Chasing New Horizons

by Alan Stern

  Alan told the team about the practice they would get on the Jupiter flyby: “Our objective here is to learn as much as we can about the spacecraft by putting it through its paces. At Pluto our objective will be to learn about Pluto, and I don’t want to learn a thing about the spacecraft there—we need to learn all those lessons here, at Jupiter, so that the Pluto flyby is flawless.” The final Jupiter priority: to take advantage of the flyby—the first of Jupiter since NASA’s Cassini flew by in 2000 on its way to Saturn—to learn more about that planet, its largest moons, and its gigantic magnetospheric cocoon of charged radiation.

  New Horizons was in many ways more capable than any previous spacecraft sent to Jupiter, and the New Horizons team was psyched to see what new things they could learn at Jupiter using their seven state-of-the-art scientific instruments. There had been previous close looks and even long-term observations of the Jupiter system, but it’s a complex and ever-changing place, so it was valuable to have another exploration of it in 2007, particularly one bringing powerful new instrumentation to bear.

  Planetary geologist Jeff Moore, the New Horizons Geology and Geophysics team lead, honchoed the New Horizons Jupiter Encounter Science Team (called JEST). Jeff was a veteran at Jupiter exploration, having served as a graduate student on Voyager, and then on NASA’s Galileo Jupiter orbiter in the 1990s.

  Planning for the New Horizons Jupiter encounter went into high gear in the fall of 2006, just after the instrument checkouts and calibrations were completed. Because their Jupiter-to-Pluto aim point was actually very far—4 million miles—from Jupiter, New Horizons wouldn’t be able to swoop close to any of Jupiter’s moons, which all orbited much closer in. Nonetheless, many important observations of them could be made using the onboard telescopic instruments.

  One goal was to create a multi-week record of volcanic activity on Jupiter’s planet-size moon Io, the most volcanically active world in the solar system (outperforming even Earth). Another was to create a similar, multi-week record of Jupiter’s relentless storm systems. Neither of these objectives had been accomplished by the Galileo mission because of problems it had returning data due to its crippled spacecraft antenna.

  And beyond what the telescopic instruments could contribute, in a stroke of luck, the post-Jupiter path that New Horizons would take on its way to Pluto would fly literally 100 million miles down the tail of Jupiter’s roiling magnetosphere. This would make possible a whole new and pioneering class of observations of the giant planet’s magnetosphere with SWAP and PEPSSI. Nothing like that had ever before been accomplished, because no other mission had flown so deeply down any giant planet’s magnetosphere. In all, almost 700 observations of the Jupiter system were planned, involving all seven instruments on New Horizons.

  The Jupiter flyby, which entailed observations while approaching, at, and then departing Jupiter, spanned January to June of 2007, and was a spectacular success. The navigation team flew through the Pluto aim point with precision. Additionally, all of the many spacecraft and instrument tests went well, and New Horizons collected a wide range of scientific observations that ultimately landed it on the cover of the prestigious research journal Science.

  Perhaps the most enchanting result at Jupiter came about through simple, dumb luck. During the flyby planning, the team realized that it would be able to make some key new studies of Jupiter’s thin, dusty rings. However, no one on the New Horizons team was an expert in this scientific area. So planetary rings specialist Mark Showalter was brought on board to help. Showalter designed a five-frame movie to be taken by the LORRI camera at just the time when Jupiter’s moon Io would be crossing in front of the main Jovian ring. His idea was to make high-resolution maps of the ring as Io occulted it. But when these images came down to Earth, the team was astounded to find that in addition to accomplishing that objective, the image sequence had caught one of Io’s large volcanoes, Tvashtar, in the act of erupting, spouting a giant plume off the north pole of Io. Volcanoes on Io had been photographed many times before, and both the Voyagers and Galileo had caught dramatic shots of volcanic plumes shooting off Io into space! But before New Horizons, no time-lapse sequence of any volcano off Earth had ever been captured. The result was remarkable—a shimmering fountain of material being ejected from Io high into space and falling ballistically back down toward its surface. It was both scientifically valuable and mesmerizing.

  JUPITER’S STING

  We pointed out earlier how radiation can negatively affect the electronics on board a spacecraft. Just days after closest approach to Jupiter, there was an unexpected and unwelcome event, which had to do with just that. New Horizons was stricken with a “safing event,” caused when something goes wrong on board the spacecraft and triggers it to go into “safe mode”—a configuration in which the errant system is turned off, its backup system is engaged, and the spacecraft radios Earth that something is wrong and it is awaiting instructions. Safe mode is common to many spacecraft; it is designed to keep spacecraft from doing anything that will cause more harm or exacerbate a problem, until the situation can be diagnosed—and resolved—from the ground.

  When Alice Bowman’s mission ops team got telemetry back from New Horizons revealing this problem, it was found to be due to the main flight computer, which had rebooted itself. Something very similar had occurred on approach to Jupiter. A second occurrence in so short a period, just three months, was worrisome. Then, several months later, the same thing happened again—another unexplained main computer reboot, and another safe-mode entry. Initially some on the team were concerned that the main computer was failing and might not make it to Pluto. As more such events occurred later in the mission, they were found to be spaced at longer and longer intervals. The engineering team developed a theory that the main computer was not failing, that instead the reset events had been caused by radiation damage associated with Jupiter’s powerful magnetosphere of charged particle radiation. They hypothesized that over time the circuits causing the resets were healing from the radiation damage. By the time New Horizons reached Pluto, no computer reset had occurred in years: Jupiter’s sting had worn off.

  TO HIBERNATE

  If a person bought a television in January of 2006 and was expecting it to be operating in mid-2015, would it be more likely to meet their 2015 needs if they left it on continuously for those nine and a half years, or if it was turned off and only checked out occasionally until 2015 came? From an engineering perspective, the latter course is more likely to succeed, and this is the concept behind hibernating the spacecraft, the next big task for the New Horizons team after Jupiter.

  Hibernation is one of the pioneering and truly innovative aspects of New Horizons. The idea behind it is to save wear and tear on most spacecraft systems by turning them off for most of each year en route from Jupiter to Pluto. Thanks to hibernation, by the time New Horizons got to Pluto it would be 9.5 years old on the clock, but most of its primary systems would only have about 3.5 years of “on time” clocked against them. Those spacecraft systems would, in effect, be many years “younger” than their actual age, less worn out, and more likely to be operating at peak performance at the Pluto flyby.

  The software designed to hibernate New Horizons was written before launch but it had been tested for only a few days when the spacecraft was at Goddard. Because the flight hibernation periods would typically last for months on end, the plan was to ease into it carefully, to make sure the spacecraft didn’t get into any trouble while being left in this hibernating state for longer and longer periods.

  The first test was for only one week in the summer of 2007. When the craft came out of hibernation it sent all the resulting, stored telemetry down to Earth for engineers to evaluate how it went, and everything looked good. So next the spacecraft was commanded to hibernate for a few weeks. Then, after the spacecraft engineers looked at that data and saw that longer test also had gone well, they tried hibernating for ten weeks, then for four months. As the team became more and more confident
, they increased the hibernations to as much as seven months at a time.

  Each time the spacecraft hibernated, from 2007 to 2014, the mission team could turn from babysitting their bird to focus on the massive task of Pluto flyby planning, which we will describe in the next chapter. Between hibernation periods, the spacecraft would be woken up to be thoroughly checked out, to conduct instrument calibrations and en route science observations, and, from time to time, receive software upgrades to correct bugs or add new capabilities. One key upgrade allowed the SDC, SWAP, and PEPSSI instruments to remain on, taking data while in hibernation so they could trace out the solar system’s dust and charged-particle environment all the way to the Kuiper Belt, a scientific bonanza en route to Pluto.

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  BATTLE PLAN PLUTO

  PLANNING THE INVASION

  Early in 2008, project manager Glen Fountain laid out a Pluto flyby planning schedule that would unfold over six years and involve virtually every person on the New Horizons flight team. Most of the flyby planning was slated to take place during lulls in mission operations, when the spacecraft was in hibernation and needed little tending.

  The undertaking was massive for such a small team. First, a series of technical design studies of the flyby’s specifics were needed for the science and mission operations teams to be ready to work on detailed objectives for the flyby. All of that was needed before the comprehensive design of hundreds of flyby observations of Pluto and its moons by the seven scientific instruments aboard New Horizons could be made. This in turn would have to be followed by exhaustive testing of all flyby operations, and an enormous effort to generate procedures to handle hundreds of possible malfunctions the spacecraft and its ground team had to be ready for. That in turn was then followed by flyby simulations and rigorous project and NASA technical reviews to inspect, critique, and insert recommendations to improve every aspect of the flyby plan.

  It’s interesting to compare the high-tech, computer-drenched twenty-first-century flyby planning effort by New Horizons, with the comparatively stone age mainframe computer–era planning for Voyager flybys back in the 1970s and 1980s. The Voyager team had just a few years to plan each giant-planet flyby, but their team was huge. Each Voyager flyby involved almost five hundred people and took most of three years to plan. New Horizons would take six years to plan its Pluto flyby, but would involve a crew of just fifty people—barely 10 percent the staffing level of Voyager.

  The Pluto flyby planning czar was Leslie Young, who had undertaken a skeletal version of Pluto flyby planning as a part of the New Horizons proposal to NASA in 2001. Having seen her impressive flyby planning work, drive, and attention to detail on the proposal, Alan asked her in 2008 to lead the planning of the real thing.

  When New Horizons was proposed, Alan had organized the science team into crosscutting “discipline” theme teams, focusing on different aspects of Pluto system science. This prevented the usual balkanization into instrument teams that had caused many other planetary missions to develop warring instrument camps. The four New Horizons theme teams were: the Geology and Geophysics team (applying the techniques and insights of the geology of Earth and other planets toward understanding the structures and motions of Pluto’s surface, and interior, as well as its satellites), led by theme team leader Jeff Moore of NASA’s Ames Research Center; the Composition team (determining what the surfaces of Pluto and its satellites are made of), with theme team leader Will Grundy of Lowell Observatory; the Atmospheres team (focusing on measurements of Pluto’s atmosphere), with theme team leader Randy Gladstone of SwRI; and the Plasma and Particles team (understanding the way that Pluto and its moons interact with the solar wind and what the ionized gases escaping from Pluto’s atmosphere are made of), which was led by Fran Bagenal of the University of Colorado. All four groups reported to Leslie for flyby planning and comprised what was dubbed the overall Pluto Encounter Planning (or PEP) team (sometimes jokingly called the “PEP squad”).

  Several other New Horizons teams were also key to the flyby planning: mission operations planning was led by Alice Bowman; mission design and navigation was led by Mark Holdridge; and spacecraft engineering was led by Chris Hersman. A big part of the spacecraft engineering team’s work involved making sure that no onboard resource, like fuel or power or data storage, ever exceeded its safe limit. As project manager, Glen Fountain herded all these cats to stay on schedule and on budget. Alan wore a few different hats during the encounter planning. He led the Alice and Ralph instruments as their instrument PI; he also sat on Leslie’s PEP executive board (which also included Cathy Olkin and John Spencer); and as mission PI he was the ultimate reviewer, critiquer, and approver of all aspects of flyby planning, contingency planning, and team training.

  FINDING JUST THE RIGHT DISTANCE AND TIME FOR A FLYBY

  Before the New Horizons team could put any kind of detailed flyby plan together, there were two big decisions to make: exactly when the spacecraft should fly through the Pluto system and at exactly what distance. So beginning early in 2008, Leslie captained detailed studies to pick the best day and best altitude to fly by Pluto.

  There was enough fuel aboard New Horizons to change the date of the flyby by up to a few weeks from its nominal date in mid-July of 2015, and Alan wanted to get the most out of the flyby by finding the absolute most optimal date. So Leslie and the PEP team evaluated every possible factor, from which parts of Pluto the spacecraft would fly over on each given day (as Pluto rotates on its axis once every 6.4 Earth days), to the distances of every moon from New Horizons on each possible flyby day, to which radio tracking stations would be in position on Earth to perform the radio science experiments to measure Pluto’s atmospheric pressure and radar reflectivity. In all, Leslie’s arrival date study looked at more than a dozen factors that varied across each day in late June and all of July in 2015 to choose the best possible arrival day. No trajectory would be perfect for every factor, so deciding among them involved a complex series of trade-offs. In the end, Leslie’s team recommended, and Alan approved, July 14, in part because on that date the spacecraft would fly over Pluto’s brightest terrain spot (which also was known to have an unusual composition). But July 14 also gave the best combination of satellite science, and it cost the least fuel—because that was the date they had originally targeted at Jupiter flyby, which was a bonus because Alan also wanted to save fuel for flybys of bodies farther out in the Kuiper Belt after Pluto.

  After settling on July 14 as the encounter date, Leslie’s team then began looking at flyby closest-approach distances. The most important science observations would be of Pluto itself, so the closest-approach distance to Pluto was key; but the distance to each satellite was also important. The four science discipline theme teams began by weighing how well each of their detailed Pluto scientific objectives would be accomplished for a range of possible Pluto closest-approach distances from about three thousand to twenty thousand kilometers, with corresponding ranges for each satellite of about twenty-eight thousand to almost eighty thousand kilometers. Going really close helped the plasma instruments detect more phenomena, but created problems for the cameras. (You might think the camera teams would want the closest view possible, but at a flyby velocity of over 30,000 miles per hour, going too close meant that images from too close would be smeared by the spacecraft’s blazing speed.) Dozens of factors were considered. In the end, flying by at a distance of 7,800 miles, deep inside the orbits of all of Pluto’s moons, was selected as the best overall way to satisfy the competing desires of the four science discipline teams.

  Calculations showed that for the 7,800-mile closest approach flyby to work—for all the images to be properly centered on their targets—the spacecraft had to arrive no more than nine minutes off target after its 9.5-year journey, equivalent to a cross-country airline flight from Los Angeles to New York landing within four milliseconds of its planned time! New Horizons also had to reach the Pluto aim point at closest approach no more than about 60 miles off cou
rse after the 3-billion-mile journey from Earth; that 60 miles is about the size of metropolitan Washington, DC, and the spacecraft had to hit this target in a journey traveling all the way from Earth to Pluto. This was the equivalent of hitting a golf ball from L.A. to New York and landing it in a target the size of a soup can! That made for a tall order.

  FLYBY PLANNING BEGINS

  To begin planning the multi-month flyby itself, the encounter was broken into stages. It would begin in January of 2015, still six months and almost 200 million miles from Pluto, with what was dubbed Approach Phase 1, or AP1. AP1 was primarily needed to gather navigation imagery to home in on Pluto, but it also included measurements of the environment that Pluto orbits in using the SWAP and PEPSSI plasma instruments and the Student Dust Counter. This far out, Pluto would just be a dot in the distance. With the start of AP2 (Approach Phase 2) in April of 2015, the distance to Pluto would be cut in half compared to the start of AP1. So in addition to continuing the same kinds of activities as in AP1, the spacecraft would then be able to start to see Pluto as well as Hubble could see it from Earth. After that, images would get progressively better as each week went by—so the first scientifically useful Pluto observations were planned for AP2.

  AP3 would begin much closer to Pluto—in mid-June, and it would last only three weeks but would include an intensive imaging campaign of Pluto and its moons as they circled the planet; it would also include the first compositional observations of Pluto and Charon by New Horizons, and intensive searches for new moons and even rings. After AP3, the so-called Core of the encounter would then begin, just seven days before flyby closest approach. Core would last until two days after closest approach. The Core flyby would then be followed by three DP’s, or departure phases, continuing until October of 2015.