Reading the Pioneer/Voyager Pulsar Map

by Wm. Robert Johnston updated 30 October 2007

This interesting question came up recently: which pulsars are used in the Voyager record pulsar map? Answering this question turned into an interesting exercise in interpreting this map, which I review here.

During planning of the Pioneer 10 and 11 missions to the outer solar system, a group of astronomers proposed including a message for any intelligent extraterrestrials that might discover the probes. These two probes would be the first manmade objects to leave the solar system. While the probability of such beings discovering the probes is infinitesimally small (even assuming they exist), a message was included on a metal plaque on the outside of each probe prior to their launches in 1972 and 1973.

For Voyagers 1 and 2, outer solar system probes launched in 1977 and also destined to leave the solar system, a more extensive message was incorporated into a record containing sounds and digital pictures from Earth. The cover of the record included a message modified from the Pioneer message.

All four messages included in particular a pulsar map. This map was designed by Frank Drake to shown the location of the Earth relative to 14 pulsars. Pulsars are rapidly rotating neutron stars which emit beacons of radio waves. The precise timing of the resultant radio pulses provide markers observable in radio wavelengths across broad stretches of the galaxy.

A quick search failed to turn up a readily accessible list of the pulsars used in the map, so I decided to experiment with interpreting the map. If the map could really be interpreted to show our location, it would then be possible to identify the pulsars from the map. This would serve to test at least a few elements (only a few) involved in its potential interpretation.

The map identifies the distance and direction of the 14 pulsars relative to the direction and distance of the center of the galaxy, along with--most importantly--their periods. The periods are expressed in binary notation as multiples of the hyperfine transition period of hydrogen (7.04024183647 x 10 -10 sec). This transition occurs in hydrogen atoms and involves a change in the relative spins of the proton and electron; the resultant radio-frequency radiation, with a wavelength of 21.1 cm, is observed in interstellar gas.

Using a diagram from p. 58 of Murmurs of Earth by Sagan et ali (which describes the Voyager record message), the data in table 1 is derived (listing the pulsars as on the map, clockwise from the line to the center of the galaxy). (Note: some measurements may not be accurate due to the quality of the diagram.) Pulsar period is given in the units of the hydrogen hyperfine transition (in both binary and base 10) and converted to seconds. The direction and distance information is less precisely indicated on the diagram. Distance in particular to most pulsars is poorly determined, even today.

Table 1: Pulsar data from the Pioneer/Voyager pulsar map

As an initial attempt, the derived periods were compared to a list of pulsars known as of June 1975. Using this sample of 147 would narrow down the possibilities, since it would at least include the 14 used to create the map around 1970. The list used was from Taylor and Manchester, "Observed properties of 147 pulsars," (1975), The Astronomical Journal , (80:794-806). Matches were found for all 14 pulsars, with data given below in table 2. Included is the difference (in parts per million) between the periods in the map and the presumed matches in Taylor and Manchester; note that most are quite good.

Table 2: Pulsars identified based on the pulsar map, 1975 data

Finally, a modern pulsar listing was used to check that the pulsars can be unambiguously identified. Using the ATNF Pulsar Catalog (2002), which lists 1,480 pulsars, the data in table 3 was obtained for the above pulsars. Celestial coordinates (RA and DEC) and galactic coordinates (l and b) are given along with period, P dot (the rate of decrease of the period), the epoch (time of period measurement), and distance.

Table 3: Same pulsars, 2002 data

All 14 pulsars could be distinguished from others, although not by period alone in one or two cases. In these cases the direction made the choice clear. Table 4 compares direction and distance information from the pulsar map to that derived from the ATNF Catalog data.

Table 4: Pulsar direction and distance information

The direction information is somewhat helpful, but the distance information is not.

The precise nature of pulsar periods largely extends to the rate of decrease of these periods. If the pulsar map periods are accurate, one can in principle determine the time corresponding to those measurements of the pulsar periods. Comparing the data from the ATNF Catalog to that from the pulsar map, this is used in table 5 to estimate the date of origin of the pulsar map.

Table 5: Derived date of map origin

Discarding the discordant value from PSR J1645-0317, this gives a date of origin of 1969.7 ± 1.2 year! This is not a bad result. (A note: I don't know whether the pulsar data was updated for the Voyager pulsar map.) Now suppose some ETI reads the map and manages to determine which pulsars we were referring to. Reading the map is not so simple in this case: the solar system is moving through space, as are the pulsars (generally with higher space velocities than main sequence stars); adjusting for the change in the pulsar periods must take into account the light travel time from the pulsars, which is hundreds to tens of thousands of years. But if these factors were successfully accounted for, the map could be used to identify the location of the solar system to within tens of light years or less.

Of course, my task of reading the map was far less daunting than it would be for some ETI that finds the map. Just a sampling of issues for interpretation that did not enter this analysis:

Epilogue: After completing this page, I received a reply from the Planetary Society which referred me to a 1972 article by Carl Sagan, Linda Salzman Sagan, and Frank Drake identifying the pulsars. The article uses yet another type of pulsar identifier in a table, data from which is given below (note: this was read from a poor reproduction, so there may be errors in my table). The last column gives the pulsar identifier from a 1970 article by B. Y. Mills, giving some indication of the then non-standardized cataloguing system.

Table 6: Pulsar list from Sagan, Sagan, and Drake

A time interval of only 3 weeks existed between the formulation of the idea of including a message on Pioneer 10, achieving NASA concurrence, devising the message, and delivering the draft message for engraving.
The large number of digits is the key that these numbers indicate time intervals, not distances or some other quantity... There are no other conceivable quantities that we might know to ten significant figures for relatively distant cosmic objects.
With 14 periods, almost all of which are accurate to nine significant figures in decimal equivalent, a society which has detailed records of past pulsar behavior should be able to reconstruct the epoch of launch to the equivalent of the year 1971. If past records of pulsar "glitches" (discontinuities in the period) are not kept or reconstructable from the physics it should still be possible to reconstruct the epoch to the nearest century or millennium... If either [of the two pulsars nearest the Earth] is correctly identified, it can be used to place the position of our solar system in the galaxy to approximately 20 pc, thereby specifying our location to approximately 1 in 10 3 stars.

(In 2007 Rick Nungester alerted me to a few errors, particularly my calculation of the period of pulsar #1; this is corrected above.)


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  • The Big Idea

NASA sent a map to space to help aliens find Earth. Now it needs an update.

The map that NASA launched in 1972 could lead extraterrestrials to Earth. A new map, nearly 50 years later, provides even better directions.

A sparkling mass containing at least half a million stars—and some two dozen pulsars—the globular cluster known as 47 Tucanae is one of roughly 150 ancient stellar clumps orbiting the Milky Way galaxy.

A half century ago astronomers designed a map that would point to Earth from anywhere in the galaxy. Then they sent it into space, reasoning that any aliens smart enough to intercept a spacecraft could decode the map and uncover its origin. Many movies and TV shows have used variations on this theme as a plot point, but we didn’t borrow it from science fiction. It’s reality.

Truth is, this tale has been part of my family’s lore since before I was born. Growing up, I’d heard stories about the map and seen its depiction on multiple interstellar spacecraft, and several years ago, I found the original, penciled-in pathway to Earth where my parents had stashed it. (More on this later.)

That was an exciting find! Then came the buzzkill: This original map won’t be good for much longer, cosmically speaking. The signposts it uses will disappear within tens of millions of years, and even if they don’t, the map would point toward our home for only a fraction of the 200 to 250 million years it takes the sun and other nearby stars to spin once around the Milky Way.

Sure, the chances of aliens intercepting the map are astronomically improbable—but if that did happen, an outdated map would be useless rather than helpful. And that wasn’t the goal.

Why on Earth does this map even exist?

It was December 1971, and NASA was getting ready to launch Pioneer 10 , a spacecraft that would sweep by Jupiter and make the first reconnaissance of the solar system’s biggest planet. More stunningly, though, Pioneer 10’s brush by Jupiter would sling it onto an interstellar trajectory, making it the first ever human-made object destined to leave the solar system.

With a little help from his friends, the astronomer Carl Sagan decided that the craft ought to carry a greeting from humanity—a message identifying and commemorating Pioneer’s makers that would be interpretable by anyone who found it. NASA agreed and gave Carl less than a month to design the message.

This is when Carl’s friend, the astronomer Frank Drake , enters the story. Frank is also my dad, and among other notable accomplishments, he is credited with conducting the first scientific search for noisy aliens and with formalizing a framework for estimating the number of detectable alien civilizations in the Milky Way galaxy. ( Read more about how Frank Drake changed astronomy. )

Carl asked Dad for help crafting the message while the two of them were in San Juan, Puerto Rico, for a meeting of the American Astronomical Society . Dad recalls that, in the lobby of the San Gerónimo Hilton, he and Carl quickly came up with ideas about what to include: line drawings depicting humans, a rendering of the spacecraft—and then, “in the next moment, we hit on the idea of a galactic map that would pinpoint the location of the Earth in space.”

Dad designed that map, and in 1972 it flew into space aboard Pioneer 10. The next year Pioneer 11 launched, ultimately carrying the map past Saturn and now on to the stars. Then in 1977 both Voyager spacecraft left Earth carrying Dad’s guide to finding our planet, which is etched onto the cover of the “ golden record .” The way Dad designed the map means that it points back to Earth both in space and in time, making it a galactic positioning system (a different kind of GPS) in four dimensions.

At the time, Dad and Carl didn’t really worry that the aliens who found their message in a bottle might be of the more malevolent variety.

How the map was made

Our galactic neighborhood has no obvious street signs, and crafting a map pointing to one planet among the billions (and billions) of worlds populating the Milky Way is no simple feat.

Finding Earth means finding the solar system , and the sun is rather unremarkable. There’s really no way to distinguish it from the other several hundred billion stars in the galaxy, each of which is tracing its own path around the galactic center and slowly shifting in location relative to its neighbors. That stellar jostling means the constellations spangling Earth’s skies won’t be the same in our near future—nor do the stars align in the same recognizable configurations from anywhere other than the solar neighborhood. In fact, in about 2,000 years, Polaris will no longer be the North Star, just as it was not the polestar for ancient Egyptian, Babylonian, and Chinese sky-watchers.

So, what to do? Though normal stars with churning nuclear engines in their cores might not have distinctive fingerprints, Dad realized that pulsars—the corpses of stars that once were much larger than the sun—are potentially uniquely identifiable. Discovered in 1967, pulsars spin very rapidly, often hundreds of times per second. Using powerful radio telescopes, astronomers can measure with extreme precision how quickly pulsars rotate, meaning that each of these spinning stellar relics writes its own signature in space. Dad selected 14 pulsars that could triangulate Earth’s position, and he coded information about their rotation rates into the map.

It’s not your typical map

Appropriately, Dad’s pulsar map looks like a fancy asterisk, a radial explosion of hatched lines that intersect at our solar system’s location. Briefly, here’s how his map works:

Each of the lines connect Earth to a pulsar. The hatch marks are binary numbers that reveal the pulsar’s rotation rate (at the time the map was designed), and line lengths are roughly proportional to distance. Some of the pulsars parked on Dad’s map—for example, the Crab and Vela—sit in the centers of beautiful nebulae created during the pulsars’ violent formations. Presumably, any civilizations sharp enough to detect and snare a quiet interstellar spacecraft would know about pulsars. And by matching the rotation periods on the map with stellar signposts in the sky, aliens could find their way to Earth relatively easily.

In addition, because the energy we see from pulsars comes from their spin and they slow down over time, Dad’s map also points to Earth in the fourth dimension. By calculating the difference between the observed and coded rotation periods—a difference that will be apparent after thousands of years—aliens could figure out how long ago the map was made.

Perhaps somewhat surprisingly, Dad’s map became lodged in the popular imagination and is now commonly found on everything from T-shirts to tattoos. I guess there’s something captivating about always being able to find your way home, even in the most cosmic sense imaginable.

Keeping it in the family: A love story

Several years ago, two significant things happened. I found the original, penciled-in pulsar map, folded away and casually tucked into a tomato box in my parents’ closet. And I linked up with a rock climber named Scott Ransom, one of the world’s more prolific pulsar astronomers.

Scott had been thinking about the Voyagers, the “golden record,” and the pulsar map since he was a 10-year-old in Mansfield, Ohio, watching Carl’s Cosmos television show. Some years and an astronomy Ph.D. later, he realized that Dad’s map has a near-future expiration date. Its Achilles’ heel is the same property that lets it pinpoint Earth in time: Pulsars slow down, and the ones Dad had chosen (from the few known at the time) would fade and disappear within several million years, give or take a few millennia.

Coincidentally, Scott had set out to make a new, more precise, and longer-lived pulsar map even before we moved in together and portmanteau’d ourselves into the Dranksomes. Now I write the words that tell our stories, and Scott does the important cartographic stuff such as choosing pulsars and deriving their binary codes. He occasionally drafts some text passages, but you’ll never catch me committing academic acts of astronomy.

A newer, better map to Earth

Scott’s new map is a GPS for the ages. It navigates to Earth using pulsars both inside and outside the Milky Way, with a twist.

(This is Frank Drake and Carl Sagan’s original map. Explore Scott Ransom’s new map here. )

Instead of the more ordinary pulsars Dad selected, the new map employs millisecond pulsars that spin faster, last longer, and have also-dead orbital companions. These binary pulsars afford a second set of identifiers: the orbital period of the system, which does not change over billions of years. And, crucially, millisecond pulsars age much more slowly than the ones in Dad’s map, meaning that it takes thousands of times longer for their spins to become unrecognizable.

In addition, Scott included another layer of signposts: pulsars in globular clusters orbiting the Milky Way. Ancient clumps of stars that predate the Milky Way, globular clusters are gorgeous and mysterious, and they are veritable millisecond pulsar factories.

By including signposts in these hard-to-miss stellar globs outside the galaxy, Scott’s map allows Earth to be discoverable for billions of years, even after the Milky Way’s stars have trekked around the galactic core multiple times, shuffling their positions and obliterating constellations.

And Dad, for the record, thinks that’s spectacular.

But first, someone has to read it

Dad’s map, of course, is still out there—but chances are slim to zero that the Pioneers or Voyagers carrying it will be intercepted. Though all four spacecraft are on interstellar trajectories, space is big, and the next stellar systems on the horizon are many thousands of years away. Plus, the spacecraft are tiny and will be completely quiet within the next couple of decades, making them extremely hard to detect.

As for sending the new map: There’s no Voyager-like space probe scheduled for launch anytime soon. But if this map did hitch a ride beyond our solar system, and if it got scooped up by intelligent space aliens, the map should be quite easy for them to read and follow.

That raises all sorts of questions: Would extraterrestrial beings at those distances have the means to reach Earth? If so and they head our way, what if they don’t come in peace? What if they’re hangry? And what if they’re not vegetarians?

Here’s the fundamental question that didn’t stop Carl and Dad: Is it a good idea to randomly send our address into the cosmos? Today, some folks would have no reservations, given that earthly transmissions already are leaking into space and, traveling at the speed of light, are detectable by anyone with a decent radio telescope living within a hundred light-years of us. Other folks, perhaps more cautiously, would hold off on announcing our presence until we know if ETs have honorable intentions. ( This is what a Martian looks like—according to Carl Sagan. )

As for the Dranksomes: We’d gladly send out the new map to Earth, as a bid to ensure that our presence as a species would live on in some form. If that message in a bottle were finally picked up, after bobbing and drifting through the galactic ocean for millions or billions of years, someone would know that Earthlings did exist—or, with luck, still do.

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