New Horizons' moment of truth
A crowd of several hundred people were having a very good time, cheering and clapping and gaily waving little American flags. They had gathered at the Johns Hopkins Applied Physics Laboratory in Laurel, Maryland, early on the morning of July 14 to witness an exceptional scientific accomplishment. After an epic voyage of nearly 10 years and more than 3 billion miles, New Horizons, a robotic spacecraft the size of a Steinway concert piano, had reached its destination—Pluto and its system of (at least) five moons. The exuberant audience counted down the last 10 seconds to 7:50 Eastern Daylight Time, the instant when New Horizons was supposed to fly closest to Pluto's surface. Wound-up space bloggers and science journalists and video personalities from around the world broadcast the same bulletin: New Horizons had done it.
Alice Bowman watched the celebration with a characteristic smile that did not reveal how tense and tired she had to be after another night without sleep. Bowman is the New Horizons mission operations manager. She is extraordinarily popular with the mission team, some of whom seem to enjoy calling her by her job's acronym, MOM. As the cheering continued, Bowman knew something that most of those in the excited crowd did not: Their celebration was paradoxically a bit behind actual events yet possibly premature. She had the insider's knowledge that New Horizons had sailed past Pluto 72 seconds before its 7:50 a.m. EDT target time—if it had sailed past Pluto at all. The media event at APL notwithstanding, at the time no one knew if the spacecraft had survived its encounter with the Pluto system. Because New Horizons could not point its instruments at Pluto and its antenna at Earth simultaneously, neither Bowman nor anyone else had heard from it in eight hours, and nearly 13 more hours would elapse before mission control got telemetry from the craft confirming that it had not smashed into a piece of Plutonian debris. That it had not inexplicably shut itself down. That its instruments had gathered scientific data. The scientists and engineers had every reason to believe they had succeeded. But for all their faith, they could not be certain that 15 years of work and $720 million of taxpayers' money had not become a piece of high-velocity space junk. And they wouldn't be certain until a huge antenna from NASA's Deep Space Network locked on to the spacecraft's signal later that night.
Before she'd left for a 7 a.m. appearance on NASA television and a subsequent press conference, Bowman had deflated the air mattress laid out on the floor of her office. After the press conference, she reinflated it and took a half-hour nap. Then, she checked the sequence of commands sent to New Horizons "once, twice, three times, a hundred times," she says. "And then I sort of stressed. I sort of freaked out waiting for that signal to come in." Thirteen hours can be a long, long time to wait for a spacecraft to radio answers to the questions: Did it get there? Did it work? The world knows the answers now. But in July Bowman and the rest of the New Horizons team were working in Pluto Standard Time, where a radio transmission at lightspeed took four and a half hours from spacecraft to Earth, where events were celebrated half-a-day before anyone was certain nothing disastrous had happened, where success required pinpoint timing after 3,463 days of flight toward a moving target.In 2001, NASA issued a challenge to the nation's space contractors: come up with a lean, inexpensive mission to Pluto that could be bolted together in time to launch in about five years. Stamatios Krimigis, then head of APL's Space Department and a man who had already been part of missions to explore every other planet in the solar system, thought the lab could pull off such a challenge, and he knew whom he wanted as principal investigator—a planetary scientist named Alan Stern. Employed by the private Southwest Research Institute in Boulder, Colorado, Stern could not remember a time when he had wanted to do anything other than study outer space. (His father told Science magazine that the first word spoken by his infant son was "moon.") He had been talking up a Pluto mission as far back as 1989, and after Krimigis' call he seized the chance to partner with APL.
He and an APL team designed and built a triangular platform, which they referred to as the bus, to ferry seven scientific instruments across the solar system. If they survived the rigors of interplanetary space, the instruments would take pictures of the planet—or dwarf planet, a sore spot with some New Horizons scientists—and its largest moon, Charon, and pursue about 30 science objectives, including detailed photographic mapping, analyzing the surfaces and atmospheres of the pair (Pluto and Charon are so similar in size, scientists consider them a double-planet system), and measuring the solar wind in that part of the cosmos. Almost all the crucial observations would be made during a nine-day flyby, and there would be only one chance to get it right. All the instruments had to work, the onboard computer processor had to work, the tiny plutonium reactor that powered the craft had to work, and the communications system had to work—after nine and a half years traversing 3 billion miles of freezing, irradiated vacuum. What could go wrong?
The New Horizons team pondered that question a lot. They imagined every possible problem, from glitches to disasters, and created dozens of contingency plans. After New Horizons was well on its way, the Hubble Space Telescope began finding new moons in the Pluto system, four in all. Hal Weaver, mission project scientist at APL, recalls, "We thought it was really cool when they discovered one more moon after Charon. Then when they discovered another one shortly after that, pretty cool. When they got to the third, everyone was kind of, 'Oh no, are we going to hit something?'" By the fourth new moon, scientists began to worry that the spacecraft might encounter a debris field, even a ring that could not be observed from Earth. The spacecraft would be barreling along at about 33,000 miles per hour, the fastest man-made object in history, and at that speed hitting something no bigger than a bead could be ruinous. So for seven weeks in 2011 the team used LORRI, the spacecraft's long-range camera, to search for other moons or smaller debris that Hubble might have missed. The science and mission teams created alternate flight paths through the Pluto system that would not be optimal for science but might preserve the craft if mission ops learned there were too many hazards on the planned course.
There were other concerns. Any computer network can be hacked, and the team had to consider the possibility of malicious vandals attempting to take the mission control center down. APL set up independent security teams in two separate buildings to guard against hackers. Natural disasters—tornadoes, hurricanes, long-term power outages—had to be planned for. So the primary Mission Operations Center at APL had a mirror MOC across the road from the main campus, a backup that could take over if the primary MOC were to go down. The mission team also set up a remote MOC at the California Institute of Technology's Jet Propulsion Laboratory. Were a natural disaster to threaten the APL campus and close nearby airports, the plan was for a team of three flight controllers to jump into a van with two drivers who were on 24-hour standby and head for JPL. "We figured that with two drivers around the clock, they could get there in three days," Bowman says.
The team rehearsed everything over and over and over. Stern even had them practice writing press releases under tight deadline. He knew what kind of media coverage they would face in July 2015, and he was intent on maximizing the opportunity to tell the world of their accomplishment and, he hoped, re-excite people about space exploration and frontier science. That is, if New Horizons got there and the instruments worked.
The mission's navigation team was tasked with putting the spacecraft where it had to be to conduct its scientific mission. That meant hitting both a temporal and a spatial target: Within 150 seconds of 7:50 a.m. EDT on July 14, 2015, New Horizons had to arrive at a point in space within a margin of error—inside a box 100 kilometers by 150 kilometers. Fifteen thousand square kilometers may sound like a pretty big bull's-eye, but before New Horizons launched, Andy Cheng, one of the lead scientists on the project, likened the challenge to firing an arrow 100 miles and hitting a dime. Were New Horizons to swing too low over Pluto's surface, the rapid changing of angular perspective would ruin the images, "smear the data" in the scientists' parlance. Arrive too soon or too late and the preloaded, precisely timed command sequence for the observations might point the spacecraft to empty space instead of toward Pluto. Furthermore, the scientists wanted to take advantage of an occultation—Pluto and Charon both passing between the spacecraft and the sun, affording a unique opportunity to study their atmospheres. All of this meant putting New Horizons in just the right place at just the right time.
There was a unique complication to the navigation team's work. When New Horizons lifted off the launch pad in 2006, the nav team was not all that sure where Pluto would be in 10 years. That is, not with the precision they needed to conduct the mission's science. Pluto is so far away, even Hubble could only be approximate in pinpointing the planet's location at any given time. Estimates of where Pluto would be when New Horizons reached its cosmic neighborhood were made imprecise by the fact that from Pluto's discovery in 1930, astronomers had been able to observe only about one-third of its elliptical 248-year orbit, which made it harder to plot where that orbit would place the planet at the time of the flyby.
At launch, the team estimated the "axis of greatest uncertainty" at around 800 to 10,000 kilometers. For the mission to succeed, they could not be off by that much. So as the flight proceeded, two separate navigation teams, using different software, monitored the data and projected where New Horizons was headed. As anticipated, the two software packages produced different answers, and by comparing the projections and resolving those differences, mission ops could plot the best course. Every day Mark Holdridge, the encounter mission manager, would say, "Where are we, guys?" He adds, "They'd put their points up on the plot. It was kind of interesting because they started quite a bit apart, but as the mission progressed they started to come together. Within the last two days they were within one second and 10 kilometers of the aim point."
Month by month, as the operations team got more and better data about where Pluto was and where it was headed, they fired the spacecraft's thrusters in a series of course corrections and speed adjustments. The first occurred in January 2006, a 4:36 burn of the thrusters to change New Horizons' velocity by about 5 meters per second. By July 2010, the nav team had figured out that the craft needed another correction, and for a remarkable reason: Its power source, the little plutonium reactor, emitted thermal photons, and those photons bounced off the back side of New Horizons' high-gain antenna with enough force to hold back the craft and require a 35.6- second burn to speed it up by about 1 mph.
The final approach had begun by January 25, 2015, when LORRI, the long-range camera, made some optical observations to re-evaluate the trajectory. The navigation teams ran the numbers and concluded that now New Horizons was going too fast—they would be off-target by several thousand kilometers and would arrive about 13 and a half minutes too soon. So they put on the brakes, firing the thrusters in mid-March to slow the craft by about 1.5 meters per second.
On the morning of the big July 14 celebration at APL, Holdridge already knew the spacecraft would arrive about 70 seconds before 7:50. "I was at my house watching," he says, grinning, "and I applauded at the right time."
To get to its destination by 2015, New Horizons needed to fly close enough to Jupiter for that planet's gravity to fling it toward Pluto at much greater speed. (Without that gravity boost, New Horizons would not arrive until 2018.) The Jovian flyby took place in February 2007 and gave scientists an opportunity to power up the instruments and make some observations. This was not only to conduct some planetary science but to test how the instruments performed after a year in space. They performed flawlessly—in one respect better than flawlessly: The ultraviolet spectrometer, named ALICE, turned out to be a far more sensitive particle detector than the scientists had expected. It was supposed to pick up ultraviolet photons, but it turned out to be sensitive to energetic electrons, as well. Four years later, mission ops put the spacecraft through a nine-day simulation of the Pluto flyby, running the full command sequence, testing navigation, guidance, control, the scientific instruments, all the craft's capabilities. Once more, all appeared to be in great shape, which made everyone happy. "Engineers like it to be boring, everything following the prescribed plan," Weaver says. "No drama."
But drama came only a month later in March 2007. Mission ops was monitoring the craft, getting telemetry, watching the MOC's computer monitors as the data scrolled by colored green—situation normal. Space missions must reserve time on the Deep Space Network antennae, and on this day everything seemed so routine and nominal, the New Horizons crew offered their last 20 minutes on the dish to the Voyager mission. Voyager declined, which was a good thing because suddenly the telemetry numbers on the MOC screens began to turn red, indicating "rule firings," a sequence of actions taken by the spacecraft's autonomy system when it senses something is wrong. The ops crew monitoring the mission recognized this set of firings—New Horizons was resetting its computer for some reason and going into safe mode, which meant it was pointing its antenna toward Earth and shutting down to await further instruction.
Ground control got the craft back to normal functioning, but this would not be the last time New Horizons' main processor reset itself and put the spacecraft into safe mode, requiring mission ops to restart it. Engineers eventually figured out that some of the resets were the result of a bug in the operating system, which was corrected by a new upload in January 2013. Some of the other resets, which continued until days before the Pluto encounter, still mystify them. Some were probably caused by radiation. Neutron stars and other celestial bodies emit high-energy charged particles that bombard anything in their path, and at various times some of these particles zipped through New Horizons. If they happened to hit the onboard processors in the right spots, they could actually change bits—change 0s to 1s or 1s to 0s. If a bit reset in only one place, an error-detection system on the spacecraft could correct it, but if two bits reset at the same time, the autonomy system would shut down the craft and signal mission ops that something was awry.
Alan Stern has a vivid memory of another episode, this one in 2010. For most of the eight years from Jupiter to Pluto, New Horizons was deliberately powered down in hibernation mode, to preserve its power plant and its instruments. Every Monday, the craft would send telemetry to Earth, a tiny health report that let mission ops know it was still asleep but OK. "One Monday, I got a call from Glen Fountain," Stern recalls. Fountain is New Horizons' project manager. "I wasn't able to take the call because I was at the podium of a big scientific meeting in front of several hundred people. I did manage to put on my little earpiece and listen to his message, and he'd said, 'We didn't hear from the spacecraft.' That had never happened before. I was stuck at that podium for an hour and a half, unable to return the call and get any details. That was a very hard hour and a half to sit through." As soon as he could, Stern raced from the room to call Fountain. "As it turned out, the DSN tracking antenna had pointed to the wrong place. They'd used the wrong list of positions for that day. The spacecraft had reported just fine but we didn't get anything because we had the antenna pointed the wrong way."
On July 4, New Horizons had begun its close observations of the Pluto system when some of the mission principals took a few hours off to have holiday lunch with their families. The craft was doing its work when, at 1:54 p.m., a computer monitor in the missions operations center suddenly displayed OUT OF LOCK. That meant the Deep Space Network station had lost contact with the spacecraft. The line had gone dead.
Bowman by now had developed a sixth sense for whether a problem was in the DSN receiving station or on the spacecraft, and she was pretty sure this one was on the craft. She called in Holdridge, and Gabe Rogers, the systems guidance and control engineer, and Christopher Hersman, the mission systems engineer. She called in Fountain, the project manager, who found himself wondering if New Horizons had hit something, even though they had finally concluded that the risk of that was only one or two in 10,000. Stern raced in. If the craft had shut down its main processor for some reason and now was operating on the backup processor, it would be transmitting at an emergency rate and on a different frequency and signal polarity. So mission ops instructed the DSN dish in Canberra, Australia, to reconfigure and search with new radio frequency parameters. Stern put out word on Twitter: "New Horizons in safe mode. We're working it, folks."
At 3:11 p.m., the word everyone wanted to see flashed on the monitors: LOCKED. Canberra had locked on the signal from New Horizons' backup processor. The spacecraft was intact, at least. By 4 p.m., the mission's Anomaly Review Board had convened to be briefed on what had transpired and to discuss the best way forward. Midway through the spacecraft's recovery, they determined there was no fault in the hardware or software. But there had been a conflict when the spacecraft tried to commit to flash memory the complete command sequence for the nine days of the flyby, which it had just received from Earth, while at the same time compressing science data that had been gathered by its instruments. All that simultaneous activity had triggered an overload of the main computer, prompting the autonomy software to switch to the backup processor, point New Horizons toward Earth, and put the craft into safe mode. An estimated 30 observations were lost. But a much bigger concern was getting the craft out of safe mode and the main processor back online in time to begin the command sequence crucial to the flyby. That had to be done by July 7. Bowman did not go home for two days, getting by with naps on the floor of her office. Engineering and mission ops sorted out the processor and got the spacecraft back to normal operations with only four hours to spare. It turned out to be the most harrowing episode of the mission.
Nearly 13 hours after the morning's flyby celebration, Bowman and mission ops team member Karl Whittenburg intently watched two monitors and a laptop in the MOC, and hundreds of people in an APL auditorium watched them on a live video feed. This would be the first moment of truth, when mission ops learned whether the Deep Space Network ground station in Madrid had received a signal from New Horizons that told them the spacecraft was functioning. Whittenburg appeared to take a deep breath as Bowman listened to something over her communications headset, then said, "In lock with signal." Seconds later, another team member behind them pumped his fist as Whittenburg leaned back and smiled and Bowman announced, "Copy that. We're in lock with telemetry with the spacecraft." The room erupted with cheers and handshakes, and in the auditorium the crowd delivered a standing, flag-waving, 55-second ovation. New Horizons was alive.
Now Bowman waited for word that all the various components of the craft were healthy and had done their jobs. "Subsystems, please report your status as you get enough data," she instructed. On the video feed, viewers saw her exhale, releasing tension, then heard a voice off-camera, "MOM, this is RF on Pluto One." (RF is "radio frequency.") "Go ahead, RF," Bowman responded. "RF is reporting nominal carrier power, nominal signal-to-noise ratio for the telemetry. RF is nominal." Then came another report: "MOM, Autonomy is very happy to report nominal status. No rules have fired." That brought a big smile to Bowman's face and more cheering, as did news from Command and Data Handling that New Horizons' solid-state recorders contained the anticipated amount of data. One after another, the subsystems checked in. "MOM, this is Propulsion." "MOM, this is Power." "MOM, this is Thermal." Then, Bowman stood up, turned to face Stern, the principal investigator, who was waiting just outside the MOC and watching through the door. "P.I., MOM on Pluto One. We have a healthy spacecraft. We've recorded data from the Pluto system and we're outbound from Pluto." An exultant Stern strode into the MOC, arms raised in triumph, and crushed Bowman in a hug. Not long after, the mission principals entered the auditorium for a press conference. As they were introduced one by one, Stern got a huge ovation and Bowman looked teary-eyed when APL staffers in the audience began to chant her name.
By the next day's press conference, Stern and some of the mission scientists already had news. They had found evidence on Charon of recent geological activity, and the moon appeared to be riven by a canyon four to six miles deep. Some areas of Pluto's surface showed a remarkable lack of impact craters, indicating recent geological activity there, too. (Activity like volcanism covers old craters with lava flows, for example.) Pluto's atmosphere appeared to extend tens of thousands of miles behind the planet, driven by the solar wind. And one of the images revealed mountains of water ice on the surface, estimated to be 11,000 feet high. Asked by a reporter how it felt to get such remarkable early results, Stern deadpanned, "It feels terrible. There's a lot of depression in the science team. We're all thinking about catching flights out." After the laughter subsided he said, "I don't think any of us could imagine it would be such a toy store."
It will take the next 14 months for New Horizons to empty its recorders of all the scientific data it has collected, all the images and readings and measurements, and beam them to Earth in a stream of 0s and 1s traversing billions of miles of space and striking DSN dishes in Spain, Australia, and California. The scientists have years of work ahead of them, trying to make sense of it all. At the press conference, one look at their faces told you they could not wait to get started.