by Jim Bell
One answer came from small new moons that were discovered in the images, two of which orbit within a gap in the rings and apparently twist and “shepherd” some of the rings with the push and pull of their combined gravity. If a block of ice in this region starts to wander too far in, one of the shepherd moons will come by and pull it back out; if a house-sized piece of ring in some other region starts wandering away, the other shepherd moon will come by and pull it back in. It was an elegant and completely unanticipated discovery.
Heidi Hammel was an undergraduate at MIT during the Voyager Saturn flybys, and she recalls one of her fellow students somehow hacking into NASAs Voyager image feed and watching all the images stream by—just like at JPL, but in his dorm room in Boston. When the professors in Heidi’s planetary science class got wind of this, they took the class on a “field trip” to this fellow’s dorm room to view the images together. “We just sat there, as part of our class, watching the Voyager Saturn images come up on this TV screen,” she told me. “I remember when the images of the F Ring, the braided F Ring, first appeared. It was all twisted and strange, and we were just staring at it. And I remember that one of my professors, the astrophysicist Irwin Shapiro—I’ll never forget this—he was looking at that, and he said, ‘Well, that’s just not possible. It’s not possible for rings to do that.’ And we all just started laughing because there it was on the TV screen.” It seemed like Voyager was making the impossible possible everywhere it went.
Another important finding had to do with the composition of the rings. Voyager found them to be made of almost pure water ice, with maybe just the slightest hint of a reddish or pink coloration from some unknown but minor contaminant. Earth-based observations in 1977 had discovered thin dark rings around Uranus, and Voyager had discovered thick dark rings around Jupiter, which suggested that icy rings get dark over time, maybe from accumulating dust or dark materials from comet and asteroid impacts with the rings. So why would Saturn alone have a gigantic system of super-bright, ultrapure rings? The question is part of a bigger debate that is still going on among my planetary science colleagues: how old are Saturn’s rings? Their brightness and clean icy nature suggest that they are very young, perhaps having formed from the catastrophic breakup of a large icy satellite just a few tens to hundreds of millions of years ago (that’s young to astronomers). However, their highly ordered structure and the amount of mass contained in the rings suggest that any such doomed precursor moon should have been broken up very early in the history of the solar system, when such giant impacts were more common. That suggests that the rings are ancient, and that they keep clean by repeatedly clumping and unclumping with their neighbors as they orbit, continually stirring up “fresh” ice in the process.
Voyager 1 found that not just Saturn’s rings but all its large moons are covered by, and mostly made of, water ice. That was not particularly surprising, given the expectations from previous Earth-based telescope observations and the fact that small bodies like moons and asteroids and comets are expected to get icier the farther out in the solar system they form. What was surprising, however, is that the geology of these moons varies so widely. One of the more distant large moons, Iapetus, has a dark hemisphere (the side always leading or facing forward as it orbits around Saturn) that is five times darker than the other, trailing hemisphere. Several of the closer-in large moons, such as Tethys, have global systems of troughs and fractures, suggesting significant but mysterious past active tectonic stresses in their icy crusts. Some of Saturn’s moons have preserved a history of intense bombardment over time and are literally saturated with the scars of impact craters formed on their icy surfaces (in the cold temperatures of the outer solar system, ice acts like a rock in many ways). But others have some places on their surfaces that are heavily cratered and other places that are not, as if some process had come along and wiped the slate clean, covering or erasing some of the ancient craters but not others.
Some team members speculated that this process may have been volcanism—but in this case a unique kind of outer solar system process called cryovolcanism where the “magma” is liquid water, formed by the melting of the “rock” (solid ice), and erupted through cracks and fissures that let it flow across the surface the same way a rocky lava flow would on Earth. “We certainly did not anticipate the level and breadth of active geologic processes we would find among the outer solar system satellites,” Larry Soderblom confessed to me. “We were expecting to explore a bunch of ancient cratered balls with not much else on their surfaces. We should have been smarter.” Fortunately, he and the other geologists on the Voyager team became smarter very quickly and learned to extend the basic ideas of volcanism—melting of material and transport of the resulting lubricants through cracks or fissures or other underground “plumbing” systems. “Now, because of Voyager, we know that no matter where you go in the solar system (or probably in the universe, for that matter) you will always find geological lubricants no matter how cold it gets,” Larry went on. “Nitrogen is the melt on Triton, methane on Titan, and ammonia and water likely provide the geological lubricants for many other icy moons. It turns out that volcanism in some form can occur anywhere—whether it is the traditional rocky kind that we know of on the terrestrial planets, or the ultracold cryovolcanic flows and eruptions that we’ve seen on many of the moons way out there.” The specific internal heat sources that could power such volcanoes is still a mystery, although many of my colleagues, like Larry, suspect that tidal heating—like that which drives Io’s volcanoes—probably plays a role.
One of Saturn’s icy moons proved to be especially enigmatic, as revealed in Voyager 1 images. Enceladus (pronounced en-CELL-uh-dus) is only about 300 miles across—driving completely around it would be like driving from Boston to Chicago—and yet it is nearly perfectly spherical, has the most reflective surface of all of Saturn’s moons, and relatively few impact craters were detected in distant Voyager images. This suggested to Voyager team members that the surface of Enceladus could be very young, and that, indeed, active resurfacing could still be occurring. In addition, the faint outer E ring of Saturn seems to be densest near the orbital distance of Enceladus, suggesting that this moon might be the source of those ring particles. It was exciting and enigmatic and just plain weird for such a small moon to show so much evidence of geologic activity. Enceladus and its neighbor Dione orbit in a 2:1 resonance (like Io and Europa in the Jupiter system), so maybe the same kinds of orbit-related tidal forces heat the inside of Enceladus, melting the icy mantle and causing cryovolcanic eruptions? Hopes were high that Voyager 2’s much closer encounter with Enceladus nine months later would provide the evidence needed to solve this puzzle.
Voyager 1’s planetary mission ended at Saturn, as the path required for a close Titan flyby, combined with the other constraints, caused the spacecraft’s trajectory to be bent upward and well away from any known potential future flyby targets. While Voyager 2 sped on to strange new worlds, Voyager 1 settled down for the long journey to the edge of the solar system, and beyond.
If Voyager 1 hadn’t been targeted so closely to Titan and had instead been able to be diverted by Saturn’s gravity toward a flyby of Pluto in the 1980s, as NASA had envisioned for some of the original Grand Tour missions proposed in 1969–1970, we would have discovered much sooner that Pluto is a small world with a thin atmosphere, a surface mostly made of nitrogen ice, and orbited by at least five moons rather than just the one large one discovered from Earth-based telescopes in 1978. Maybe active plumes or nitrogen-powered volcanoes would have been discovered.
Charley Kohlhase is less sanguine. “I don’t think a far-off little, now–Kuiper Belt Object like Pluto, and the trip time and whether you could make it there or not, would ever have beaten out Titan. Most of the people I knew did not regret giving up on Pluto. We were happy to go with Jupiter-Saturn-Titan for Voyager 1, and the smaller Grand Tour Jupiter-Saturn-Uranus-Neptune with Voyager 2.”
Ed Stone is similarly pragmatic. “Giving up something that you know you can do—Titan—for something where you’re not sure you could get there . . . there was no real science controversy about that decision,” he recalled. “And we didn’t really know Titan at all. We probably wouldn’t have had a probe landing on Titan if we hadn’t focused on it with Voyager 1,” he added, referring to the successful Cassini-Huygens Titan landing mission in 2005. Regardless, we will, hopefully, know for sure what Pluto is really like up close after the July 2015 Pluto flyby of NASA’s New Horizons spacecraft. Will our biases against the possibilities of life on small, distant worlds continue to be shattered by New Horizons as they have been shattered by Voyager?
CLOSE CALL
After Voyager 1’s successful November 1980 Saturn flyby, all of the team’s efforts became focused on planning for Voyager 2’s pass through the system in August 1981. With Voyager 1’s successful Titan flyby in the can, and no hope of gaining useful additional close-up imaging coverage of Titan from Voyager 2 because of the thick haze layer, the path was cleared for Voyager 2 to attempt the Grand Tour. By directing the spacecraft to a very close pass by Saturn, the team could use Saturn’s gravity to give the spacecraft a 90-degree turn to aim it toward a 1986 encounter with Uranus, and then, if all went well, possibly on to Neptune in 1989. But getting the required course change meant getting very close to Saturn, close to the region where the bright A, B, and C rings orbit around the planet. Pioneer 11 and Voyager 1 had shown that the ring particles were (generally) sparsely separated, and that the ring plane could be traversed that close-in to the planet. Still, it had to happen in order to bring about the eventual encounters with Uranus and Neptune, plus it offered close encounters with the enigmatic moons Enceladus and Tethys. The successes of Pioneer 11 and Voyager 1 emboldened the team to take the risk—Saturn’s rings or bust! So the course was set.
Despite the overall feeling that success at Saturn had already been generally achieved by Voyager 1, for some of my colleagues who were involved in the mission, the days surrounding the closest approach of Voyager 2 to Saturn were among the most harrowing on the mission. Team members who studied the rings, in particular, were nervous. Voyager 1 had made a rather distant flyby past the rings, but the images still revealed strange waves, ripples, and twisting patterns in the rings that defied explanation (“Rings don’t do that!” they said). This was partly because the resolution of the images was too low to show adequate detail. It seemed like some fundamental piece of information about how rings form and evolve, and maybe how waves and wakes form within them, was missing. So they needed to get even closer to the rings with Voyager 2 to try to understand what was going on. They were nervous, though, because they knew that the closer the spacecraft got to the main rings, the closer and denser the ring particles became, increasing the risk of ring particles impacting the spacecraft.
Even a tiny ring particle, no larger than a speck of silt or a grain of sand, could cripple Voyager, which was traveling faster than 31,000 miles per hour relative to the rings. It’s the same principle of relative velocity and kinetic energy that explains how you can stop a tractor trailer traveling at 50 miles per hour with a common housefly—as long as the fly is going a million miles per hour! Tiny particles might not seem dangerous, but if they are moving extremely fast, they can carry an enormous amount of energy. Thus, getting closer to the rings offered great scientific potential but also presented great risk—a double-edged sword.
The first half, or inbound, part of Voyager 2’s flyby past Saturn was as routine as flying a tight trajectory past a gas giant planet can ever be, and made it possible to capture more of Andy Ingersoll’s giant planet atmosphere movies (covering Saturn’s northern hemisphere), and distant flybys of Titan and the icy moons Mimas, Dione, and Rhea—all of which had been photographed at higher resolution by Voyager 1. It was then that things got . . . interesting.
As the spacecraft fell deeper into Saturn’s gravity well and started to speed up (Voyager 2 was eventually accelerated from 36,000 miles per hour to nearly 54,000 miles per hour by flying a gravity-assist trajectory behind Saturn) and get closer to the planet and the rings, the kinds of maneuvers that the sequencing team had to build into the instrument observations became more and more complex. Specifically, the cameras and other instruments on the spacecraft’s scan platform had to be pointed around more rapidly, and over a wider range, than they had been pointed in a very long time. This was a simple result of parallax—the fact that the closer you are to something, the larger it appears to be. Imagine trying to take a picture of the Statue of Liberty, for example, from a mile away on the Staten Island Ferry. The ferry is moving slowly relative to Lady Liberty, who is far enough away to be leisurely photographed without much need for panning your camera. Now imagine trying to get that same photo from the passenger’s seat of an F-15 fighter jet, passing within a hundred yards of the statue and at 1,000 miles per hour! You would have to work fast and avoid craning or whiplashing your neck to try to catch Lady Liberty in a quick shot without blurring her out. It’s the same kind of challenge faced by the Voyager 2 sequencing team during the closest, fastest part of the Saturn flyby.
All was going well as the spacecraft neared its closest approach to Saturn and—as planned—went into eclipse behind the planet as seen from the Earth. This was the start of a series of carefully orchestrated observations from a spectacular vantage point inside the Saturnian system that had never before been witnessed. Critical observations would be made of Enceladus and Tethys. Saturn’s rings would be viewed edge-on. And the clouds and storms in Saturn’s southern hemisphere would be observed up close and personal. The spacecraft would also go into a solar eclipse while behind Saturn, with sunlight shining through its outer atmosphere and scattering through the dense gases as it made its way back to the cameras and instruments on Voyager 2. In this way, the spacecraft could measure the chemistry of Saturn’s clouds as never before. On its journey, the spacecraft would navigate its way through the plane of the rings, not far from the main rings themselves. Voyager 2 was on its own for most of this time, hidden behind Saturn, obscuring all possible communications. The intricate series of scan platform slews and camera exposures had therefore been preplanned and uploaded ahead of time. The data would be stored on the spacecraft’s 8-track tape recorder and then played back later, when there was time again for dedicated communications back to Earth. A similar scheme had worked flawlessly for both Voyagers at Jupiter, and while it would get colder than normal during the eclipse, crossing through the ring plane was a risky maneuver. It didn’t help that for about ninety minutes the spacecraft would be out of contact with the Earth. Despite all these dangers, the team was confident that everything would go as planned. Unfortunately, it didn’t.
It was after ten p.m. back in Pasadena, and the team knew that Voyager 2 would be out of communications with the Earth until about midnight. Some team members went home to catch some sleep. Imaging team member Candy Hansen, exhausted from a hectic day of last-minute planning and first-minute data analysis, made it as far as her car in the JPL parking lot and just fell asleep there for the night. Candy says it was pretty common for her and other team members to catch some sleep there at work during the height of the planning and during the encounters themselves. “It was really hard to leave JPL, because you didn’t want to miss anything!” she told me. “At each of the encounters I just went out and slept in my car for a few hours. At the Jupiter flybys it was the backseat of my ’55 Chevy. By the time of Saturn, I had a little Toyota pickup truck with a camper shell to sleep in. At Neptune I had a van, so sleeping in my car was really comfy by then.”
While she and others slept, a skeleton crew, including Rich Terrile, kept watch for Voyager 2’s signal to come out from behind Saturn. Rich had invested significant time in planning for the ring-plane-crossing images that would be taken during the spacecraft’s pass behind Saturn, as well as the close-up images of Tethys and Enceladus. He couldn
’t sleep without knowing how those had gone. It is a common theme among the Voyager group: “You didn’t want to leave work because something new was just around the corner. The next camera move was going to show something unique. And you wanted to be there to see it, to interpret it. It was an electrifying experience.”
When Voyager 2 did emerge from behind Saturn, there were some cheers and perhaps even a few unconfirmed Champagne corks popped among the small group still on shift. But soon it was clear that something had gone wrong. “All of a sudden, the pictures stopped coming. They were sort of frozen,” recalls Rich Terrile. “Oh my gosh, we’ve got a big problem here.” Telemetry showed that a series of hardware and software errors had occurred while the spacecraft was behind the planet, and that the spacecraft was not what the engineers call “healthy.” Something had happened to cause the preplanned sequence to stop taking data.
“It was an amazingly frustrating, shocking, kind of ‘What do I do? I’m totally helpless’ kind of experience,” Rich Terrile recalls, “where suddenly the spacecraft is not doing something when it’s supposed to be doing something. You don’t have any idea what’s going on, and it’s a billion miles away.” He remembers feeling as if he went through the various stages of grief (shock, denial, anger, and so on) compressed into a few minutes. “This can’t be happening!” he felt.
