by Jim Bell
Ed Stone had been catching up on a little sleep at home that night as well, but got up early to head back to JPL to be in a morning East Coast media interview about Voyager’s results and status. “I think it was probably four in the morning,” he recalls, “and I showed up and they said”—he whispers—“‘Scan platform—it’s stopped!’ So I had to go on this live TV interview and talk about it, even though I’d just heard, and didn’t know anything!” No one else knew much yet either. Ed remembers that the team’s primary feelings were of real concern for the spacecraft and of course the future of the mission. “We’d had a wonderful Saturn encounter, so Saturn wasn’t really the issue. The issue was, is this a spacecraft that’s going to get to Uranus and Neptune?”
At first, many people’s thoughts turned to the ring-plane crossing. Maybe Voyager 2 had rammed into a chunk of ice or dust floating among the outer rings? The Plasma Wave Subsystem instrument’s PI, the late Fred Scarf of TRW, reported to the Voyager science team the day after the scan platform anomaly that his instrument had detected activity “a million times the normal energy level very close to the time of the ring-plane crossing.” Scarf played a cassette tape recording of the “sounds of the ring plane crossing” and hypothesized that they were detecting bursts of energy from the high-speed impacts of small dust grains with the spacecraft. As reported by Voyager imaging team member David Morrison in his almost minute-by-minute account of the Saturn flybys, “The quantity of such impacts was truly staggering—thousands upon thousands, not just at the moment of ring-plane crossing, but extending for several minutes on either side. The roaring sound of these impacts on the tape that he played, sounding almost like a hailstorm striking a tin roof, sent chills down the spines of the seventy-five scientists attending the meeting. But did this unexpected plasma activity really have anything to do with the scan platform failure? No one could tell.”
Had it been too much of a risk to take? If Voyager had crashed into a larger-than-normal (a micrometer or two in size) ring particle, it must nevertheless have been an extremely small particle, because the spacecraft’s trajectory when it came out from behind Saturn was still exactly on target for where it was supposed to be. Still, even tiny impacts with ring particles could cause localized problems with some of the spacecraft’s instruments and subsystems, so the possibility was taken seriously as a potential culprit.
Another possibility was that the scan platform mechanism had failed because of the larger temperature contrast encountered when Voyager 2 went behind Saturn. As JPL’s testing of the flight scan platform showed, as its mechanisms and components were used, they heated up. Meanwhile, the outside temperature of the spacecraft itself got colder than it had ever been while it was passing though the eclipsed darkness of Saturn’s shadow. It was possible that wear and tear and “normal” aging of the scan platform’s gears and drive shafts and lubrication in the deep-space environment could respond to sudden, more extreme temperature changes in unpredictable ways—so that hypothesis had to be taken seriously too.
Ultimately, most team members believe that the reason for the failure was the sheer complexity of what the spacecraft and its mechanisms, especially the scan platform, were being asked to do—pointing the cameras and other instruments quickly and over wide angles from target to target during the high-speed, high-parallax closest approach. “We were cranking that thing around a lot,” recalls Rich Terrile. Overwork of the gears within the scan platform, in particular, was treated as a serious possibility, and it was a hypothesis that could be directly tested using the flight spare scan platform that was back on Earth, at JPL. Based on that work, the spacecraft engineers quickly devised some tests that could be done on Voyager 2’s actual scan platform, to try to recover at least some science as the spacecraft quickly receded from Saturn. Commands were sent to Voyager to move the scan platform back toward Saturn, but in smaller, slower steps than it had been previously commanded. Easy does it. . . .
“If it responded properly, the first picture would be received at 5:38 p.m.,” reported Dave Morrison in his diary of events for August 28, 1981. “All over JPL, people gathered around the television monitors as 5:30 approached. The three-minute delay between pictures always seemed long, but in these final moments time seemed almost to stand still. Then, at last, the critical picture began to be displayed, line by line, on the screens. And there it was! Clearly visible was a bit of the planet with the rings, now seen for the first time on their dark side, arching up across the field of view. A vast collective sigh of relief was expressed all around the Lab.” A work-around had been found. Some data were lost but the spacecraft, camera, and scan platform were responding well to triage. We might get to Uranus and Neptune after all!
What exactly happened when Voyager 2 was behind Saturn remains a mystery. The favored hypothesis for the scan-platform failure behind Saturn, based on generally good simulations and reproduction of the failure in the spare scan platform at JPL, is overwork and overheating from high-rate slew, and the seizing of a small gear on a tiny shaft within the mechanism. Still, it is hard to let go of the coincidences associated with the spacecraft’s cold pass through Saturn’s shadow, and especially of the daring plunge through the ring plane so close to the planet itself. In the years that followed, mission planners would wonder if future missions, such as the planned joint NASA/ESA Cassini Saturn orbiter and Huygens Titan lander, should take a more cautious approach to studying the Saturn system.
In 2004 the author Richard Hoagland, who covered the Voyager 2 Saturn flyby for the media and JPL and who by that time had become somewhat famous (or infamous, depending on whom you talked to) for identifying the so-called Face on Mars from NASA Viking orbiter images, accused NASA of sending the impending Cassini orbiter on a “suicide mission” to Saturn, knowing the fatal risks that Voyager 2 had barely avoided when crossing through the ring plane. Or, Hoagland asked in a more ominous tone, could NASA be covering up some remarkable, revolutionary physics discovery that Voyager 2 secretly made during that pass behind Saturn in 1981? About NASA’s Cassini plans he asked, “Is the spacecraft actually more prepared for such a ‘secret mission’—including being able to survive its highly dangerous ring plane crossings—than we have been publicly led to believe . . . ?” Well, secret mission or not, Cassini survived its own daring plunge through the ring plane of Saturn in July of 2004, and, building on the discoveries by the Voyagers, has gone on to orbit the ringed gas giant more than two hundred times, revolutionizing again our view of the planet, its moons, and its rings, as the Voyagers had done twenty-five years earlier.
Voyager 2’s troubles on the far side of Saturn were an acute reminder to science team members of the risks inherent in space exploration, and of the fact that there are often psychologically painful costs associated with those risks. Dave Morrison reported that “a sense of gloom pervaded the Imaging Science area. Everyone realized that these would be the last close-up views of Saturn or its satellites they would ever receive—what Rich Terrile called ‘our last best data.’ It was like watching each labored breath, waiting for the sick scan platform to expire.” Candy Hansen remembers the sad scene. “That particular day after the anomaly, my boss and fellow imaging team planner Andy Collins and I commandeered the browse room and sadly went through every image that had been taken but missed its target. . . .” Linda Spilker was similarly distraught. “I remember feeling a tremendous sense of loss of all the science observations that I had helped to plan,” she said.
While many high-resolution images of Saturn and its rings were successfully acquired before the scan platform stopped moving, particularly painful for the planetary geologists on the team was the fact that the platform had failed just prior to the highest-resolution imaging of Tethys and Enceladus. “Instead of satellite images,” reported Morrison, “only a blank screen appeared.” Given that these would be the final planned planet or moon images of Voyager’s nominal Jupiter-Saturn-Titan mission, the ability of the spacecr
aft to potentially continue on to successful encounters with Uranus and Neptune was now in grave doubt. And given the fact that budget cuts were decimating the planetary exploration program in general (with no new outer solar system missions then in the works), an anonymous team member also watching the parade of blank images march by the screens lamented out loud that these could be “the final images of the planetary program.”
In 2005, Cassini Saturn orbiter photos of that tiny moon finally explained the enigmas noted by the Voyager 1 team back in 1980: Enceladus is indeed geologically active, and plumes of water vapor mixed with organic molecules were seen spewing out of a series of “tiger stripe” cracks in that moon’s south polar crust. Heated by tidal forces, the interior of tiny Enceladus is partially molten ice—liquid water—that makes its way out via fractures in the crust and helps supply the particles that make up Saturn’s faint E ring. Instantly, Enceladus was vaulted to a prime position, along with Mars and Europa, on the solar system’s short list of astrobiology “hot spots”—places where liquid water, heat sources, and organic molecules could have made the environment habitable for life as we know it. Maybe Voyager 2’s scan platform was dinged by one of these pieces of Enceladus back in 1981? Maybe the Enceladusians just weren’t ready to be discovered yet?
In any case, Voyager 2 had survived mostly unscathed and still on course, so JPL Voyager engineers and science team members did what they often do best: they improvised, and devised work-arounds, to solve a problem by remote control. “Fortunately, we had five years to sort it out,” said Ed Stone. Through testing of the spare JPL scan platform, strategies were devised to enable its effective use in the future (though never again at high speed), if Voyager 2 could survive to Uranus and beyond. In fact, the engineers devised a way to use the entire spacecraft as one big scan platform, in what Torrence Johnson called an “anti-smear campaign,” by using gentle puffs (each with a thrust of about 3 ounces) from its attitude-control jets to impart a slow, gentle roll. “We always knew that the light levels at Uranus and Neptune would be low,” Charley Kohlhase says, “so you’d have to use longer exposures. If you wanted to get good pictures, you’d have to avoid smearing them. The flight team knew that by firing a few pulses from the attitude-control thrusters, you could put the spacecraft in a gentle roll in a direction that compensated for the apparent motion of a target.” While it was known early on that Voyager could be controlled this way, “We just didn’t deal with it,” Charley told me, “until we had to.” Luckily, in some ways, the years it would take Voyager 2 to travel out to Uranus—twice as far away from the sun as Saturn—would give the team plenty of time to test these strategies and to prepare for an encounter with the truly unknown.
6
Bull’s-Eye at a Tilted World
WHEN I WAS a kid, we’d take long summer family car trips through New England or up and down the East Coast. We had a variety of interesting cars (a different one seemingly every week—my father ran a junkyard and auto-repair shop), but the ones I remember the most were the kind of “family truckster” station wagons made famous in movies like Chevy Chase’s National Lampoon’s Vacation. My sisters and I would romp and play in the backseat, in the seat wells, or in the “back back” among the luggage and spare tire, inevitably playing games that somehow involved us hitting each other. Good times. I’m not sure that any of these cars even had seat belts—not that we’d have known how to use them. It was a different age, to be sure.
Members of the Voyager team found their own ways to pass the time during the long interplanetary cruises between flybys. Many team members planned vacations to coincide with the end of each planetary flyby as a way to blow off some steam from the stress (and sometimes, frustration) of planning for and acquiring “one-time-only” observations as the Voyagers sped past. Such respites were usually short-lived, however, because once back from vacation, the stresses of another impending flyby would once again slowly start to accumulate. Some team members took on new jobs with new projects, either using their Voyager experience to land a better position at JPL or elsewhere, or were forced to seek alternate work at JPL because the project’s budget was being trimmed yet again.
Between Saturn and Uranus, Candy Hansen took a two-and-a-half-year leave of absence from JPL and went to work as a science and operations liaison at the German Space Operations Center in Oberpfaffenhofen, learning how other countries operate their spacecraft and exploring Europe in a little camper during the generous German summer vacation seasons. Some team members also retired during the course of the mission, having started on Voyager when it was just a gleam in their eyes fifteen or twenty years earlier. Some team members even decided to time their weddings and the births of their kids to coincide with “downtime” on Voyager, which of course could be predicted years ahead of time. Between Uranus and Neptune, Candy Hansen had a daughter and went back to graduate school at UCLA to get her master’s degree (followed by her PhD after the Neptune flyby). Linda Spilker recalled how she had to interweave her personal life with her professional life, including starting her family. “I tell my daughters, Jennifer and Jessica, that their births are based on the alignment of the planets, and I mean that! There was about a five-year window between the Voyager Saturn flyby in 1981 and the Uranus flyby in 1986. It was in this window that I had both of my daughters. Other Voyager moms made similar choices and our kids grew up together.” This is sort of a modern-day version of astrology—people’s lives being dictated by the positions of the planets. Those Voyager kids, in particular, were born when they were born only because the outer planets happened to align every 175 years or so, the most recent alignment happened to occur when our species (finally) had the technology needed to exploit it, and at least one of their parents happened to have a job that also depended on the alignments of those planets. Voilà! Astrology works.
The years between the Voyager Saturn and Uranus encounters were formative years for me as well, as I graduated high school and moved out west to become an undergraduate at Caltech. I had to take the required math and physics classes, but for my electives I dabbled in astronomy, engineering, and planetary science. Many faculty members in the Division of Geological and Planetary Sciences were directly involved with the Voyager missions, and in class they would sometimes take us students on joy rides to test-drive some of their scientific models or explanations for the images and other data that had only relatively recently been taken by the Voyagers at Jupiter and Saturn. Viewgraphs, sometimes hand drawn, 35mm slides, even film-loop movies would be bandied about while the professors argued with one another and with the grad students in the class, and the grad students argued among themselves. Most of us undergrads in the classes were blown away by the jargon and the complexity level of the material, but still, we were in hog heaven. Here was the process of science and discovery, happening right in front of us! We struggled to get B and C grades but felt good about passing at all.
I remember the debates about Uranus were particularly poignant, because the professors and grad students were so keenly attuned to the fact that this basically unknown planet and its rings and moons were going to be almost magically revealed to us all by Voyager 2 very soon. There was a lot of speculation about what Voyager would see, and about how this smaller, more blue-green giant planet would compare to its larger cousins, Jupiter and Saturn. Are all giant planets the same? they wondered. Are there patterns in the ways that giant planets behave depending on their size? Or on their distance from the sun? Out at Uranus, sunlight is about four times fainter than at Saturn, and more than thirteen times fainter than at Jupiter. What difference would that make?
BIG SCIENCE
Uranus (pronounced by professional astronomers as YUR-uh-nus rather than the way eleven-year-old boys pronounce it, while giggling, your-ANUS) is special among the planets. To start with, it is the first planet that was discovered by telescope. The ancient astronomers of Greece, Persia, China, and Babylon didn’t know it existed. It is so far away fr
om us, orbiting at an average distance of almost eighteen times farther than the distance between the Earth and the sun, that it is usually too faint to be visible to the naked eye. Even if it were naked-eye visible, it would still have appeared starlike and would have been moving so slowly relative to the real background stars (taking eighty-four Earth years just to orbit the sun once) that it would not have been recognized as a particularly different or special star. There are reports of some very astute ancient astronomers, and perhaps even Galileo Galilei, the inventor of the first astronomical telescope in 1610, having seen a starlike object where Uranus should have been at the time. But no one recognized it as a planetes asteres, a wandering star.
At least, not until March of 1781. That’s when the German-born English musician and astronomer William Herschel (1738–1822) single-handedly doubled the size of the solar system. Herschel is one of those almost mythic characters from the early history of Western astronomy. Like Isaac Newton in the century before him, Herschel was a prominent polymath—a person with many different kinds of technical and academic skills. He focused much of his energy on, and made his living by, composing and performing music (twenty-four symphonies, fourteen concertos, and many other pieces) and also dabbled in astronomy, optics, and other areas of science. This dabbling eventually led him to design large mirrors and telescopes (based on Newton’s design) and to begin making systematic observations of the night sky. Soon enough, using a 6.2-inch-diameter, 7-foot-long telescope from his home observatory in Bath, he noticed an object with a “nonstellar disk” moving ever so slowly through the stars. Instead of appearing as a point of light like the stars, this object had a circular nature to it. Collaborating with others, Herschel was able to show that the object moved on a planetary path, well beyond the orbit of Saturn.
