The Interstellar Age

by Jim Bell

  Voyager 2 flew right through the gap between the two outermost rings but was still impacted by hundreds of tiny dust particles per second for several minutes. Luckily, it emerged unscathed, perhaps because the dusty ring particles are so small—only about 1/100th the width of a human hair. It’s still not known what makes the rings clumpy, though planetary scientists suspect that Neptune’s ring clumps may be getting “shepherded” in their orbits, like some of the thin rings of Saturn, by little moons that were too small and faint for Voyager to see. The rings were eventually named after early astronomers who were instrumental in the initial discovery and characterization of Neptune, including Le Verrier, Galle, and Adams, and the thickest clumps in the outermost Adams ring have been named Liberté, Égalité, and Fraternité, in honor of the fact that it was France that in 1846 took victory in the race to discover Neptune.

  I spent the week surrounding the Voyager Neptune flyby back in Pasadena, having been granted a magic access badge through Fraser Fanale’s invitation, and maybe also owing to the fact that enough of the team knew and remembered me from the Uranus flyby that they figured I wouldn’t cause too much trouble. And this time, as a graduate student in the field, I might even be useful. Upon setting foot once again in those rooms where, brand-new, the images were streaming in from the farthest reaches of our solar system, I happily filled the role of gofer and errand boy for the real members of the imaging team. I was dutifully taking lunch orders, making copies, and otherwise trying to stay out of the way but still soak it all up, as if through some sort of academic osmosis. This would be Voyager’s last stop on the way to the stars.

  THE NON-PLANETARY SOCIETY

  By Neptune, Voyager team members like Rich Terrile and others had become very good at hunting for small new moons or strange patterns in the rings around giant planets, with the team having so far discovered twenty moons around Jupiter, Saturn, and Uranus, and thus having increased the number of then-known moons in the entire solar system by nearly 70 percent.

  “That was a lot of fun,” recalls Rich. “As an astronomer, you’re trained in trying to pull some useful signal out of noisy data, and hunting for new small moons was exactly that kind of problem.” One of his Voyager team colleagues challenged him at one point, stating flatly that “it’s no big deal to find these things; they’re just in the data.” So Rich challenged that person back, giving them some images with recently found moons to look through as a test—which they promptly failed. The reason why imaging team members like Rich Terrile could spot these subtle little moons so quickly was because they’d had years and years of experience studying all the quirks and characteristics of stars, cosmic-ray hits, and camera artifacts in the Voyager images, and they could tell when a new little dot in an image wasn’t any of those—especially if it was moving from image to image.

  True to form, as the final pre-flyby Voyager approach images were streaming in, Rich and other team members were busily starting the process of discovering six small new moons of Neptune that hadn’t been seen or confirmed from previous telescopic observations. Candy Hansen recalls the lighthearted teasing that Rich got from the team about his prowess for discovering new moons or features in Voyager images. “Like the time we told Rich that he couldn’t come drinking with us at the Loch Ness Monster [a bar nearby the lab] until he discovered something,” she recalled, laughing. “And then he showed up at that seedy, seedy bar with a hard copy of a Voyager photo showing an elliptical ring!” Rich earned his drink that night, and many others.

  “That really happened,” Rich confirms. “I was perfectly OK to stay at work and find more things, though. I was on a roll!” Combined with the giant moon Triton (about the size of Europa or our own moon), and the smaller, elliptically orbiting moon Nereid, these new moons took Neptune’s then-known moon count up to eight. Since Voyager, six more smaller, fainter moons have been discovered by more sensitive ground-based and space-based telescopes.

  If the moons of Neptune were to be the last planetary hurrah for Voyager, it would head for the stars with fanfare. After swinging just 3,000 miles above the cloud tops of Neptune (closer than to any planet that the spacecraft visited since leaving Earth), Voyager 2’s trajectory would take it past one final, glorious, unknown destination: Triton. The moon was discovered shortly after Neptune itself in 1846 (because it is so big and so bright), and Voyager imaging team members figured it would be an oddball of some kind because of its unusual, backward orbit. The spacecraft would fly within 25,000 miles of Triton, taking pictures of surface features as small as 5 to 10 miles across, and no one really knew what to expect. Because it is so bright and so far from the sun, Triton’s is among the coldest natural surfaces in the solar system, with an average temperature only about 38 degrees above absolute zero (or an incomprehensible –391°F). Triton’s brightness suggested that there would be relatively clean ice on the surface, perhaps even including exotic, low-temperature ices other than water ice. And its strange backward orbit suggested that it may have been through some sort of planetary-scale trauma, such as being captured by Neptune, or had its course changed by some sort of giant impact. It was a great way to end the surface-imaging phase of a great mission—with an encounter that would be surprising no matter what was revealed.

  Last Port of Call. Voyager 2 flyby trajectory past Neptune. (NASA/JPL)

  About five hours after closest approach to Neptune, Voyager 2 flew past Triton. Several days later, I remember being in the JPL workroom with imaging team member Larry Soderblom, looking over the first high-resolution images of Triton that had come in. Larry is a friendly, outgoing, sometimes mischievous, and highly respected member of the planetary science community who works at the US Geological Survey’s Astrogeology Science Center in Flagstaff, Arizona (I’ve always been confused about the name of that group, as astrogeology technically means “the geology of stars,” but stars don’t have geology—planets and moons and asteroids and comets do. But then again, “astronauts” don’t travel to the stars . . . at least, not yet . . .). Larry was one of the Voyager imaging team members who would occasionally give me a nod and a wink and beckon me away from some dark, out-of-the-way corner to come sit at the big table and look at some images. How could I resist?

  Luckily, I didn’t feel too stupid, because what bizarre images they were! Larry was as mystified as I was. Instead of a surface covered with classic impact craters, cracks and ridges, or other typical features like those that had been seen on many icy moons before, Triton was determined to be different. The part of Triton’s southern hemisphere that was sunlit during Voyager’s flyby was split into two very weird kinds of terrains: a darker one consisting of pits and dimples and ridges reminiscent of the skin of a cantaloupe, and a brighter one consisting of smoother plains materials interspersed with terraced depressions that looked like frozen lakes. A translucent, reddish layer of nitrogen ice in some places, and nitrogen snow or frost in others, appeared to drape much of the terrain. That, and the relative lack of impact craters, suggested that the surface was geologically very young. It was as strange and unexpected a place as Voyager had ever revealed, and I remember Larry laughing to himself more than once. “Isn’t that just beautiful?” he would ask rhetorically.

  To top it off, Voyager’s ultraviolet spectrometer team had recently discovered a surprise—Triton has a very thin atmosphere (less than 0.001 percent of Earth’s pressure) made mostly of nitrogen and methane. Larry was looking for some evidence of the interaction of that thin atmosphere with the surface. He and others had already noticed dark wind streaks near the south pole—places where that thin atmosphere appeared to be moving sediments across the surface. Using software that he and his USGS colleagues had developed, Larry was making short time-lapse movies of Triton’s surface as Voyager sped past, looking for evidence of any changes that the wind might be actively making. That’s when they noticed something remarkable: dark plumes, rising up more than 5 miles above the brighter surface of Trito
n and then spreading out more than 60 miles downwind. Four of them had been caught in the act of erupting as Voyager 2 flew past, and photographing them at different angles during the flyby is what allowed Larry and colleagues to view them in stereo and determine their heights.

  “I was analyzing newly received Triton images with longtime USGS Flagstaff friend and colleague Tammy Becker,” Larry recalls, thinking back to that wonderful moment of discovery. “We were building a new map of Triton’s surface, pasting the overlapping Voyager images together into a mosaic. Because the flyby images were all taken from different angles as we flew past, we had to try to paste the images onto a spherical model of Triton’s surface so they could be aligned into a global map. But some dark and bright streaks seen in two particular overlapping images just would not line up. We puzzled a bit and the reason soon became clear—the streaks were not on the surface but were above it! We put those two images together into a stereo viewer, grabbed our red-blue glasses, and Triton’s plumes popped out of the surface and into full view!”

  Active geysers on Triton! Larry and the team came up with a model to explain what they were seeing: sunlight warms the bottom of a relatively transparent 3-to-5-foot-thick layer of seasonal surface nitrogen ice, causing it to sublime (evaporation of ice directly from a solid to a gas) and collect under pressure under the ice. When the ice cracks somewhere, the nitrogen gas is explosively released, carrying dark dust and mineral grains along with it up to high altitudes under the low gravity and atmospheric pressure. It was electrifying to be around the team that was discovering and trying to explain, as the discovery was made, what was then only the third known place in the solar system with active eruptions of some sort (the others at the time were the Earth and the Voyager-discovered plumes of Io). I’ve since worked with Larry on the Mars rover missions as well as being the PhD dissertation advisor of his (equally mischievous) son, Jason. The baton is passed along.

  We haven’t been back to Triton since Voyager 2 zipped past, but we have made some progress in trying to figure out where Triton may have come from. The discovery of Pluto, in 1930, was among the earliest pieces of evidence that astronomers used to argue for the existence of a large disk of similar small bodies extending well beyond Neptune’s orbit. Among these visionaries was one of the fathers of modern planetary science, the Dutch-born American astronomer Gerard P. Kuiper. In the early 1990s, planetary astronomers began discovering the first members of that population beyond Pluto—the Non-Planetary Society, you might say. Today, taking advantage of substantial improvements in telescopes and camera detectors over the past few decades, more than 1,200 of these Kuiper Belt Objects, or KBOs, as they are now known, have been discovered. Many of them, like Pluto, are in an orbital resonance dance with Neptune that always keeps them far away from that giant planet’s gravity (Neptune orbits the sun exactly three times for every two times Pluto orbits the sun). Over the 4.5-billion-year history of the solar system, it is hypothesized that some fraction of KBOs that were in unfortunate orbits bringing them too close to Neptune either crashed into the planet or were slingshot out of the solar system or even into the sun. But, just possibly, one of them—Triton—survived that close encounter and was captured by Neptune’s gravity.

  Voyager’s flyby of Triton, then, may have been humanity’s first encounter with a Kuiper Belt Object, a small planetlike body that originally formed in the cold reaches of the outer solar system, and which may provide a glimpse into the ices and rock that formed the original cores of all the giant planets. To illustrate how rare and precious the Triton flyby was, we would have to wait twenty-six years for our second encounter with a KBO, when the NASA New Horizons spacecraft flies by Pluto in July of 2015. After more than nine years of traveling through space as the fastest mission ever launched, New Horizons will have to do all its best science, and take all its best images, within about a thirty-minute period around closest approach to Pluto (no pressure on that team, eh?).

  There’s a lot of speculation that Pluto will resemble Triton. The two are comparable in size, and Pluto is already known to have a thin atmosphere like Triton’s, as well as a surface dominated by nitrogen ice. But there are also significant differences. For example, though smaller than our own moon, Pluto has five moons of its own, including a relatively large one (half the size of Pluto itself) called Charon, which has a surface dominated by water ice instead of nitrogen. While Pluto itself may end up showing some similarities with its possible cousin Triton, my bet is that the Pluto system overall will turn out to be just as new, strange, and alien as every other place that we’ve encountered in our travels out into the solar system. Planets, dwarf planets, moons . . . it really doesn’t matter what we call them. They are a diverse, interesting, and just plain cool lot of neighbors that we share our solar system with.

  With so many hopes pinned on the fast-approaching flyby of Pluto, I can’t help but think back to the Voyager 2 Neptune flyby where almost instantly so much was revealed. And yet we all felt a mixture of exhilaration and regret as we looked in the rearview mirror, wanting more. Larry Soderblom recalls, “Our feelings as Voyager 2 completed its last solar-system encounter and receded from Neptune toward the deep void of interstellar space were wistful and depressed. That evening I remember remarking to imaging team leader Brad Smith, on a positive note, ‘Well, Brad, you only explore the solar system for the first time once.’”

  The Voyager 2 Neptune flyby would end up being the capstone to one of the greatest voyages of exploration ever conducted by our species. As I prepared to head back to Honolulu a few days after the Neptune flyby, I wandered one last time through the Science and Mission Operations areas of JPL Building 264. Someone had printed out a large-format image of one of Voyager’s final Neptune photos, a beautiful, parting view of the thin crescents of both Neptune and Triton as Voyager turned and looked back.

  What was Charley Kohlhase thinking when he first saw that “last port of call” receding photo of the crescent Neptune and Triton, back in 1989? “I felt . . . what is it when you have an epiphany?” he says. “I felt nostalgia, I felt sadness that we were saying good-bye to the last worlds . . . but also great satisfaction. All the years that had gone by . . . and we had pulled it off. It was a big success. . . .” His words slowed, and he gazed upward, outward, focusing well beyond the confines of the room. “And to look at those limbs, of Neptune and Triton, as we’re departing . . .”—tears welled up in Charley’s eyes—“I won’t ever forget that.”

  Jon Lomberg was similarly wistful about the end of one phase of Voyager’s mission and the beginning of the next. “One of the images I’ve always imagined,” he told me, “is riding along with Voyager, looking in the reflective gold surfaces of the record and seeing the volcanoes of Io, the cracks of Europa, the braided rings of Saturn, and so many other wondrous sights. It was bearing witness for all of us. It was bearing witness to all of these new worlds.” Perhaps the simplest, most direct summary of Voyager’s influence on the many people touched by the adventure came from Rich Terrile: “Voyager was the most amazing experience of my life.”

  Part Three

  LOOKING BACK,

  LOOKING AHEAD

  8

  Five Billion People per Pixel

  “SELFIES” SEEM TO be all the rage these days, and I am constantly amazed at the dexterity of my kids and their friends manipulating a smartphone held at arm’s length to take these sometimes artfully framed self-portraits. I’ve also just recently learned about, and am fascinated by, the concept of people taking “dronies” of themselves and friends from remote-controlled cameras hovering above them on balloons, kites, or powered quad-copters. This fascination with self-portraits is not just a fad, however, and it extends far beyond just a subset of young, tech-savvy individuals. For as long as we’ve been looking upward and outward, we’ve also been looking inward, seeking clues that might help illuminate our place in the universe.

  We tend not to think about the fa
ct that the Earth is round, a sphere of rock and metal surrounded by a thin shell of air and water, floating in space and moving swiftly under the gravitational influence of the sun. Moving swiftly? It doesn’t feel like that at all, but the numbers tell us otherwise: Earth is spinning at more than 1,000 miles per hour at the equator, and we’re all traveling more than 67,000 miles per hour in orbit around the sun, and the whole solar system is moving at about 450,000 miles per hour as the sun orbits around the center of the Milky Way galaxy. We don’t notice any of these motions acutely, however, because gravity wins. The mass of our planet and its gravity holds us, and the air and the oceans and the mountains and everything else, to the surface, counteracting by far any other forces that might be acting to fling us off. And of course the Earth is a round sphere, right? We’ve all seen the pictures of our Big Blue Marble suspended against the inky blackness of space that the Apollo astronauts took on their round-trip voyages to the moon back in the ’60s and ’70s.

  But that knowledge was not at all obvious to most of the estimated 100 billion human beings who have lived before us, and who may have imagined but never had the chance to see the results of the simple experiment (simple once you’ve developed the technology for space travel, that is): if you want to find out what the Earth is really like, look at it from the outside. The sixth-century BCE Greek philosopher, mathematician, and astronomer Pythagoras of Samos (the same guy of a2+b2=c2 fame) is generally acknowledged as among the first scientists to recognize that the Earth is spherical, without, of course, the benefits of perspective provided by the modern space age. His evidence was indirect: Greek sailors saw southern constellations rising higher as they sailed south; when they got really far south, the sun shone from the north instead of the south (as it does north of the equator); and when the full moon passed into the Earth’s shadow during a rare lunar eclipse, the outline of the Earth’s shadow appears curved.