Chasing New Horizons

by Alan Stern

  To remedy this situation, the team put in place some careful new controls on how often the spacecraft would turn in the future, hoarding the use of the two underspecced thrusters, and also began to “load-share” these thrusters by using their backups. The backups suffered the same problem, but by alternating which sets were used, and by limiting their use for some kinds of maneuvers, the flight control team could husband them to keep both the prime and backup sets well under their 500,000-cycle limit all the way to Pluto. Bullet dodged, lesson learned. But it meant a new wrinkle that the flight team would have to live with for the entire journey.

  Another key task in those first weeks of flight was perfecting the trajectory toward the Jupiter-gravity-assist aim point that would propel the spacecraft onward to Pluto. After two weeks of careful radio tracking of New Horizons followed by orbit calculations, it became even more clear that the Atlas had put them on a near-perfect trajectory, and that only a tiny spacecraft engine burn correction would be needed to make it perfect. As a result, trimming the trajectory up after launch would take much less fuel than had been allotted when the propulsion system was designed. Those fuel savings gave them a bonus to put in their pocket, either for an asteroid flyby in June, or for more Kuiper Belt exploration after Pluto. Alan chose the bird in the bush—saving the fuel for the Kuiper Belt, a mission objective—which an asteroid flyby, though soon and seductive, was not.

  To fine-tune the trajectory to Jupiter, the navigation team estimated the need to burn the onboard engines to change the spacecraft’s speed by just eighteen meters per second, or about 40 miles per hour. Not bad, considering it was traveling almost 40,000 miles per hour! But if this correction wasn’t made, that 40 miles per hour would mount up—over all the hours until it got to Jupiter on March 1, 2007—to a whopping 400,000-mile error, which in turn would cause them to miss Pluto by several hundred million miles!

  To be surefooted with their first trajectory correction of the mission, the team made the adjustment in two parts. First the spacecraft would make a very small engine burn of about five meters per second, just to suss the engines out and see how everything worked. Then once the telemetry from that burn was downlinked and it was confirmed by engineers that it had gone well, the rest of the maneuver would be completed a few days later, putting New Horizons perfectly on track for the Jupiter aim point to Pluto.

  With the spacecraft fully checked out, and the engine maneuvers to get precisely on course completed, it was finally time to check out and test the seven scientific instruments that would be humankind’s eyes and ears and sense of smell at Pluto. Each instrument team followed a cautious series of steps to carefully turn on their sensor, make sure its computer worked, that it was operating at the right temperature, and that it could be powered and could communicate with the spacecraft’s prime and backup systems. Next, the protective launch doors in front of the three telescopic instruments—Alice, Ralph, and LORRI—were opened. Then each instrument was tested to determine if it was performing as it had back on Earth, pointing them at calibration targets, like specific stars in the sky with known brightnesses.

  Over a period of many weeks, all seven scientific instruments checked out perfectly, but a near miss with disaster occurred during the LORRI instrument checkouts. Recall that LORRI is a high-powered telescopic camera. The problem: during one checkout it was accidentally pointed directly at the Sun for a few seconds. Just as you can blind yourself by looking at the Sun through a telescope, LORRI’s camera can be blinded in the same way. Of course, there was flight software to prevent this from ever happening, but there was a subtle mistake in the way that software was designed. The way the instrument protection software was written, it said, “Anytime we are pointing at a target, make sure that it’s at least 20 degrees from the Sun, or don’t point there.” What it didn’t do was ask whether, at any time during the maneuver to swing to a target, LORRI would sweep across the Sun. Unfortunately, as the team was testing LORRI’s performance, the spacecraft did just that, pointing LORRI briefly at the Sun during a turn. LORRI, sensing the Sun’s extreme brightness, shut itself off to try to protect itself, but not before the Sun briefly poured right down the barrel of LORRI’s telescope.

  Thankfully, LORRI escaped damage, but the fact that its protecting against scans across the Sun had been overlooked in the “watchdog” protection software produced a real scare, so the flight control team wrote new software protections for LORRI, and the other instruments as well, to make sure none of them could do anything similar again.

  During a lessons-learned review of this incident, the New Horizons Pluto flyby mission manager, Mark Holdridge, said to Alan, “Well, we sure dodged a bullet there. I’m glad we learned this, and knock on wood nothing like it ever bites us again.” When Mark said that, Alan looked around mission control and realized that everything there was made of something artificial. There was no wood to knock on! So he bought some kitchen cutting boards and sawed them into a dozen little three-by-three-inch blocks. Then he placed a New Horizons mission sticker on the top of each one and attached a little plaque on the bottom of each that read: “Anytime you need to knock on wood for New Horizons, here it is! Keep this until 2015!” Alan then sent a dozen of these little wooden blocks to New Horizons mission control, where they sat on the consoles and office desks throughout the entire flight to Pluto.

  Now months into flight, things seemed to be going pretty well. But something odd was brewing in Europe that the New Horizons team did not anticipate.

  THE ASTRONOMERS EJECT PLUTO—2006

  In August of 2006, barely seven months after the launch of New Horizons, a meeting of an astronomer’s organization called the International Astronomical Union (or IAU) was held in Prague. At that meeting the definition of the word “planet” came up for a series of votes.

  The Kuiper Belt was turning out to be full of small icy planets that were much like Pluto and were neither gas giants like Jupiter, ice giants like Neptune, or rocky “terrestrial planets” like Venus, Earth, and Mars. Pluto, it turned out, was not alone out there as a large body; it was simply the first discovered and brightest (and therefore the easiest to detect) of this new class of worlds. At the same time, new planets of many other types were being discovered around faraway stars. Most of these were very large planets—as large as or larger than Jupiter. The discovery of smaller planets around other stars was limited by technology, but it was (and still is) widely expected that smaller planets would also be discovered around distant stars in coming years.

  Many planetary scientists had long been referring to the rich harvest of newly discovered small planets in the Kuiper Belt as “dwarf planets,” a term Alan coined in a 1991 research paper mathematically calculating that the solar system might contain as many as one thousand of them. He chose the term “dwarf planet” in analogy to the well accepted astronomical term “dwarf stars,” like the Sun, that are the most common type of stars in the universe.

  Then, in 2005, the discovery was reported of a Kuiper Belt planet later named Eris that was thought by its discoverer, Caltech scientist Mike Brown, to be slightly larger than Pluto (later this turned out to be wrong). This in turn resulted in the IAU appointing a planet-definition committee, which included the award-winning science writer Dava Sobel, along with six eminent astronomers. After long deliberations and debates, this august committee proposed a simple and straightforward planet definition: a planet is an object in orbit around any star that is large enough for gravity to make round but not so massive that it ignites in nuclear fusion to become a star. In this scheme, dwarf planets in the Kuiper Belt were recognized as a new class of small planets in line with what many planetary scientists thought.

  But what happened next was more than just a little strange. Back in 1980 a British astronomer named Brian Marsden had famously told Clyde Tombaugh that Pluto wasn’t a planet in his view, and that it would be his mission to erase Tombaugh’s legacy by having Pluto reclassified as an asteroid. No one we’ve asked remem
bers why Marsden felt so strongly about this, but people did report that, for some reason, Marsden didn’t like Tombaugh. Then, at the 2006 IAU meeting, a gaggle of astronomers led by Marsden procedurally objected to the IAU committee’s newly proposed planet definition. Next followed a series of hastily drawn up amendments and redefinitions, all of which were voted down. But on the final day of the weeklong meeting, when most of the attendees had already left the meeting (only 4 percent of the IAU membership remained), those tired few still in Prague voted on a newly proposed definition over the carefully considered one drafted by their own planet definition committee.

  Unfortunately, the definition that they voted on was sloppy, awkward, and inelegant, and resulted in Pluto and all dwarf planets, along with all the planets around other stars, being cast out. The hastily arranged voting process the IAU used that day has since nearly universally been regarded as flawed, and the definition it adopted is disliked both by many astronomers, and even more so, by a wide cross section of planetary experts—planetary scientists.

  The adopted IAU definition is flawed on multiple grounds. For example, it contains the stipulation that a planet must orbit our Sun. This is silly, as it ignores the marvelous discovery that our universe is full of exoplanets, circling nearly every star. Thus, the IAU vote defined “planet” in a way that excludes virtually all the planets in the universe. The IAU further defined “planet” in such a way as to deliberately manage the number of planets in our solar system to be small, on the rationale that schoolchildren would not be saddled with memorizing long lists of planet names if planets were plentiful. (Yes, this argument was actually made with straight faces!) That was accomplished by demanding that a planet must have “cleared its zone” in the solar system. This is a strange way of thinking. If one wants to determine if something is a planet, one should consider its attributes rather than what other bodies it is near or not near in space. But the IAU definition doesn’t consider what the object looks like or what its main properties are: for example, whether it has an atmosphere or moons or mountains or oceans. What’s key in their definition is where the object is located and what is orbiting or not orbiting near it. By this definition, if the Earth itself was surrounded by a swarm of debris (which it was for its first 500 million years of existence, and arguably still is, even today), it would not be considered a planet.

  Adding direct insult to their flawed definition, a final stipulation added by Marsden’s group at the end of the IAU’s resolution was the vindictive and linguistically nonsensical statement “A dwarf planet is not a planet.” With that, Marsden had accomplished his longstanding goal: Pluto would no longer be a planet in the eyes of astronomers, or in astronomy texts, and Clyde Tombaugh’s pioneering legacy would essentially be erased.

  The IAU vote created a firestorm in the press, primarily focusing on Pluto’s “demotion,” which is not a neutral term; as “demotion” implies a diminishment of status, a lessening of importance.

  It quickly became clear that there had been an effort to cast out the new crop of planets, to remove Pluto and its counterparts from the pantheon of important bodies in the solar system.

  When word of the astronomers’ vote in Prague reached the New Horizons team, reactions ranged from indifferent (“Who cares what astronomers think? They’re not the experts in this.”), to bemused, to annoyed, to seriously pissed off. As Fran Bagenal succinctly put it, “Dwarf people are people. Dwarf planets are planets. End of argument.”

  Many planetary scientists found it particularly annoying that mainstream press outlets seemed to report the reclassification as a fait accompli, accepting without question the authority of the IAU, an organization composed mostly of astronomers, rather than planetary scientists, to define a commonly used word like planet.

  Within two weeks of the astronomers’ vote, hundreds of planetary scientists—more than all the astronomers who had voted in Prague—signed a petition stating that the IAU’s definition was so flawed they would not use it. The press largely also ignored this, and we do not understand why. But as a result, many in the public began to picture Pluto more or less as an asteroid, rather than the small planet it is.

  LONG JOURNEY AHEAD

  The astronomers’ folly aside, the New Horizons team had a great deal of work ahead of them in 2006. In total, their nearly decade-long cruise to Pluto was divided into two distinct cruise phases, each with its own character, work, and pace. The 13-month sprint to Jupiter was jam-packed with spacecraft commissioning, early course corrections, instrument commissioning and calibrations, and the flurry of activity to plan the Jupiter flyby. After Jupiter would be an eight-year-long flight to Pluto, in which the spacecraft would hibernate most of each year while the mission team would plan the flyby of Pluto. Years before, when Tom Coughlin retired as New Horizons project manager and Glen Fountain took over his position, Alan named these two flight phases in honor of the mission’s two project managers: the flight to Jupiter was dubbed “Tom’s Cruise”; the flight onward to Pluto became “Glen’s Glide.”

  With New Horizons on its long voyage across space, the project team shrank dramatically. In the four-year buildup to launch, more than twenty-five hundred people were involved in building, testing, and launching the spacecraft, its ground system, its RTG, and its rocket. But within a month after launch, the majority of the staff were no longer needed, and went to work on other projects. The big city that was New Horizons was reduced to a small town.

  During the long years of flight to Pluto, only a skeleton crew of flight controllers and planners, a handful of engineering “systems leads,” the two dozen members of the science team, their instrument engineering staffs, and a small management gaggle was needed. Alan recalls, “Just weeks after launch nearly everyone went their own way, and the project was reduced to a little crowd of about fifty belly buttons. All of a sudden I looked around and it hit me: there are just a few of us—a tiny team—and we’re the entire crew that’s going to fly this thing for a decade and 3 billion miles and plan the flyby of a new planet.”

  You might think given the vast distance between Earth and Pluto, and all the years of flight time ahead, that boredom or impatience could have been a problem for the mission team. But in actuality, advances in automation and the plan to hibernate the spacecraft for most of the journey meant the New Horizons mission team was almost ten times smaller than the vast Voyager crew of 450. As a result, the stripped-down New Horizons team remained remarkably busy for ten straight years.

  This was particularly true during the first leg of the journey. The thirteen-month trip out to Jupiter was lightning fast, and there was so much to do. Hal Weaver remembers:

  There was no rest for the weary. After we got the spacecraft checked out, the first course corrections made, and the instrument payload checked out, the schedule was still hairy, because we had to plan and execute a complex Jupiter flyby almost immediately. We had to navigate to the right keyhole in space near Jupiter to fly on to Pluto, and we also wanted to have a practice at Jupiter for all of the steps and through the process that we would be using for planning the Pluto encounter, and we wanted to plan a gangbuster science flyby at Jupiter as well. All of that had to be prepared and tested and executed within just thirteen months after launch.

  A longer-term concern was retaining the corporate memory over the nearly ten years of flight ahead—all the fine details about how the spacecraft and instruments are constructed and operated. This was especially important because more than nine out of every ten people originally on the project went on to other projects after New Horizons launched. What would happen a decade hence when the mission had to be staffed up again for the flyby, with many new people who had never been a part of designing and building the bird? To counter this and other prudent concerns about the long road ahead, team members documented as much as possible about every aspect of the spacecraft and its mission control. They also made training plans for those they would hire eight or nine years in the future, and they created
spare parts inventories for the mission control and spacecraft simulators. They even created videos describing (classroom style) how every detail of the spacecraft and mission control worked.

  This work to make sure all of that corporate knowledge was captured while still fresh, and made ready for that distant year of 2015, was in addition to the work to learn to fly New Horizons, to get it and its scientific instruments checked out, and to plan the Jupiter encounter. Hal Weaver was right: there was no rest for the weary flight team during the entirety of 2006 and most of 2007.

  BY JUPITER

  For the science and mission operations teams, the largest single activity of late 2006 was planning their 2007 Jupiter flyby. There were three separate reasons why this flyby was key. First and foremost, they had to fly through a tiny window in space near the precise aim point at Jupiter, and at just the right moment, to get the gravity assist needed to target Pluto. If that didn’t succeed, the spacecraft would be headed elsewhere, and it would be game over for Pluto exploration. Second, Jupiter was the only encounter with anything before Pluto, so that flyby was the one opportunity to practice an actual planetary encounter, running all the spacecraft systems through their flyby paces, and trying out all the instruments on a planet and its moons. Among the many examples of this were tests of the optical navigation capabilities that would be needed at Pluto. The Jupiter flyby would be used to perfect how to determine the precise range and aim point offset to their flyby target by comparing the changing position of Jupiter against the faraway stars in successive images.