by Jim Bell
With my wrists duly slapped but the study green-lighted, I went to work on a project to map the different kinds of materials on the five major icy satellites of Uranus using not just the high-resolution black-and-white photos that Voyager 2 had taken, but also the lower-resolution (more distant) color photos that were taken on approach. In retrospect, it was actually a really dopey and naïve kind of project, because the moons are very gray and show only small and not particularly diagnostic color variations on their surfaces. Voyager team members recognized this quickly, which is why (I believe) no one had yet gone off and worked on and published the project that I had proposed to do. Maybe NASA officials and my proposal reviewers realized this too, but since my proposed budget was so small, it was treated as a worthy student training project rather than the path to the next major discovery in planetary science. I had to learn, and in some cases create, image-processing software to work with the images, to do the color analysis, and to make the resulting maps. My results were essentially null—no major color variations and thus no new clues to the moons’ compositions—but I found somewhere to publish my null result and in the process learned how to write a peer-reviewed scientific journal article. It felt good to have an official scientific connection to Voyager, albeit a weak one.
In the two decades since Voyager 2 flew by Uranus, we’ve learned a lot more about the place from a variety of ground-based and space-based telescopic observations of the planet and its moons and rings. Perhaps the most striking discovery has been that we were fooled by the Voyager 2 images into thinking that the planet’s atmosphere is always boring. It turns out that Voyager happened to fly by at an anomalously boring time.
“We flew by at the peak of the summer solstice,” Heidi Hammel says, “when the whole polar region was enveloped in haze. You couldn’t see discrete cloud features in the Voyager images, and we didn’t have an infrared camera that would let us see through that haze to the cloud layers below.” In Hubble Space Telescope images as well as those from one of the giant 10-meter Keck telescopes on Mauna Kea taken since the Voyager flyby, many more visible bright and dark bands, clouds, and small storm systems have been tracked, revealing more complex and dynamic weather than originally thought. Voyager 2 had flown by Uranus when the southern hemisphere was nearly continuously sunlit and the northern hemisphere was dark. Since then, the seasons have advanced through the southern fall/northern spring equinox (in 2007), and telescopic observations have revealed some significant seasonal changes. “It’s a very different kind of planet now than the one Voyager saw, because of its dramatic seasonal changes,” says Heidi Hammel. “Uranus has now traveled a quarter of the way around the sun since 1986, and now almost the entire planet is in sunlight, rather than just the south pole. If we were to fly by Uranus with Voyager now, we would see a much more active planet than we did back then. It’s kind of neat to watch those changes.”
The Voyager flybys of Jupiter were followed up with a dedicated orbiter mission, Galileo, including an atmospheric entry probe that further revolutionized our understanding of the Jovian system. The Voyager flybys of Saturn were followed up with a dedicated orbiter mission, Cassini, which also carried a small probe, Huygens, that successfully descended through the clouds and hazes of Titan and landed on its surface. Similarly, many realize that the same path will have to be followed at Uranus (and Neptune), with a dedicated planetary orbiter being the next logical ramp-up in the exploration of that world. In the Interstellar Age, we know that to truly get to know a place, you’ve got to spend real time there, among the locals, learning their strange, alien ways.
7
Last of the Ice Giants
THE DISCOVERY OF Uranus in 1781 not only doubled the size of the then-known solar system, it also made English astronomer William Herschel a household name and a scientific pop star in his day. The idea of discovering an entirely new planet, and the resulting scientific immortality that would ensue, helped compel a revolution in eighteenth- and nineteenth-century telescope technology—an arms race, of sorts—among wealthy European gentlemen scholars and the patrons and potentates who supported them. Herschel had shown that cutting-edge technology, in this case building and having sole use of the largest telescope in the world, was a straightforward (though challenging and expensive) way to make new discoveries. Everyone wanted in, and new telescopes began popping up all over the continent. It was a growth industry.
Meanwhile, the mathematicians were in on the game too. Now that it was possible to accurately track the positions of Uranus and the other planets against the background stars with unprecedented precision, it was also possible to search for tiny deviations in their positions that could arise from the gravitational attraction that might be exerted by some new, as-yet-undiscovered planet. In some sense, just like the Voyagers, all of the planets, moons, asteroids, and comets in the solar system are constantly going through mini gravity-assist flybys with one another, slightly tweaking their positions relative to what would otherwise be the kinds of perfectly predictable orbital motions that Johannes Kepler and Isaac Newton had long ago described.
When my colleagues on the navigation team at JPL, for example, want to study a possible trajectory for a new space mission, they load their computers with the positions and masses of the sun, all the planets and their fifty or so large moons, and more than a half million asteroids, to make sure that every single possible “perturber” of the spacecraft is taken into consideration in their calculations. When astronomers and mathematicians like Edmond Halley and Pierre-Simon Laplace were working out the theory of motions of comets and asteroids, they were working on what physicists call the three-body problem, for example needing to account for the gravity and motions of the sun, Jupiter, and one of the Galilean satellites; or maybe the sun, Jupiter, and a newly discovered comet. Today’s more sophisticated computer modeling of solar-system motions search for solutions to what is known as the n-body problem, whereby n is some very large number of objects. Everything exerts a force on everything else; we are all moved by the planets.
Two especially talented nineteenth-century mathematicians, John Couch Adams from England and Urbain Le Verrier from France, were particularly well connected with the astronomy community and the best available data on the positions of Uranus. Without the use of computers or mechanical calculators of any kind, both men noted slight differences between the predicted and actual positions of Uranus, and both set about calculating the predicted position of a hypothetically more distant new planet whose gravity could be perturbing the orbit of Uranus, thereby explaining the differences. It was a classical three-body problem in which the bodies were the sun, Uranus, and some unknown but suspected massive new planet beyond Uranus. Although Adams and Le Verrier were working independently, and unbeknownst to each other, on the same problem, it was still a pitched scientific battle between the British and the French the likes of which as had occurred on any actual battlefield over the course of their histories, with nothing less than solar-system glory on the line. Adams asked his telescope colleagues at Cambridge to search for the putative new planet in a particular broad region of the sky. Le Verrier convinced colleagues at the Berlin Observatory in Germany to search a narrower predicted region (after failing to persuade his own institution, the Observatory of Paris, to spend more than a token effort on the search). Late in 1846, the French won the battle, as German astronomer Johann Galle found Le Verrier’s “perturber” (on the first night that he looked for it), and confirmed its identity as the eighth planet.
The successful prediction and then discovery of a new planet was hailed as a triumph of modern physics and mathematics; according to French mathematician and politician François Arago, Le Verrier had discovered a planet “with the point of his pen.” Le Verrier and Galle were jointly credited with the discovery of the new planet (even by the gracious Adams), and Le Verrier chose to name it Neptune, after the Roman god of the sea. With the new planet orbiting at an average distance of thirty
times the distance from the Earth to the sun, once again, but not for the last time, the size of the solar system was roughly doubled. While it was a modern triumph of math and physics, there had also been a bit of luck involved—the same kind of luck, in fact, that would enable Voyager’s Grand Tour mission to take advantage of the discovery of this new planet some 130 years later. Both Adams and Le Verrier were searching for perturbations in the orbit of Uranus using measurements of the position of Uranus taken between 1846 and the time of its discovery, back in 1781. During that specific time period, Uranus happened to have passed Neptune in its orbit (around 1821, actually, before Neptune was discovered), and thus Neptune happened to have had its maximum influence on the orbit of Uranus right when the telescopic and computational “technology” of the day would allow such tiny tweaks to be recognized. Such an alignment hadn’t occurred since the year 1650, when neither planet was known, and wouldn’t occur again until the year 1993—by which time technology had advanced so far that we had actually visited both worlds with robotic spacecraft. That same time interval of about 175 years between alignments of Uranus and Neptune is what Gary Flandro and others recognized, in the 1960s, sets the timing of potential spacecraft Grand Tour trajectories to all four giant planets.
OHANA
As Voyager 2 sailed on toward Neptune after its 1986 encounter with Uranus, my own trajectory moved me distinctly westward, to find my way as a professional planetary scientist via a graduate education in Hawaii. The late 1980s and early 1990s were a particularly challenging time for NASA’s exploration of the solar system. The Viking missions to Mars, highly successful orbiters and landers, were over, and the future of new “flagship”-class missions was in doubt because of government belt-tightening. The loss of the Space Shuttle Challenger and her crew in 1986, just a few days after Voyager 2’s flyby of Uranus, threw NASA’s human exploration program into a state of disarray, and along with it many of the robotic planetary science missions, such as the Magellan Venus orbiter and the Galileo Jupiter orbiter, that had been planning to use the Shuttle as a launch vehicle. The idea of intermediate- and smaller-class, “better, faster, cheaper” planetary space missions hadn’t been invented yet. In short, it was a great time to be focusing on the use of “sure bet” facilities, such as high-powered ground-based telescopes, to try to push the envelope in planetary science. So that’s what I did, and there was no better place than Hawaii, as I had learned during my summer spent at Mauna Kea Observatories between my junior and senior years in college.
In yet another example of the lingering power of astrology—the “undeniable” ability of the positions of the planets to influence people’s lives—the quirk of the timing of my birth in 1965, combined with the typical track of elementary, secondary, and college education in the United States, put me in my first years of graduate school in the mid- to late-1980s, right when, coincidentally, the Earth and Mars happened to be going through a series of close passes every few years that provided outstanding opportunities for the best telescopic observations yet made of the Red Planet. My PhD research group was working on just that, and so it became incredibly convenient, as well as fun and interesting, to consider that as a potential thesis topic. If I’d been born five or ten years earlier, maybe I would have pursued some other planet, or the more “pure” astronomy of stars and galaxies. If I’d been born five or ten years later, maybe I would have skipped learning how to research with telescopes entirely and tried to go straight into space missions, like many modern planetary science students do today. But no, my birth in July of 1965 set me on an astrological collision course with Mars via telescope in the late 1980s, and then a lifelong involvement in Mars via spacecraft thereafter. Is it a coincidence that the world’s first spacecraft flyby mission past Mars, Mariner 4, also occurred in July of 1965? Well, yes, in fact it is.
Though I was primarily studying Mars in grad school, I couldn’t let go of Voyager. I wasn’t at Caltech anymore and no longer had a direct connection to the mission via my mentor Ed Danielson or any of my other former professors involved in the mission, like Andy Ingersoll. Still, I had to figure out a way. . . .
Planetary science is a rather small, close-knit field. When I was in graduate school, there were maybe five hundred or so professional planetary scientists, including grad students, in the field. Nowadays the number is something like three or four times that, but it’s still small enough that most people know one another, or at least know of one another. There’s a bit of a family feel to the endeavor, with most of the community getting together sort of “over the holidays”–style (at one or the other of the major conferences held during the course of the year), or for special occasions, like a spacecraft launch, flyby, or landing.
After moving to Hawaii from Caltech, I was doubly fortunate to find two amazing families there in the islands—one made up of close friends and mentors who helped me in my work world at the university, and the other made up of close friends and mentors who helped me after hours when I learned how to paddle as part of a Hawaiian outrigger canoe club. Paddling with my brothers and sisters in the waves off Waikiki Beach, learning about ancient Polynesian navigation and other local traditions, and kicking back afterward to soak in some “island style” music and food taught me not just a new word—ohana, Hawaiian for “family”—but the inner spirit behind the word as well.
If any one person was the embodiment of ohana, it was Fraser Fanale, a professor (now retired) of planetary science who specialized in thinking about the history of water and other “volatile” molecules in the solar system—on Mars, on the satellites of Jupiter . . . anywhere. He would talk to anyone, at any time, about volatiles, where to find them, how they move around, and what they tell us about how planets, moons, asteroids, and their environments evolve over time. Fraser was one of the most disorganized, but kindhearted, souls I have ever met. He defined the stereotype of the absentminded professor. In the days before cars had those high-tech key fobs that let you flash the lights or honk the horn, I’d run into Fraser at conferences in the parking lot to the hotel, wandering around trying to find his rental car, which he’d misplaced. He showed up to give a talk once with a stack of viewgraphs—handwritten overhead projector notes—and proceeded to drop and scatter them while being introduced. Undaunted, he delivered his talk, legibly but out of order, from the randomly reassembled stack. Sometimes it was almost comical, but—my goodness!—the man had an impressive and intuitive grasp of what is going on out there in the solar system.
Fraser hadn’t been directly involved with Voyager, but he was a member of the Galileo Jupiter orbiter mission team, helping plan for observations of volatiles on the Galilean satellites. One of the ways ohana manifests itself in planetary science is that, almost always, members of a spacecraft team who are preparing for some publicity associated with an upcoming event, like a launch or, say, a flyby of Neptune, will often invite members of other mission teams, and their families, to participate in the event. So, early in the summer of 1989, Fraser found himself invited to attend the events at JPL surrounding the historic Voyager 2 flyby of Neptune in August. Fraser wasn’t a big fan of traveling, and by this time the first prototype versions of something called the Internet were appearing on college campuses and government labs, allowing colleagues to exchange “electronic mail” messages and to send digital photos back and forth over a strange new interconnected series of dedicated communication lines called the World Wide Web. Fraser was going to sit it out, watching the Neptune images come in over the web. But his invitation and badge were transferrable, and he knew about my work with the Voyager images of the Uranian moons. Would I like to go? he asked.
Dude.
THE OTHER BLUE PLANET
Voyager 2 passed Uranus in 1986 at exactly the right place and time to slingshot onward to where Neptune would be in August of 1989. The precision with which the JPL navigation team guides spacecraft like the Voyagers, and others, still astounds me. Three separate “needles
” had to be threaded—at Jupiter, Saturn, and Uranus—precisely, to get the spacecraft where it needed to be for our first and only encounter with Neptune. The physics behind the feat had mostly been worked out in the seventeenth century by Isaac Newton. But Newton could take us to the planets only in a hypothetical, theoretical way. “It’s not rocket science,” joked one of my JPL friends when I tried to gush over the remarkable and historic exploration achievements that she and the extended Voyager team had helped pull off. Oh, but it is. Working out the mission on paper is one thing (and an impressive thing at that). But to actually get there required technologic feats and innovations not possible until the late twentieth century, not to mention the largesse of the taxpayers of an entire nation to help foot the bill.
Just like in the lead-up to the Uranus flyby, improvements were made remotely to the spacecraft as well as to the ground stations that would be needed for the encounter. Voyager’s thrusters were throttled down even more to allow the spacecraft to slew even more slowly and gracefully—essential for the dimmer level of sunlight at Neptune compared to Uranus. Improvements were made to the image motion compensation software and to the camera software to enable it to compress images better and to take longer-exposure images without smearing. And improvements in NASA’s Deep Space Network were needed to reliably detect the extremely faint radio signals from the spacecraft. Voyager 2 had traveled so far from home that signals from Earth to and from the spacecraft as it neared Neptune were now taking more than four hours, each way, traveling at the speed of light. Larger radio antennas (now nearly 230 feet across), and new receiving stations in New Mexico and Japan, helped guarantee high-quality communications with the spacecraft.
