Chasing New Horizons

by Alan Stern

  At that great distance, the Sun’s gravitational hold is much weaker and planets orbit much slower. As a result, Pluto takes 248 years to make it just once around the Sun. Think about this: Roughly one Pluto year ago, Captain Cook was just setting sail on his first voyage from England, and only a little over one-third of a Pluto year ago Clyde Tombaugh was first eyeing the dot, Pluto, jumping in the frames of his blink comparator at Lowell Observatory.

  Pluto’s path is also highly elliptical (i.e., noncircular), more so than any closer planet. Because of this, from the time of its discovery to the late 1980s, Pluto had been slowly heading inward, ever closer to the Sun and to Earth. In the 1950s its increasing brightness, along with the development of new tools for precisely measuring the brightness of astronomical objects, allowed the first detailed measurements of Pluto’s “light curve”—that is, the way its brightness changes as it rotates on its axis. What jumped out from this analysis was a regular pulsing, a brightening and dimming, that occurs precisely every 6.39 Earth days. By spotting this slow, steady rhythm, scientists had discovered the length of Pluto’s day. Whereas Earth takes twenty-four hours to rotate once, Pluto spins at a comparatively stately pace, taking about 6.4 times as long between each successive sunrise—more slowly than any planets except Venus and Mercury.

  As technology improved in the early 1970s, planetary astronomers also succeeded in recording the first crude spectrum of Pluto, meaning they succeeded at determining its brightness as a function of wavelength. This revealed that its overall color is a reddish hue.

  Then in 1976, planetary astronomers at a mountaintop observatory in Hawaii discovered the subtle spectral fingerprints of methane frost (frozen natural gas!) on Pluto’s surface.1 This provided the first evidence that Pluto’s surface is made out of truly exotic stuff. The team that discovered methane on Pluto realized that the discovery also had an important implication for the size of Pluto, which was then unmeasured. All scientists knew at that time was how much light Pluto reflected in total. From this they could derive its size if they knew—or assumed—how reflective the surface was. Because methane frost is bright and reflective, its discovery meant that Pluto was likely small.

  Next, in June 1978, James “Jim” Christy, an astronomer at the U.S. Naval Observatory, observed “bumps” on some of his images of Pluto. Were these real features? Or were they imperfections in the images? Christy noticed that the stars in the same images had no such bumps, only Pluto did. So he analyzed the time between appearance of bumps and found a familiar period—6.39 Earth days, the same as Pluto’s rotation period! Other observers, alerted by Christy, found something similar. He had discovered that Pluto has a moon that circles it closely in an orbit that has precisely the same period as Pluto’s day. He later named it Charon (most often pronounced “Sharon”), after the mythical Greek ferryman who carried the dead to the Plutonian underworld. Choosing this name also cleverly allowed Christy to name Pluto’s moon to sound like his wife’s name, Charlene. After all, scientists are people, too.2

  Christy’s discovery of Charon turned out to be a mother lode for learning more about Pluto. Careful observations of the changing position and brightness of Charon revealed the size of its orbit. This, and the laws of physics, allowed the elusive question of Pluto’s mass to finally be nailed down, and the result was a bit shocking. Whereas Lowell, Tombaugh, and many subsequent Pluto researchers had expected a planet of roughly Earth mass or even higher, Pluto turned out to be only about 1/400th the mass of Earth. Rather than being another giant like Neptune, it was a tinier planet than any previously discovered.

  And the surprises didn’t stop there. Compared to Pluto Charon was huge, with a mass almost 10 percent of Pluto’s: the pair literally formed a double planet (sometimes also called a binary)—a first in our solar system! A double planet was something completely unknown in planetary science before the discovery of Charon. Pluto, it seemed, became more exotic every time we learned something new about it.

  But beyond what Charon immediately taught us, one other aspect of Charon’s discovery turned out to be bizarre, yet scientifically convenient. It turned out that when Charon was discovered, its orbit was about to enter into a very unusual geometric arrangement: essentially it was about to point right at us. Yes, that sounds strange—and it is strange. Because the tilt of Charon’s orbit remains steady while Pluto makes its slow circuit around the Sun, occasionally, but only briefly, Charon lines up to repeatedly pass (in our view) directly in front of and then behind Pluto. Such a perfect alignment happens for just a few Earth years per 248-year-long Pluto orbit. Amazingly, this chance alignment was just about to begin only a few years after Charon’s discovery. So, by pure luck, as seen from Earth, the newly discovered moon and its planet were about to start repeatedly eclipsing each other—a scientific bonanza that would teach us many things about the distant planet Pluto and its big moon Charon.

  Estimates indicated that these eclipses would start happening in 1985, give or take a few years, and should continue for about six years. During this “mutual event season” such an eclipse should occur every 3.2 days—that is, every half Charon orbit period of 6.4 days. Those eclipses would allow the sizes and shapes of both bodies to be determined for the first time, and would provide many additional clues about their surface brightnesses, compositions, colors, and possible atmospheres. In the pantheon of strange and lucky coincidences, the fact that Charon was discovered just before it was about to swing into place for this mutual event season—after all, another such set of eclipses wouldn’t occur for more than a century—is right up there with the grand-tour alignment of the planets appearing just when humans were ready to take advantage of it by mastering spaceflight.

  One of the Plutophiles primed for observing these mutual eclipses was a young scientist named Marc Buie, another child of the Space Age who had been deeply and permanently touched by black-and-white television images of people in rockets blasting off in his youth. As a college student, he’d caught the Pluto research bug and never got cured.

  Marc then went off to grad school at the University of Arizona in Tucson. Finishing his Ph.D. in 1985, his timing was perfect, because Pluto and Charon were about to start eclipsing each other. Marc:

  We were lucky enough to have discovered Charon right before the mutual eclipses, so we could do this wonderful six-year observing effort, which put Pluto on the docket for every major planetary science conference from then on, and raised Pluto up another notch in the collective scientific consciousness.

  In the mid-1980s several observing groups had been watching for the predicted eclipses to start, but nobody knew exactly when the first ones would occur. The first detection of them was observed by a young scientist named Richard “Rick” Binzel in February 1985. Then, once it was known the events had begun and Pluto and Charon started casting shadows on each other every 3.2 days, more observers got in on the game, and they provided a rush of new results, including hints of interesting surface features and surprising differences between the pair of bodies. As Marc put it, due to these eclipses, “Pluto hit the main stage.”

  THE UNDERGROUND

  As the 1980s drew to a close, Voyager 2 was nearing the end of its landmark exploration of the giant planets, culminating in its flyby of Neptune and Neptune’s planet-size satellite Triton.

  As the end of Voyager’s exploration of the four giant planets approached, a group of young scientists, all at the beginning of their careers, and conditioned by the boldness and near-mythical success of Voyager, started dreaming and scheming about how to keep the spirit and the quest of first-time exploration alive, how to go farther still—to explore Pluto.

  In 1988, and still a grad student, Alan started to ponder the possibility of sending a spacecraft mission to Pluto. He could see that the first, and in some ways biggest, hurdle to getting such a mission started would be more social and political than technical or scientific.

  To succeed, the concept would need a critical mass of support withi
n NASA and the scientific community. Alan knew that the mutual events between Pluto and Charon had yielded many exciting results that painted a new picture of Pluto, showing it to be a highly exotic planet. And the late-1980s discovery of Pluto’s atmosphere had added more fuel to that fire. Alan knew that this increasing scientific interest in Pluto might be channeled to support a spacecraft mission there.

  But did NASA know this? Not really, it seemed. Pluto never appeared on the short lists of high-priority missions on the reports that came out from influential committees guiding NASA policy.

  Yet Voyager’s decision not to go to Pluto had come before the discovery of the atmosphere, the discovery of its giant moon, and the growing evidence that it had a complex and varied surface. Pluto had, in the years since the Voyager decision not to visit it, become a lot more enticing.

  Moreover, the six-year season of Pluto-Charon eclipses had already revealed a lot, including dramatic surface variations between bright and dark areas on Pluto and the fact that the surfaces of Pluto and Charon are, surprisingly, made out of very different ices. Pluto’s surface features frozen methane, whereas Charon’s was found to be made of water ice. Continued observations of “occultations”—the temporary dimming of stars when Pluto passed in front of them—had also revealed some weird atmospheric structure, possibly indicating a low-altitude haze, hinting even more at a surprisingly complex planet.

  Given all these factors, Alan saw Pluto as the obvious next place to explore, and he sensed that it was a good time to see if he could drum up support for that exploration among planetary scientists.

  He wasn’t quite sure how to do it, but Alan thought that a good first step would be to gather Pluto scientists together in a highly visible scientific forum. At the time, there were only about one thousand active planetary scientists, and most attended the annual spring and winter meetings of the American Geophysical Union (AGU), where scientists congregate for a week to attend “sessions” of talks organized around different topics. Alan and a few colleagues decided that they would organize a technical session about new discoveries and insights into Pluto, and would propose this to the committee arranging the upcoming spring AGU meeting, in Baltimore in May of 1989.

  For this task of organizing and recruiting other scientists, Alan enlisted the help of Fran Bagenal, a young, hotshot British planetary scientist who had made a big impression with her work on the Voyager science team. Fran had just been hired for her first “real job,” a junior professorship of the Department of Atmospheric and Planetary Sciences at the University of Colorado, where Alan was finishing grad school and where David would soon become a professor.

  Fran was not a Plutophile. In fact, at first she needed convincing that a Pluto session at AGU even made sense. Back then, Fran thought of Pluto as merely a distant curiosity, and it was not the kind of place for her particular genre of scientific wizardry, which involves magnetic fields. To someone who looks at a planet and first sees those magnificent, magnetic structures, what good was a little ice ball like Pluto, a world too small to likely have any magnetic field? Fran:

  To be honest, I didn’t think much of Pluto. It was a small object in the outer solar system. It probably didn’t have a magnetic field. What kind of interaction with the solar wind was it going to have, and why bother to study it? Was it worth going all that way to visit some battered chunk of ice?

  But Fran was a rising star of planetary science, and Alan wanted her on his team. Looking back on it now, she sees her initial involvement as stemming from a combination of Alan’s advocacy and the impending need, after Voyager, for inspiring and daring new exploration:

  Alan was recruiting and galvanizing a group of people to work on Pluto. I actually remember thinking, “Oh, Voyager’s going to be over; now what do we do?”

  So at Alan’s urging, Fran attended some Pluto science meetings, and she soon started to catch the Pluto bug:

  There was a transition to my thinking, and that happened before the AGU conference in May of 1989. I found that indeed there were smart people who had been looking at Pluto’s atmosphere and surface and finding curious things. So I brought in Ralph McNutt, and we sat down and looked at it and realized there could be quite an interesting interaction with the solar wind. That’s how we started to get curious about the physics that could be occurring on Pluto.

  Fran and Ralph McNutt had met when they were both grad students at MIT, both working with the Voyager plasma instrument team, studying magnetic fields. After a period working for “the dark side” at Sandia National Laboratories in New Mexico (where nuclear weapons are developed), Ralph ended up back at MIT as a faculty member, where he was during the Voyager flybys of Uranus and Neptune. Ralph recalls:

  At Alan’s urging Fran and I submitted an abstract to that 1989 AGU meeting discussing Pluto’s possible solar-wind interactions. She gave the talk, and we wrote up a paper about it. We both started to understand that the interaction between the solar wind and Pluto’s atmosphere of evaporating methane was actually a really important thing to study. Then I realized that we needed a mission to figure it out. I got on board.

  After that, anytime anybody said “Pluto” to me, I would say, “We need to send a spacecraft there. We have to finish the exploration of the solar system, and we need to do it right.” I got caught up in the enthusiasm of Fran and Alan and the others who realized Pluto’s scientific potential. None of us were going to take no for an answer, and we were all so young that we didn’t know any better than to try.

  With Ralph’s encouragement and collaboration, Fran got involved with Alan in organizing that first ever AGU Pluto session. Then they put the word out to the community of scientists doing research on Pluto to “vote with their feet” by submitting research talks and attending the session to show interest in a possible mission to explore Pluto.

  Among those who heard the call was a bright and iconoclastic geophysicist named William “Bill” McKinnon. Bill had just recently been hired as an assistant professor at Washington University in St. Louis. His specialty is planetary geophysics, which means he applies knowledge and techniques used to study the internal structure and movements of the Earth to make sense of the same on other planets and moons. It took some imagination and courage to apply geophysics to a body like Pluto that was still barely more than a point of light. But Bill was fascinated with the origin and geology of icy worlds like the satellites of the giant planets and distant Pluto.

  Very tall and a little gaunt, with angular features and long dark hair, Bill looked a little like Frank Zappa, or at least more like someone you’d run into at a Zappa concert than at a planetary science conference. Bill was and is a serious rock-music fan, and he has a dry, slightly dark sense of humor. He’s also one of the smartest people anyone could ever meet and one of those lovable nerds with whom you just want to talk science forever.

  Bill had authored a pivotal research paper published in Nature in 1984—“On the Origin of Triton and Pluto.” Decades later, it’s still regarded as a classic. The paper refuted a then-common idea for Pluto’s origin, that it could have started out as a satellite of Neptune—a twin to Triton—that had escaped into orbit about the Sun. Bill’s work modeled all the gravitational and tidal jostling that could have occurred between these bodies, and his convincing and clever calculations showed that the only plausible origin scenario for Triton and Pluto was exactly the opposite. Bill showed the story was one of capture, not escape. Pluto was not a runaway moon of Neptune. Rather, Triton had started off like Pluto, a small, freely orbiting planet of the solar system, but it had been caught by Neptune’s gravity and drawn into orbit about that giant planet. His paper concluded, “The simplest hypothesis is that Triton and Pluto are independent representatives of large outer solar system planetesimals.”

  Throughout the 1980s, McKinnon had continued to work on the origin of Pluto, becoming convinced that if we could ever get there, we’d discover much more than just the random idiosyncrasies of one small world. Pluto,
McKinnon realized, could provide a glimpse behind the veil of a whole kingdom of new worlds containing missing information on the construction of our entire solar system. So he certainly did not need to be persuaded about the need for a mission there. When he learned of Alan’s AGU session, he contributed a talk entitled “On the Origin of the Pluto-Charon Binary.”

  Planning for the AGU session was coming together well—with many research-talk contributions and almost all of the most prominent scientists studying Pluto planning to attend. With this success almost in hand, Alan sensed that it might also be a good time to try to plant the seed for a Pluto mission directly in NASA Headquarters.

  So about a month before the AGU members gathered in Baltimore, he requested and was granted a one-on-one meeting with Dr. Geoff Briggs, then NASA’s director of the Solar System Exploration Division. It was not your usual meeting request by a graduate student, but having been older than average and having had a previous career as a spacecraft engineer, Stern knew Briggs and leveraged that relationship.

  The week before the AGU meeting, Alan visited Briggs in his office at NASA Headquarters in Washington, DC. He told Briggs about the upcoming Pluto session at AGU, about how much great new science there was and about the growing wave of interest in Pluto. He asked Briggs, “With Voyager winding down, why don’t we complete the job of exploring the solar system? Would you fund a study of how to do a mission to Pluto?”