Chasing New Horizons

Inside the Epic First Mission to Pluto

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Contents

DREAMS OF A GRAND TOUR

This book tells the story of a small but sophisticated machine that traveled a very, very long way (3 billion miles) to do something historic — to explore Pluto for the first time. It achieved that goal through the persistence, ingenuity, and good luck of a band of high-tech dreamers who, born into Space Age America, grew up with the audacious idea that they could explore unknown worlds at the farthest frontier of our solar system.

The New Horizons mission to Pluto had many roots. They reach back to the astonishingly difficult discovery of Pluto in 1930. They then extend, over half a century later, to the delightful discovery of a host of other worlds orbiting at the edge of our planetary system, and to an underdog proposal to NASA by a determined team of young scientists bent on historic exploration and new knowledge.

Scientists don't necessarily believe in destiny, but they do believe in good timing. So we begin in 1957, the year that the first spacecraft, called Sputnik, was launched into Earth orbit.

KICKING TO GET STARTED

Sol Alan Stern arrived on Earth in New Orleans, Louisiana, in November 1957, the first of three children born to Joel and Leonard Stern. His parents say it was a very easy pregnancy, except for the final few weeks. Then he suddenly began kicking, like crazy. Alan's father maintained, years later at his son's fiftieth birthday party, that Alan had apparently been hearing people talking about the launch of Sputnik, and was clearly impatient to get out and get going to explore space.

Alan grew up interested in science, space exploration, and astronomy, from his earliest days. He read everything he could get his hands on about space and astronomy, but eventually ran out of library books — even in the adult section.

When Alan was twelve, he watched newsman Walter Cronkite on television describing one of the early Apollo landings while holding up a detailed NASA flight plan. "You couldn't actually read it on TV," said Alan, "but you could see it ran hundreds of pages and was filled with all kinds of detail, with every activity scripted, minute by minute. I wanted one, because I wanted to know how space flight was really planned. I thought 'If Walter Cronkite can get one from NASA, then I can get one too.'"

So Alan wrote to NASA, but when told he wouldn't be receiving a copy because he wasn't an "accredited journalist," he decided to double down and fix that issue. Over a year, he researched and wrote by hand a 130-page book. The title was "Unmanned Spacecraft: An Inside View," which — as Alan is the first to note — was "a pretty funny title for a kid who was entirely on the outside and learning as he went."

But it worked. Not only did Alan receive a whole set of Apollo flight plans from NASA, he ended up being taken under the wing of John McLeish, the chief NASA public affairs officer in Houston, often heard narrating Apollo missions on TV. In fact, McLeish began sending Alan a steady stream of Apollo technical documents: not just flight plans, but command-module operation handbooks, lunar-module surface procedures, and much more. Alan became hooked on a space career, but knew he'd have to study for a decade to get the technical skills to join the space workforce after college.

THE GRAND TOUR

Around the same time that John MacLeish was befriending him, Alan also got hold of the August 1970 issue of National Geographic, with a cover depicting Saturn as it might appear from one of its moons. The painting, showing the giant, ringed planet cocked at an angle, floating against the black of space over a cratered, icy, alien landscape, seemed at once both realistic and utterly fantastic. The cover story, "Voyage to the Planets," is something that many planetary explorers of Alan's age remember paging through as kids. It contained a level of magic — robotic spaceflight — that today would be found in Harry Potter.

The article described how in the decades to come, NASA planned to launch a series of robotic spacecraft that would explore all the planets and transform knowledge of them from science fiction fantasies into actual photographs of known worlds.

The exploration of the solar system was portrayed as an ongoing sequence of journeys. The article was accompanied by profiles of the first generation of planetary scientists — Carl Sagan among them — who conceived, launched, and interpreted the data from those first voyages. By 1970, NASA had managed to launch only seven spacecraft beyond Earth to reach other planets — three to Venus and four to Mars. These first interplanetary crossings had all been "flybys," missions which simply sent a spacecraft zooming past a planet, with no ability to slow down to orbit or land, gathering as many pictures and other data as possible during a few hours near closest approach. (Note: we say "simply," but, as the following pages of this book illustrate, there is actually nothing simple about it.)

That National Geographic article described how the 1970s promised to be "the decade of planetary investigation," with an ambitious list of planned and hoped-for NASA missions that would open up the rest of the solar system to humanity. First, in 1971, would be a pair of orbiters to Mars. Next would be the first missions to the immense uncharted realm of what was then called the outer solar system, as Pioneer 10 and 11 would reach Jupiter in 1973 and 1974 and then travel on to reach Saturn in the distant year of 1979.

Shortly after, Mariner 10 would make the first visit to Mercury, traveling there by way of Venus, where it would make the first ever use of a "gravity assist," a nifty trick that has since become indispensable for getting around the solar system. In a gravity-assist maneuver, a spacecraft is sent on a near-miss trajectory to one planet, which pulls it in and then speeds it toward its next target. It seems too good to be true — like getting something for nothing, but it's not — the equations of orbital mechanics do not lie. For the planet, the tiny loss of orbital speed it trades with the spacecraft has no meaningful effect, but the spacecraft gets a whopping shove in just the right direction. Pioneer 11 was slated to use this same trick during its planned flyby of Jupiter, allowing it to then go on to Saturn.

If all these missions were successful, then before that decade was out, spacecraft from Earth would have visited all five planets known to the ancients — Mercury through Saturn. And what's more, Pioneer 10 and 11, sped up from their close encounters with Jupiter and Saturn, would be racing outward with enough velocity to eventually escape the Sun's gravitational hold entirely, becoming the first human-built artifacts to leave our solar system (along with their uppermost rocket stages).

And then what? There would still be three other planets left to explore, but at the vast orbital distances of Uranus, Neptune, and Pluto it would take an impossibly long time to reach them. Unless ...

The National Geographic article described an ambitious plan to launch a "grand tour" mission that could use multiple gravity assists to visit each of these planets. In theory, a spacecraft could be launched outward toward Jupiter, relayed toward Saturn, and then again relayed successively to each more-distant world. Such a mission would allow all the planets, even distant Pluto, to be reached in less than a decade, rather than the multiple decades such a journey would otherwise take.

But this trick cannot be attempted at any random time, even in any random year or century. The planets, each one on its own orbit around the Sun, need to be arranged in just the right way, like beads strung on an arc, stretching from Earth to Pluto. Like a secret passageway appearing only briefly every couple of centuries, the motions of the planets line up to create such a conduit only once every 175 years.

It just so happened that one such rare opportunity would soon present itself, and it was dubbed the "Grand Tour." Using it, a spacecraft launched by the late 1970s could quickly travel all the way across the solar system, visiting every outer planet in turn and arriving at Pluto by the late 1980s. It was fortuitous that at that moment in history, in the late twentieth century, when humans had just figured out how to launch spacecraft to other worlds, such a rare chance would be coming around.

There were lessons here for a young reader: The laws of physics can be our friends. They can be used to achieve things that would otherwise be beyond reach. And sometimes things line up just right to provide opportunities that, if not seized, won't come around again for a very long time.

That National Geographic was illustrated with early spacecraft photographs of Mars and Venus, and artists' depictions of the planets as yet unexplored. It also contained a table summarizing the known facts about all nine known planets, and one planet stood out from the others as completely mysterious. In the column for Pluto, most of the boxes were filled in with just question marks. Only the details of its vast and distant orbit (taking 248 Earth years to complete one of its own) and its length of day (spinning on its axis once every 6.4 Earth days) were given. Number of moons? Unknown. Size? Unknown. Atmosphere? Surface composition? Both also unknown. There was nothing to give us much of a clue about what it might actually be like on Pluto. Alan remembers reading that article and seeing that table, and thinking about spaceships one day exploring mysterious Pluto, the most distant unknown of all the planets.

VOYAGERS

Back then, most interplanetary missions launched as pairs of spacecraft, to guard against the possibility that one might fail. There was good logic in that, because the cost of building a second, identical spacecraft is much reduced by borrowing the design and much of the planning for the first. For example, Mariner 9, the Mars orbiter that finally revealed the "Red Planet" in all its detail and glory was successful. But its twin Mariner 8 ended up crashed beneath in the Atlantic Ocean due to rocket failure. A similar fate had met Mariner 1, though Mariner 2 made it to Venus, and Mariner 3 had failed, but Mariner 4 got to Mars.

NASA's planned grand tour of the giant planets included two pairs of identical spacecraft that would visit three planets each. One pair, to be launched in 1977, would fly by Jupiter and then be ricocheted on to Saturn and Pluto. The other pair would launch in 1979 to visit Jupiter, Uranus, Neptune, and Pluto. The grand tour would complete what Carl Sagan referred to as "the initial reconnaissance of the Solar System."

It was a wonderful plan, but sending four spacecraft to each visit three planets was just too expensive. The projected cost to design, build, and fly this mission, lasting well over a decade and traveling much farther than any spaceflight in history, was more than $6 billion of today's dollars. Sadly, at that time NASA's budgets were falling, and in that environment such an expensive mission was a nonstarter. The grandiose grant tour was canceled before it ever got off the drawing board.

Recognizing that the opportunity would not come again in their lifetimes, the science community scrambled to reduce cost and rescue the grand tour, producing a scaled-down version called the "Mariner Jupiter-Saturn" mission, with the more modest goals of exploring only the two largest and closest outer-solar-system planets: Jupiter and Saturn. This twin-spacecraft mission, at just under $2.5 billion in today's dollars, was approved in 1972. A contest was held to formally name the spacecraft, and they were christened Voyager 1 and 2 just months before their launches in August and September 1977.

Although the original grand tour had been canceled, the Voyager 1 and 2 launch dates and trajectories were cleverly chosen to enable the craft to keep going after Saturn, using gravity assists to reach all the other planets. The nuclear power source was also designed with enough energy to fly the spacecraft for many years after the "primary mission." So, potentially, these craft could continue on to Uranus, Neptune, and Pluto if funds could later be found to pay for their extended flights.

The Voyager mission would be considered a complete success if it just succeeded in exploring the Jupiter and Saturn systems. Yet its designers planned that — with luck, and future resources they couldn't count on — it just might be possible to keep it going for years longer and billions of miles farther, completing all of the grand tour's objectives after all. And indeed, the Voyagers ultimately did just that. Launched in the late 1970s, each completed its primary mission at Saturn by 1981, and both are still operating today — four decades after launch. Voyager 2 traveled in the direction of Uranus and Neptune, but the wrong direction to reach Pluto, but Voyager 1 headed in the right direction.

So why didn't Voyager 1 go on to Pluto? One of the big prizes, and one of the official metrics for success for Voyager, was the exploration of Saturn's unique and enigmatic, giant moon, Titan. As the only moon in the solar system with a thick atmosphere, even thicker than Earth's, and like the air we breathe made mostly of nitrogen, it naturally stood out as a place scientists wanted to know better. Titan also possessed hints of some interesting organic chemistry (the kind of chemistry, involving carbon, that on Earth enables life to exist), and its atmosphere was known to include the carbon-containing gas methane. This had been discovered in 1944 by astronomer Gerard Kuiper, one of the founders of modern planetary science and someone whose name we'll see again soon.

There was a problem, though, and Titan forced a difficult trade-off. Voyager 1 could only do a really good job of investigating Titan if it made a close flyby immediately after flying by Saturn. Executing such a maneuver would pull the spacecraft permanently off the grand-tour trajectory, flinging Voyager 1 toward the south, veering sharply out of the plane of planetary orbits. This post-Saturn flight direction would make a continuing journey outward to Pluto impossible. At the time, no one could really argue successfully that Voyager 1 should skip Titan. It was a body relatively near at hand compared to Pluto, and scientists knew Titan was fascinating. By contrast, the risky, five-year journey onward to distant Pluto, a body about which so little was known that no one could say it would be worth the effort. Picking Titan over Pluto was a good, and logical, choice. And even today, no one regrets this decision, especially now that Titan has proved to be a world of wonder with methane clouds, rainfall and lakes, and vast fields of organic sand dunes — truly one of the most enticing places ever explored. It was indeed the right decision, but it also closed the door on humanity's chance for a visit to Pluto in the twentieth century. If Pluto were ever to be visited, it would be left for another time, and another generation.

SCHOOL DAYS

Alan finished college, at the University of Texas, in December 1978. Just as Voyager 1 was approaching Jupiter, in January 1979, he then started grad school in aerospace engineering. His fascination with space exploration continued, but he did not see himself becoming a scientist. Even today he remembers hearing about the decision for Voyager 1 to study Titan rather than attempt the longer, riskier journey to Pluto. "I remember thinking back then, 'They made a smart choice, but it's too bad — we'll probably never have the chance to see Pluto.'"

Alan maintained a keen interest in the way spacecraft missions work, but his master's program, with a focus on orbital mechanics, was strategically designed to build a résumé that would enable him to be selected by NASA's astronaut program. What would be the right next move for that?

Alan wanted to show NASA he was versatile, so he went for a second master's in another field, planetary atmospheres. The choice turned out to be pivotal. Alan recalls:

There was a young, hotshot planetary research professor at Texas who also wanted to be an astronaut, Larry Trafton. He had come out of Caltech and had made some pretty big discoveries. He also had a reputation for rigor and toughness. I remember going to Trafton's office and knocking on the door and feeling very intimidated by his reputation, but telling him I would work for free if he had any ideas for a project we could do together. He told me about a paper he had just written about Pluto that made some calculations about the behavior of Pluto's atmosphere and the high rate that it was escaping into space, which indicated that Pluto should have completely evaporated over the age of the solar system. Of course, this didn't make sense — because Pluto was still there, indicating something else was going on we didn't understand. Trafton just happened to be puzzling over this when I knocked on his door in late 1980, asking for a good research problem to work on. So he said, "Why don't you work on Pluto?" and that eventually became my master's topic. We did some explorations of the basic physics of what Pluto's atmosphere might be like. Very simple computer modeling by today's standards, but illuminating for its time.