space station years ago. They joined the test flight after the original crew bowed out as the delays piled up. Wilmore, 61, is a former combat pilot from Mount Juliet, Tennessee, and Williams, 58, is a helicopter pilot from Needham, Massachusetts. The duo have been involved in the capsule’s development and insist Starliner is ready for prime time, otherwise they would not strap in for the launch.

“We’re not putting our heads in the sand,” Williams told The Associated Press. “Sure, Boeing has had its problems. But we are the QA (quality assurance). Our eyes are on the spacecraft.”

Boeing Crew Flight Test crew members Suni Williams and Butch Wilmore work in the Boeing Starliner simulator at the Johnson Space Center in Houston on Nov. 3, 2022. The first flight of Boeing’s Starliner capsule with a crew on board is scheduled for Monday, May 6, 2024. (NASA/Robert Markowitz)


Starliner will blast off on United Launch Alliance’s Atlas V rocket from Cape Canaveral Space Force Station. It will be the first time astronauts ride an Atlas since NASA’s Project Mercury, starting with John Glenn when he became the first American to orbit the Earth in 1962. Sixty-two years later, this will be the 100th launch of the Atlas V, which is used to hoist satellites as well as spacecraft.

“We’re super careful with every mission. We’re super, duper, duper careful” with human missions, said Tory Bruno, CEO of ULA, a joint venture of Boeing and Lockheed Martin.

Starliner should reach the space station in roughly 26 hours. The seven station residents will have their eyes peeled on the approaching capsule. The arrival of a new vehicle is “a really big deal. You leave nothing to chance,” NASA astronaut Michael Barratt told the AP from orbit. Starliner will remain docked for eight days, undergoing checkouts before landing in New Mexico or elsewhere in the American West.


Both companies’ capsules are designed to be autonomous and reusable. This Starliner is the same one that made the first test flight in 2019. Unlike the SpaceX Dragons, Starliner has traditional hand controls and switches alongside touchscreens and, according to the astronauts, is more like NASA’s Orion capsules for moon missions. Wilmore and Williams briefly will take manual control to wring out the systems on their way to the space station.

NASA gave Boeing, a longtime space contractor, more than $4 billion to develop the capsule, while SpaceX got $2.6 billion. SpaceX already was in the station delivery business and merely refashioned its cargo capsule for crew. While SpaceX uses the boss’ Teslas to get astronauts to the launch pad, Boeing will use a more traditional “astrovan” equipped with a video screen that Wilmore said will be playing “Top Gun: Maverick.”

One big difference at flight’s end: Starliner lands on the ground with cushioning airbags, while Dragon splashes into the sea.

Boeing is committed to six Starliner trips for NASA after this one, which will take the company to the station’s planned end in 2030. Boeing’s Nappi is reluctant to discuss other potential customers until this inaugural crew flight is over. But the company has said a fifth seat will be available to private clients. SpaceX periodically sells seats to tycoons and even countries eager to get their citizens to the station for a couple weeks.

Coming soon: Sierra Space’s mini shuttle, Dream Chaser, which will deliver cargo to the station later this year or next, before accepting passengers.

The Associated Press Health and Science Department receives support from the Howard Hughes Medical Institute’s Science and Educational Media Group. The AP is solely responsible for all content.

difficulties with space travel


New NASA Black Hole Visualization Takes Viewers Beyond the Brink

Ever wonder what happens when you fall into a black hole? Now, thanks to a new, immersive visualization produced on a NASA supercomputer, viewers can plunge into the event horizon, a black hole’s point of no return.

“People often ask about this, and simulating these difficult-to-imagine processes helps me connect the mathematics of relativity to actual consequences in the real universe,” said Jeremy Schnittman, an astrophysicist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, who created the visualizations. “So I simulated two different scenarios, one where a camera — a stand-in for a daring astronaut — just misses the event horizon and slingshots back out, and one where it crosses the boundary, sealing its fate.”

The visualizations are available in multiple forms. Explainer videos act as sightseeing guides, illuminating the bizarre effects of Einstein’s general theory of relativity. Versions rendered as 360-degree videos let viewers look all around during the trip, while others play as flat all-sky maps.

To create the visualizations, Schnittman teamed up with fellow Goddard scientist Brian Powell and used the Discover supercomputer at the NASA Center for Climate Simulation . The project generated about 10 terabytes of data — equivalent to roughly half of the estimated text content in the Library of Congress — and took about 5 days running on just 0.3% of Discover’s 129,000 processors. The same feat would take more than a decade on a typical laptop.

The destination is a supermassive black hole with 4.3 million times the mass of our Sun, equivalent to the monster located at the center of our Milky Way galaxy.

“If you have the choice, you want to fall into a supermassive black hole,” Schnittman explained. “Stellar-mass black holes, which contain up to about 30 solar masses,  possess much smaller event horizons and stronger tidal forces, which can rip apart approaching objects before they get to the horizon.”

This occurs because the gravitational pull on the end of an object nearer the black hole is much stronger than that on the other end. Infalling objects stretch out like noodles, a process astrophysicists call spaghettification .

The simulated black hole’s event horizon spans about 16 million miles (25 million kilometers), or about 17% of the distance from Earth to the Sun. A flat, swirling cloud of hot, glowing gas called an accretion disk surrounds it and serves as a visual reference during the fall. So do glowing structures called photon rings, which form closer to the black hole from light that has orbited it one or more times. A backdrop of the starry sky as seen from Earth completes the scene.

As the camera approaches the black hole, reaching speeds ever closer to that of light itself, the glow from the accretion disk and background stars becomes amplified in much the same way as the sound of an oncoming racecar rises in pitch. Their light appears brighter and whiter when looking into the direction of travel.

The movies begin with the camera located nearly 400 million miles (640 million kilometers) away, with the black hole quickly filling the view. Along the way, the black hole’s disk, photon rings, and the night sky become increasingly distorted — and even form multiple images as their light traverses the increasingly warped space-time.

In real time, the camera takes about 3 hours to fall to the event horizon, executing almost two complete 30-minute orbits along the way. But to anyone observing from afar, it would never quite get there. As space-time becomes ever more distorted closer to the horizon, the image of the camera would slow and then seem to freeze just shy of it. This is why astronomers originally referred to black holes as “frozen stars.”

At the event horizon, even space-time itself flows inward at the speed of light, the cosmic speed limit. Once inside it, both the camera and the space-time in which it's moving rush toward the black hole's center — a one-dimensional point called a singularity , where the laws of physics as we know them cease to operate.

“Once the camera crosses the horizon, its destruction by spaghettification is just 12.8 seconds away,” Schnittman said. From there, it’s only 79,500 miles (128,000 kilometers) to the singularity. This final leg of the voyage is over in the blink of an eye.

In the alternative scenario, the camera orbits close to the event horizon but it never crosses over and escapes to safety. If an astronaut flew a spacecraft on this 6-hour round trip while her colleagues on a mothership remained far from the black hole, she’d return 36 minutes younger than her colleagues. That’s because time passes more slowly near a strong gravitational source and when moving near the speed of light.

“This situation can be even more extreme,” Schnittman noted. “If the black hole were rapidly rotating, like the one shown in the 2014 movie ‘Interstellar,’ she would return many years younger than her shipmates.”

By Francis Reddy NASA’s Goddard Space Flight Center , Greenbelt, Md. Media Contact: Claire Andreoli 301-286-1940 [email protected] NASA’s Goddard Space Flight Center, Greenbelt, Md.

Related Terms

  • Astrophysics
  • Black Holes
  • Galaxies, Stars, & Black Holes
  • Galaxies, Stars, & Black Holes Research
  • Goddard Space Flight Center
  • Supermassive Black Holes
  • The Universe

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A Groundbreaking Scientific Discovery Just Created the Instruction Manual for Light-Speed Travel

In a first for warp drives, this research actually obeys the laws of physics.

If a superluminal—meaning faster than the speed of light—warp drive like Alcubierre’s worked, it would revolutionize humanity’s endeavors across the universe , allowing us, perhaps, to reach Alpha Centauri, our closest star system, in days or weeks even though it’s four light years away.

The clip above from the 2016 film Star Trek Beyond showcases the effect of a starship zipping through space inside a faster-than-light warp bubble. You can see the imagined but hypothetically accurate warping of spacetime.

However, the Alcubierre drive has a glaring problem: the force behind its operation, called “negative energy,” involves exotic particles—hypothetical matter that, as far as we know, doesn’t exist in our universe. Described only in mathematical terms, exotic particles act in unexpected ways, like having negative mass and working in opposition to gravity (in fact, it has “anti-gravity”). For the past 30 years, scientists have been publishing research that chips away at the inherent hurdles to light speed revealed in Alcubierre’s foundational 1994 article published in the peer-reviewed journal Classical and Quantum Gravity .

Now, researchers at the New York City-based think tank Applied Physics believe they’ve found a creative new approach to solving the warp drive’s fundamental roadblock. Along with colleagues from other institutions, the team envisioned a “positive energy” system that doesn’t violate the known laws of physics . It’s a game-changer, say two of the study’s authors: Gianni Martire, CEO of Applied Physics, and Jared Fuchs, Ph.D., a senior scientist there. Their work, also published in Classical and Quantum Gravity in late April, could be the first chapter in the manual for interstellar spaceflight.

Positive energy makes all the difference. Imagine you are an astronaut in space, pushing a tennis ball away from you. Instead of moving away, the ball pushes back, to the point that it would “take your hand off” if you applied enough pushing force, Martire tells Popular Mechanics . That’s a sign of negative energy, and, though the Alcubierre drive design requires it, there’s no way to harness it.

Instead, regular old positive energy is more feasible for constructing the “ warp bubble .” As its name suggests, it’s a spherical structure that surrounds and encloses space for a passenger ship using a shell of regular—but incredibly dense—matter. The bubble propels the spaceship using the powerful gravity of the shell, but without causing the passengers to feel any acceleration. “An elevator ride would be more eventful,” Martire says.

That’s because the density of the shell, as well as the pressure it exerts on the interior, is controlled carefully, Fuchs tells Popular Mechanics . Nothing can travel faster than the speed of light, according to the gravity-bound principles of Albert Einstein’s theory of general relativity . So the bubble is designed such that observers within their local spacetime environment—inside the bubble—experience normal movement in time. Simultaneously, the bubble itself compresses the spacetime in front of the ship and expands it behind the ship, ferrying itself and the contained craft incredibly fast. The walls of the bubble generate the necessary momentum, akin to the momentum of balls rolling, Fuchs explains. “It’s the movement of the matter in the walls that actually creates the effect for passengers on the inside.”

alcubierre drive model

Building on its 2021 paper published in Classical and Quantum Gravity —which details the same researchers’ earlier work on physical warp drives—the team was able to model the complexity of the system using its own computational program, Warp Factory. This toolkit for modeling warp drive spacetimes allows researchers to evaluate Einstein’s field equations and compute the energy conditions required for various warp drive geometries. Anyone can download and use it for free . These experiments led to what Fuchs calls a mini model, the first general model of a positive-energy warp drive. Their past work also demonstrated that the amount of energy a warp bubble requires depends on the shape of the bubble; for example, the flatter the bubble in the direction of travel, the less energy it needs.

☄️ DID YOU KNOW? People have been imagining traveling as fast as light for nearly a century, if not longer. The 1931 novel Islands of Space by John W. Campbell mentions a “warp” method in the context of superluminal space travel.

This latest advancement suggests fresh possibilities for studying warp travel design, Erik Lentz, Ph.D., tells Popular Mechanics . In his current position as a staff physicist at Pacific Northwest National Laboratory in Richland, Washington, Lentz contributes to research on dark matter detection and quantum information science research. His independent research in warp drive theory also aims to be grounded in conventional physics while reimagining the shape of warped space. The topic needs to overcome many practical hurdles, he says.

Controlling warp bubbles requires a great deal of coordination because they involve enormous amounts of matter and energy to keep the passengers safe and with a similar passage of time as the destination. “We could just as well engineer spacetime where time passes much differently inside [the passenger compartment] than outside. We could miss our appointment at Proxima Centauri if we aren’t careful,” Lentz says. “That is still a risk if we are traveling less than the speed of light.” Communication between people inside the bubble and outside could also become distorted as it passes through the curvature of warped space, he adds.

While Applied Physics’ current solution requires a warp drive that travels below the speed of light, the model still needs to plug in a mass equivalent to about two Jupiters. Otherwise, it will never achieve the gravitational force and momentum high enough to cause a meaningful warp effect. But no one knows what the source of this mass could be—not yet, at least. Some research suggests that if we could somehow harness dark matter , we could use it for light-speed travel, but Fuchs and Martire are doubtful, since it’s currently a big mystery (and an exotic particle).

Despite the many problems scientists still need to solve to build a working warp drive, the Applied Physics team claims its model should eventually get closer to light speed. And even if a feasible model remains below the speed of light, it’s a vast improvement over today’s technology. For example, traveling at even half the speed of light to Alpha Centauri would take nine years. In stark contrast, our fastest spacecraft, Voyager 1—currently traveling at 38,000 miles per hour—would take 75,000 years to reach our closest neighboring star system.

Of course, as you approach the actual speed of light, things get truly weird, according to the principles of Einstein’s special relativity . The mass of an object moving faster and faster would increase infinitely, eventually requiring an infinite amount of energy to maintain its speed.

“That’s the chief limitation and key challenge we have to overcome—how can we have all this matter in our [bubble], but not at such a scale that we can never even put it together?” Martire says. It’s possible the answer lies in condensed matter physics, he adds. This branch of physics deals particularly with the forces between atoms and electrons in matter. It has already proven fundamental to several of our current technologies, such as transistors, solid-state lasers, and magnetic storage media.

The other big issue is that current models allow a stable warp bubble, but only for a constant velocity. Scientists still need to figure out how to design an initial acceleration. On the other end of the journey, how will the ship slow down and stop? “It’s like trying to grasp the automobile for the first time,” Martire says. “We don’t have an engine just yet, but we see the light at the end of the tunnel.” Warp drive technology is at the stage of 1882 car technology, he says: when automobile travel was possible, but it still looked like a hard, hard problem.

The Applied Physics team believes future innovations in warp travel are inevitable. The general positive energy model is a first step. Besides, you don’t need to zoom at light speed to achieve distances that today are just a dream, Martire says. “Humanity is officially, mathematically, on an interstellar track.”

Headshot of Manasee Wagh

Before joining Popular Mechanics , Manasee Wagh worked as a newspaper reporter, a science journalist, a tech writer, and a computer engineer. She’s always looking for ways to combine the three greatest joys in her life: science, travel, and food.

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