by Jim Bell
The PI of the imaging team, a planetary astronomer from the University of Arizona named Brad Smith, was a force to be reckoned with: sometimes jovial, other times stern, and clearly—like everyone else in the room—worried about it all going well. He kicked us interlopers out several times, for special “team members only” meetings or just because he thought there were too many people in the way. Sometimes he was nice about it; sometimes he wasn’t. My former Cornell University colleague and research mentor Joe Veverka, a member of Brad Smith’s Voyager imaging team, tells a story about how, at an early team meeting, Brad had everyone shake hands and apologize to one another in advance for the nasty things they were probably going to say and do in the heat of the upcoming stressful flybys. Joe would later give the same advice to the imaging team that he led on NASA’s Near Earth Asteroid Rendezvous mission, and as a member of that team I can vouch for Joe’s (and Brad’s) sage advice.
I would see Project Scientist Ed Stone, who seemed to me, from the back of the room, to have that gracious and kingly sort of personality, make little speeches now and then to encourage the team, or intervene and occasionally break up some of the somewhat-too-heated debates that would pop up among the tired and overworked instrument teams. I asked Ed how he kept his cool during those stressful, exciting days.
“I don’t know!” he said. “It was just such a great thing. There was so much hard work; there were discoveries every day. It was just incredible. And wonderful.”
I had to conclude that Ed must have just had the right genetic disposition to lead a group of talented and highly motivated people through such stressful experiences. My friend and colleague Ann Harch, who was a Voyager sequencer and the science operations coordinator during the Uranus and Neptune flybys, says that she and others viewed Ed Stone as an incredibly fair and approachable leader. “He did an amazing job of making sure that all of the science investigations got their important science into the plan,” she recalls.
The Uranus flyby itself, on January 24, 1986, went stunningly well (much better than the Super Bowl a few days later, in which, sadly, my Patriots got crushed by the Bears 46–10). Uranus itself was fairly bland, showing no evidence of the stunning clouds and storms like Jupiter and Saturn. It was the Uranian moons that stole the show—tiny, icy worlds with staggeringly high cliffs and cracks and super-dark plains interspersed with bright (icy?) cratered debris. Uranus is tilted on its side (rolling around the solar system instead of spinning), and so its dark rings and five large moons make for a sort of bull’s-eye dartboard pattern that Voyager was aiming for. The spacecraft passed through the bull’s-eye close to the tiny, jumbled-up moon Miranda just before making its closest approach to Uranus. Images streamed in nonstop. A few of them would later make their way into newspapers or onto the network news, but those of us there at JPL were the fortunate ones who got to experience, live, our first encounter with these exotic worlds.
And then, just like that, Uranus was in the rearview mirror. We watched the blue-green world that we’d been seeing head-on for weeks wane into a thin, ghostly crescent as we passed behind it. The planet’s rings were revealed in all their glory by looking back at them, into the sun, making their tiny particles light up for the cameras like a wet or frosty windshield lights up when you’re driving into the sunlight. Many team members started to pack up to head back to their homes, with data tapes and stacks of photos in hand and dreams of discoveries to make and scientific papers to write.
Ed Stone and other Voyager team leaders began to prepare for a NASA press conference in which they would share the “greatest hits” from the Uranus flyby—stunning images and other measurements of the planet, moons, and rings, as well as initial results about the planet’s magnetic field and its interactions with the solar wind. Ed Stone thought it would be a wonderful celebration of one of the greatest achievements to date of the space age—the first encounter with a new world.
But the celebratory mood and all that press-conference planning stopped suddenly on the morning of January 28, when the Space Shuttle Challenger exploded just seventy-three seconds after launch, killing all seven crew members. I remember it vividly, watching on TV from my college dorm room. I never missed a shuttle launch on TV, and I was even lucky enough once to hop in a car with a bunch of college friends at the last minute and catch a shuttle landing at Edwards Air Force Base in the Southern California desert. Seeing Challenger explode live on TV was jarring, not just for me but for my Voyager colleagues, everyone else involved in the space program, and the nation as a whole. Management and design flaws in the shuttle system were uncovered, and it forced major debate and soul-searching about the importance of human space exploration for America’s future. The media became instantly consumed by the Challenger disaster, fleeing Pasadena in droves and leaving the rest of the Voyager Uranus story untold. Ed Stone and the rest of the project leaders knew they had to postpone their press conference. The “greatest hits” made it out there, eventually, but they were released without as much fanfare. The remaining somber team members slowly drifted off and headed back home.
A few weeks later, back on campus, I finally had a chance to see Ed Danielson again for more than a fleeting hello. He had been working hard as an imaging team liaison to the JPL navigation team to figure out how much Voyager’s trajectory had been bent (ever so slightly, but measurably) by the gravity of Uranus and its moons. This would enable the navigation team to estimate the mass of the moons, which, when combined with estimates of the moons’ volume from the images themselves, would let the team estimate their densities. The moons turned out to have very low densities, close to that of water ice. Perhaps not surprising, given their location in the cold outer reaches of the solar system, but still, Ed wanted to get the numbers right. I tried to express the depth of my gratitude for getting me that badge.
“It was nothing,” he said.
After graduation, Ed and I stayed in contact, catching up at various conferences or during my occasional trips back to Caltech. He took a leading role in the development of the first high-resolution planetary camera, the Mars Orbiter Camera (MOC) on the Mars Observer mission. Unfortunately, that spacecraft, and Ed’s darling camera, blew up just three days before getting to Mars. Undaunted (“It was nothing”), Ed and his MOC teammates from Malin Space Science Systems in San Diego built another one a few years later for the Mars Global Surveyor mission, and that MOC got to Mars safely and went on to discover gullies and deltas and massive sedimentary layers—photos of which would forever change our perception of that world as well.
Ed retired in 2004 and, after battling complications of a stroke, passed away in late 2005. I still feel his influence on me every day. Using the skills Ed taught me as we pored over those early Voyager Saturn images in his workroom, I was later able to work on a project of my own, mapping the geology of Miranda and the other moons of Uranus. I also developed a sense of the important role that hardworking scientists like Ed Danielson and Ed Stone can play behind the scenes in the enterprise of space exploration. The grunt work of science—planning the observations, calibrating the images, processing the data, making the mosaics, training the newbies, balancing the budgets, and so on—is critical for the team to get it right. Missions like Voyager succeed because of people like them. This world needs its tinkerers as much as it needs its theoreticians.
2
Gravity Assist
AS A KID launching model rockets in my backyard, spending hours carefully gluing parts together, applying stickers, painting the fuselage, packing the parachute, and installing the engine, I would wonder, Did I balance it right so it will launch and fly straight? Most times it did, but sometimes it never got off the pad or launched sideways, causing me and my sisters to run for safety. How far will it fly? Sometimes completely out of sight, never to be found, maybe swallowed up by the trees . . . Can I figure out how to mount a small film camera to the nose cone? I never did, too heavy . . . Repairs or rebuilds had to
be done, then another long lead-up to the launch, and then another suspenseful countdown as family and friends stood by—often watching from indoors to stay safe.
It turns out that many of the problems that need to be solved to launch model rockets are the same kinds of problems that engineers and scientists involved in NASA space missions have been working to figure out since the 1960s. How do you design the craft to withstand the stresses of launch and the harsh cold vacuum of deep space? How do you figure out how to communicate with it and control it once it’s out there? How do you get pictures and other measurements sent back from it? How do you design its mission? Early on, there was another question that arose once we started thinking about space travel: can we use a planet’s gravity to speed up and turn a corner? That could enable our rockets, our spacecraft, to go even farther. . . .
Even though it’s a cliché, lots of times a space mission really does start with scribbles on the back of a cocktail napkin. Or with conversations among colleagues over beers after a day at a professional conference. Or sometimes the idea comes in a dream, or in a flash of realization akin to making a discovery. Indeed, the Voyager mission appears to have begun in such a flash of inspiration.
In the mid-1960s, Gary Flandro was a graduate student in aeronautics at Caltech studying instabilities in rocket combustion and working part-time at JPL helping to study the aerodynamics and trajectories of missiles. His supervisor was one of the key members of the JPL Mission Analysis Group that was working to devise the upcoming Mariner 10 Venus–Mercury gravity-assist flyby mission, and he suggested to Gary that he help explore the possibility of similar gravity-assist trajectories being used for outer solar system missions. It was an area that almost no one else was looking into yet at JPL, as the lab was focused almost exclusively at the time on lunar, Mars, and Venus mission work.
Gary was also a fan of rocketry and of the history of the academic field known as celestial mechanics—the calculation and prediction of the orbits of planets, moons, asteroids, comets, and eventually spacecraft. In an interview published by Voyager mission chronicler and University of Hawaii sociologist David Swift, he pointed out that “the basic ideas behind gravity assist were known as far back as the 1800s.” These ideas were partially based on analysis by early celestial-mechanics pioneers, such as Urbain Le Verrier from France, of deviations of the orbits of comets passing by Jupiter. Le Verrier would later go on to use the same calculations to deduce in the 1820s that the planet Uranus had performed a distant gravity-assist flyby of a massive but as-yet-unseen object, which altered its orbit slightly. With Le Verrier’s help, that massive mystery object was eventually revealed to be the planet Neptune. It was on the shoulders of such giants that Gary Flandro began to search for similar ways to use gravity boosts to speed up the decades-long interplanetary travel times that would be required for direct-from-Earth outer solar system robotic missions.
One of Gary’s goals was to find out if gravity assists could be used to get to Saturn, Uranus, Neptune, and/or Pluto quicker than direct trajectories (essentially by using a close flyby of Jupiter as a slingshot), while still allowing a spacecraft to carry a significant amount of mass for propellant, power/communications/thermal systems, and science instruments. His flash of insight, in the spring of 1965, appears to have been to check if the alignments of the outer planets in the near future could, perhaps, allow not just one slingshot, but multiple slingshots that might permit a spacecraft to be fast-tracked to more than one outer planet after swinging by Jupiter. His investigation quickly showed that there would be a rare alignment of all four giant planets, plus Pluto, on the same side of the solar system in the 1980s. “So, why not look for a single trajectory that would pass each planet with the shortest possible trip time between?” he thought. Indeed, his calculations showed that it would be possible for a single spacecraft to visit Jupiter, Saturn, Uranus, and Neptune, or just Jupiter, Saturn, and Pluto, if it were launched about a decade hence, in the mid-1970s.
The Uranus Flyby of Neptune in 1821. Schematic diagram of the positions of Uranus and Neptune between 1810 and 1830, showing how Uranus made a distant flyby of the then unknown Neptune in 1821. The resulting tweak of the orbit of Uranus allowed mathematicians to predict the existence of Neptune, and then for Johann Galle to discover Neptune in 1846. (Jim Bell; SkyGazer 4 [Carina Software])
The potential efficiency of the gravity-assist process, which had been worked out in detail by others at JPL and elsewhere well before Gary Flandro began his work on the problem, nonetheless appeared in full bloom in his calculations: nearly twenty years could be shaved off direct-flight times to Neptune or Pluto by using well-timed gravity assists. Gary could “distinctly remember the feeling of awe as it first dawned on me that this mission was available at just the right time, leaving about ten years to market the mission concept and to design and build a spacecraft. Yes! Here is the way to do it! The next opportunity would not appear until about 175 years later!”
He is still excited about that profound Eureka moment from fifty years ago when he first imagined what JPL’s chief scientist at the time, Homer Joe Stewart, would dub “The Grand Tour.” “Wow—bang! There it was, right there! This is wonderful!” He describes the feeling of “flying this whole thing in my mind,” imagining, for example, a spacecraft traveling between Saturn and its innermost ring in order to get the maximum possible gravity assist. “When it actually happened later, it felt like, ‘I’ve already been there!’” Back in the summer of 1965, Gary excitedly described his results to his boss, who encouraged a more detailed study of the opportunity.
It was a spectacular cosmic coincidence that the technology needed in order to perform such a Grand Tour mission happened to have developed to a sufficient degree on the small planet from which the mission would be launched at precisely the time when the planets would line up just right to make it possible. Through the halls of JPL, Gary’s trajectory for a Jupiter, Saturn, Uranus, and Neptune flyby, augmented and enhanced by the work of others, became the “Grand Tour” trajectory. By July of 1965, while JPL’s Mariner 4 probe was making the first robotic flyby of Mars (and while I was busy being born), Flandro had worked out the details of the Grand Tour, including the best times to launch the spacecraft from Earth (the fall of 1977 or 1978).
Even though his multiplanet trajectory calculation work at JPL was far earlier than his PhD dissertation work at Caltech, he still had the good sense to write up his findings and get them published in an academic journal called Acta Astronautica in 1966. However, the response to Gary’s work was rather negative. “Many openly scoffed at the idea,” he said. At the time, JPL was being pushed to the limits to successfully operate missions to the moon and nearby planets that would last a mere several days to several years. The Grand Tour would call for a spacecraft that could operate for maybe a decade or more. Such longevity was unheard-of at the time and difficult for many to imagine. But Gary was now on a quest.
When I was a student at Caltech, one of my friends was a fellow student named Katie Swift, who was also studying astronomy and planetary science. We kept in touch after graduation, partly because she went back to her home state of Hawaii, where I went to graduate school. I got to know her father, David Swift, whose book about the many members of the Voyager team included a profile of Gary Flandro.
Despite the fact that the actual trajectories of the Voyagers’ Grand Tour were selected based partly on Gary’s 1965 research, he received little credit for his role at the time. He was still a graduate student, without any real authority or power. His contribution might have gone nowhere—certainly not into interstellar space—and even if it had, his role could well have been completely forgotten. Nevertheless, Gary is pragmatic about the part that he played in making the Voyagers happen and, still, rightfully, considers himself a part of the mission. “Many myths have arisen about the origins of the Voyager mission,” he told David Swift in his interview. “Since I was pretty low on the tot
em pole . . . no one felt much need to mention my connection with the discovery of the mission. I accepted those misconceptions, but it was sometimes difficult. As a rather naïve young fellow, I could not conceive of the possibility that I would not in time be acknowledged in some fashion for the work I had done. I thought this would happen automatically, since I had properly documented my work and presented it to many people both at the Caltech campus, at JPL, and in various technical meetings.” Despite what would be justifiable sour grapes over his lack of recognition, Gary was still gracious about the project overall: “Those at JPL who brought everything together certainly deserve major credit for the magnificent job that they did.” It wasn’t until 1998 that NASA finally did recognize his contributions, awarding him their Exceptional Achievement Medal. When I first heard this story as a graduate student myself, one line of Gary’s jumped out at me: “You have to learn not to be discouraged by experts.”
When it came to trying to actually get the Grand Tour mission off the ground, there would be plenty of discouragement to go around. In 1969 and 1970, JPL proposed an aggressive series of missions that would accomplish the Grand Tour. Four spacecraft would be launched: two in 1977 to fly by Jupiter, Saturn, and Pluto; then two in 1979 to fly by Jupiter, Uranus, and Neptune. The spacecraft would be based on JPL’s successful Mariner series (which by 1969 had successfully flown by Venus and Mars, and which would soon fly by Mercury). It would include probes launched into the atmospheres of the giant planets, and multiple launches per opportunity would help reduce one of the engineers’ biggest concerns and risks: keeping the spacecraft alive and functioning for more than a decade. This set of Grand Tour missions was not only ambitious, it was also expensive, with an estimated cost in 1970 of more than $900 million, equivalent to almost $5.5 billion today.