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Macmillan Childrens Publishing Group

Chasing New Horizons

Inside the Epic First Mission to Pluto

Alan Stern and David Grinspoon





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.


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.


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.

Copyright © 2018 by Alan Stern and David Grinspoon