by Jim Bell
At their current speeds—and at 10 miles per second, they are the fastest objects that humans have ever sent into space—the Voyagers will still take about 30,000 years to reach the outer edge of the Oort Cloud (they should reach the inner edge in “only” about 300 years). The distances between asteroids and comets out there are so vast that it is highly improbable that either spacecraft will pass anywhere near any of them. Another 10,000 years later Voyager 1 will pass only about 100,000 AU past the red dwarf star Gliese 445, which is now moving toward the sun and will by then be one of the closest stars to our solar system, just under four light-years away. Around the same time, Voyager 2 will pass only about 111,000 AU from another red dwarf star, Ross 248, which will actually by then be the closest star to the sun in the sky. If there are inhabited planets around those stars—unlikely, given their small size and very faint output of energy as compared to our own sun, but still, who knows?—I wonder if anyone will notice these little emissaries zipping past from the star next door?
Indeed, Carl Sagan and his Golden Record colleagues speculated about whether it might be possible to avoid “the near certainty that left to themselves, neither Voyager spacecraft would ever plummet into the planet-rich interior of another solar system.” Both they and I wonder if it might be possible to command one final “empty-the-tank” thruster firing, just before final communication with each Voyager is lost, to “redirect the spacecraft as closely as possible so that they will make a true encounter [with these stars]. If such a maneuver can be affected then some 60,000 years from now one or two tiny hurtling messengers from the strange distant planet Earth may penetrate into their planetary systems.” If no one else does, I will try to remember to make this request to Suzy Dodd or whoever is running the Voyager Project in a decade or so, as the spacecraft power levels wind down. We have the fuel. Feel free to mention it to your congressperson.
It might also be interesting to see if we can upload images back onto Voyager’s tape recorders before we lose communication with them forever. While their Golden Records tell of their home world, there is nothing onboard each spacecraft that tells the stories of their magnificent adventures within their solar system. It’s the same sentiment that motivates Jon Lomberg to pursue his “One Earth” New Horizons digital message project. “One thing I wish could have been on the Voyager Record,” he told me, “which we are going to remedy with the New Horizons digital message, is that I wish we could have had something of ‘here’s what Voyager was and here’s what Voyager found,’ because it’s one of the best things human beings have ever done. If they ever find Voyager they won’t know about its mission. They won’t know what it did, and that’s sad.” So I say let’s try to upload the Earth-Moon portrait; the historic first close-up photos of Io’s volcanoes and Europa and Ganymede’s cracked icy shells; the smoggy haze of Titan; the enormous cliffs of Miranda; the strange cantaloupe and geyser terrain of Triton; the swirling storms of Jupiter, Saturn, and Neptune; the elegant, intricate ring systems of all four giant planets; the family portrait of our solar system. Let’s arm our Voyagers with electronic postcards so that they can properly tell their tales, should any kind of intelligence ever find them.
INTELLIGENT BITS AROUND THE GALAXY
The idea of astronomers on other planets potentially noticing the Voyagers, or other signs of our civilization, is not as far-fetched as it used to be. In the past two decades, astronomers have discovered the first evidence of planets around other nearby stars that are like our sun. The first such planets were found using sensitive instruments on ground-based telescopes to search for tiny wobbles in a star’s motion caused by the gravitational tug of planets orbiting around it. That technique, called the radial velocity method, is most sensitive to really big planets that are really close to their parent stars, causing a big wobble. And indeed, astronomers began finding dozens and dozens of these so-called hot Jupiters (named because they’re Jupiter-sized but much hotter than our Jupiter because they are much closer to their parent stars) orbiting nearby stars. Some stars even have multiple close-in giant planets. Strange indeed, and nothing like our own solar system, where the giant planets are far away from their star. So is our solar system an oddball, and most stars have giant planets orbiting close-in? Or is our system typical, and the only reason we’re finding so many hot Jupiters is the streetlight effect: we’re finding what we’re seeking only because we’re looking where it’s easiest to find exactly those things (like looking near the streetlight at night for your lost car keys, because that’s where the light is). In this case, just because hot Jupiters are the easiest planets to find using the radial velocity method, that doesn’t mean that they are the most common kind of planet out there.
Other methods were needed, to counteract the biases of the radial velocity method and to help find out what is a “typical” solar system in our galactic neighborhood. One promising idea, which had been advocated for decades by my former NASA Ames Research Center colleague Bill Borucki and others, was to build a camera that would patiently stare at the light from many stars and hope that some of them have planets that, occasionally, will pass in front of their star as seen from our perspective and dim that star’s light by a tiny but measurable amount. This is called the transit method, because the planet passes across—or transits—its parent star. We can see this from time to time from Earth in our own solar system: both Mercury and Venus occasionally transit in front of the sun. Mercury last transited the sun in 2006 and will next transit in 2016. Venus last transited the sun in 2004 and 2012 and won’t transit the sun again from our perspective until the years 2117 and 2125. Transits are rare events, but they do happen if you’re in the right place at the right time, and if you were to observe huge numbers of nearby stars, then even rare events like that should happen to some of those stars now and then.
That was exactly the philosophy that Bill Borucki had in mind when he and colleagues pitched the Kepler space telescope mission (named after the planetary-orbit discoverer Johannes Kepler) to NASA in 2001. Bill’s idea was to launch a very sensitive camera and telescope—so sensitive that they could detect a 0.002 percent change in the light of a star corresponding to the dimming caused by transiting Earth-sized or smaller planets—and to literally stare at the same 150,000 stars or so for years to detect such transits. On the one hand, the Kepler mission has got to represent the most boring mission ever conducted, as it would orbit far beyond the Earth, and simply stare at the same region of space (a random patch of sky about as big as your fist held out at arm’s length, in the northern constellations Cygnus and Lyra) over and over and over, radioing the same picture back to Earth again and again and again. But some of those stars in the picture should, statistically, show occasional transiting planets, NASA and planetary science colleagues realized, and by carefully tracking those statistics, we could figure out what percentage of stars have Earth-sized (or smaller) to Jupiter-sized (or larger) planets, and how far from their parent star these new worlds most commonly orbit. So, maybe in the end, Kepler could turn out to be one of the most exciting missions ever conducted.
Happily, that is what has come to pass. Kepler was launched in 2009, and within a few months of taking pictures, we began to see transits occurring from the “low-hanging fruit”—the easiest planets to find—the same kinds of Jupiter-sized worlds orbiting close to their parent star that the radial velocity method was so good at finding. But over the course of several years, as more images were taken and as the Kepler team got better at working with their data, smaller and smaller planets, orbiting farther from their parent star, were revealed. To date, Kepler has found more than 1,000 planets around nearby stars. The most exciting findings have been for a small fraction of those planets that are close to Earth in size and orbiting in the so-called habitable zone around their parent star. The habitable zone is the region around a star where an Earthlike planet with an Earthlike atmosphere could have stable liquid water on its surface and thus potentially support
life as we know it. In our own solar system, the habitable zone runs from about the orbit of Venus to about the orbit of Mars. The surfaces of Venus and Mars themselves are not presently considered habitable because their atmospheres are significantly non-Earthlike (Venus’s CO2 atmosphere is hot and 100 times thicker than Earth’s; Mars’s CO2 atmosphere is cold and 100 times thinner than Earth’s). But they are still in roughly the right part of our solar system to have been habitable, if their atmospheric conditions were different in the distant past, for example. And Mars, as we now know from rovers and other missions sent there, could very well still have habitable zones lurking underground, where water, heat, and organic molecules might still all be found.
Earth is, of course, the Goldilocks planet—not too hot, not too cold, just right—and our planet defines what it means to be a habitable world in a star’s habitable zone. A luckily placed alien astronomer on a nearby star might be able to see the Earth transit in front of the sun from their perspective. But they might not be able to tell the difference between a transiting Earth and the transiting of a sunspot, so they’d want to see it happen again. So they’d have to wait for a year—there it would go again! No good scientist would be able to convince skeptical colleagues that this wasn’t just coincidence, however, so that alien scientist would have to predict the next transit . . . wait another year . . . and voilà! If a slight drop in the star’s light is seen again right when predicted, they know they’ve bagged a real planet. That same philosophy is used by the Kepler team, which is why they often need three or four years of data to find proof of the discovery of Earth-sized planets in the habitable zones around nearby sunlike stars.
I had a short involvement with the Kepler Project and the search for extrasolar planets in the early 1990s, after I graduated from the University of Hawaii and moved on to a postdoc position at the NASA Ames Research Center at Moffett Field, just north of San Jose, California. Bill Borucki gave a lunchtime talk about his (then) crazy-sounding idea for a space-based mission to use transits to find Earthlike worlds around other stars. Bill is a soft-spoken guy but from the beginning has been one of the world’s most passionate advocates of the possibilities for discovery using the transit method. I asked if there was some small way I could get involved, some minor project I could help to advance in some way. He mentioned that not many people had been thinking yet about what kinds of transits could be detected around binary star systems. Since most stars in the galaxy that are similar to our sun are members of binary (or more) star systems, it seemed like a relevant topic to explore. I reminded myself of Kepler’s laws of motion, wrote some computer programs to simulate stars in orbit around each other, and added some computerized planets in different places in the system. I found very different, but distinctive, transit signatures from planets orbiting both stars, and planets orbiting just one of the binary companions.
We wrote up some short descriptions of our results as abstracts for a few conferences, including a very unconventional and eclectic workshop called the Bioastronomy Symposium: Progress in the Search for Extraterrestrial Life, held in Santa Cruz in 1993 and attended by a wild mix of scientists, artists, philosophers, musicians, science-fiction writers, biologists, cosmologists, and, it seemed, perhaps even some actual clinically insane people, all thinking about the search for habitable planets and life beyond Earth. Unfortunately, I got bogged down in other projects and never wrote up our results in a peer-reviewed journal. After Bill finally got his Kepler mission approved and he and other colleagues finally did find evidence for planets around binary star systems—the first was reported in a paper published in Science magazine in September 2011—I was excited that the kinds of solar systems that we had dreamed about back then actually turned out to exist, and some had the kinds of transit signals that we had predicted in our long-ago study.
Some of the most interesting kinds of planets that Kepler is discovering are dubbed super-Earths or mini-Neptunes. Approaching or comparable in size to Neptune, these worlds are either ice giants like Uranus and Neptune or much larger, rocky planets like Earth that have become large enough that their gravity can help them hold on to thick atmospheres as they are forming. It turns out that these kinds of planets—and ice giants in particular—are, so far, the most common kinds of worlds seen around other nearby sunlike stars in the Kepler transit images. How fortunate, then, that Voyager 2 was able to characterize in detail two such planets right here in our own solar system. Indeed, the discovery from Voyager 2 data that Uranus and Neptune are not gas giants but are instead a separate, new class of planets called ice giants has turned out to be fundamental to understanding the figurative zoo of planetary types being found by the Kepler team. And importantly, the realization from Voyager and other missions that many of the moons of the giant planets are worthy of consideration as potential habitable worlds should be a warning to astronomers to not consider the idea of a close-to-the-star habitable zone as a firm and fixed constraint on where, in other solar systems, we might need to go looking for life.
THE END?
What does the future hold for the Voyagers themselves, the robotic harbingers of our new Interstellar Age? Most experts predict that by the mid- to late-2020s or so, their slowly decaying plutonium-238 power supplies will not be able to produce enough power to keep them heated and in contact with the Earth. They will fall into a long, permanent, silent deep freeze. The cold and vacuum of space will silence their hearts but will not cease their traveling on to the stars. Micrometeorites and high-energy cosmic rays will occasionally strike them, creating tiny pits or other local-scale metallurgic damage probably too small to notice for any particular event, but certainly more significant when summed over thousands and millions of years. The Voyagers’ Golden Records are better protected than many other parts of the spacecraft, nestled inside their gold-plated aluminum cases on each vessel. The records’ outer-facing surface is predicted to be pitted by micrometeorites over less than 2 percent of its surface by the time the ships are a full light-year away. Because of the lower density of particles in interstellar space, it should take an additional 5,000 light-years of travel, or about 100 million years, for the outer surface of the records to accumulate an additional 2 percent damage. During this time the inner surface of the records, protected even more from the “elements” of space weathering, will remain essentially pristine. Thus, barring a highly unlikely catastrophe such as a chance impact with a rogue asteroid or comet, or an extremely low-probability capture and subsequent burn-up of the spacecraft by a nearby star, there is nothing that should destroy or seriously degrade the physical integrity of the Voyagers and their precious messages from Earth. Will they, then, continue to ply the stars forever?
I am not so sure, because forever is an awfully long time. I believe that someday, in the far future, it will be humans, rather than extraterrestrials, who might have the highest probability of catching up to the Voyagers. It seems inevitable, and I can imagine many possible scenarios for that encounter. Imagine, for example, that 50,000 years from now an enormous generational starship, launched by a consortium of privately funded gajillionaires seeking new adventures and new opportunities, heads off for the solar system around the star Gliese 445. That particular star will be only about 3 light-years from the sun at the time, and astronomers have discovered an Earthlike moon orbiting one of its gas giant planets. As they near the star, the settlers decide to search for the ancient Voyager 1 probe, which is thought to be traveling in the same general vicinity. They find and gently scoop up the frail, spindly spacecraft and lovingly set it up as a monument to the beginning of the Interstellar Age in their ship’s main atrium. Plans start to be formulated about a more permanent monument to be built around the spacecraft when they arrive at their new home world.
In a little less than 300,000 years, Voyager 2 will pass about 270,000 AU (about 4.3 light-years) from the famous young, hot, blue star called Sirius. Sirius is famous partly because it is the brightest star in the sky,
aside from the sun, and partly because ancient civilizations, such as the Egyptians, Greeks, and Polynesians used Sirius for timekeeping and navigation. Voyager 2 will be half as far away from Sirius as we are now, and so what many of us call the Dog Star (the heart of the constellation Canis Major) will be four times brighter to the spacecraft. An impressive sight that would be, if we could somehow plan to turn the cameras back on in the year 298,015. Beyond then, it’s hard to know exactly what the Voyagers will pass and when, because of uncertainties in the relative motions of the sun and the nearby stars. Certainly over future millions of years the spacecraft will make other encounters within several light-years of other nearby stars. The thrusters may fire one more time and propel one or the other of our Voyagers into another solar system, or maybe not. Maybe there will be some form of intelligent life in that solar system. Who knows.
More generally, though, both spacecraft are destined to follow long, relatively circular, 250-million-year-long orbits around the center of the Milky Way galaxy, just like the sun and many other nearby stars. The Voyagers are interstellar travelers, but they are not intergalactic travelers—they would have to have been accelerated about fifteen times faster than they’re currently going, or about 1 million miles per hour, to escape the gravity of the Milky Way. It’s nice to know that even in the far future, though long dormant, the Voyagers will still be making graceful trips around the galactic center with us.