Chasing New Horizons

by Alan Stern

  A second engineering challenge RTGs pose to spacecraft designers is that the radiation the plutonium produces is bad for spacecraft electronics. As a result, all of the systems aboard the spacecraft had to be designed and tested to withstand this radiation, as had been done for previous RTG missions, like Voyager, Galileo, and Cassini. That in turn added complexities and cost to the development of the spacecraft, but there was no choice: without an RTG there would be no way to power New Horizons when it was far from the Sun.

  EYES, EARS, AND EVEN A SENSE OF SMELL TO STUDY PLUTO

  As the principal investigator, Alan was in charge of all aspects of the New Horizons project. All the teams of scientists and engineers, as well as others such as public relations and project management, ultimately reported to him.

  One key support role requiring both expertise and diplomacy was the project scientist position. Every NASA science mission has such a position, and larger missions like New Horizons also have deputy project scientists. These scientists are responsible to the PI for representing and negotiating the interests of the PI and the larger science team in the seemingly endless day-to-day stream of meetings that are needed to coordinate a complex space mission.

  The New Horizons project scientist position was filled by a highly talented, affable, and diplomatic planetary scientist named Hal Weaver. He was selected because he is extremely broad scientifically, and conversant in all the different sublanguages of all the subspecialties that make up the New Horizons science—from geology to surface chemistry, and from atmospheric science to plasma physics. Hal was also selected in part because he was a seasoned experimentalist who understood the engineering and operations of the many kinds of scientific instrumentation used on New Horizons.

  Hal had not been part of the original Pluto Underground. His background was instead primarily in the study of comets. But Hal had long been fascinated with the Kuiper Belt as the region of the solar system where certain comets originate. He had known Alan since the 1980s and they had collaborated together on some research. Hal describes his role as project scientist this way:

  The role is to be the PI’s right-hand person on the project, to keep the PI informed about what’s happening on the ground, and as design or test or other issues get raised, to be the scientific voice there to help the engineers figure it out. My job was to be the scientist who knows the engineering too, so that when the engineers tell me, “This is really hard,” or “This is going to cost a lot to implement,” I can help them find a design that not only meets the scientific objectives but also meets the project’s mass, power, cost, and schedule needs.

  When humans send scientific sensors to distant worlds, we’re launching proxies for our eyes and our other senses. This is most obviously true with cameras, which allow us to “see” landscapes where no human eye has actually been. But it is also true with other kinds of instruments, which allow us to “listen” to the vibrations of distant magnetic fields and “sniff out” the gases in an alien atmosphere, to discover what these landscapes are made of, what lies beneath their surfaces, and what hidden forces, flows, and fields can tell us about the history and nature of other worlds.

  A key design decision that was made early on New Horizons was that the spacecraft would not include a “scan platform,” a movable turntable that points the cameras and other instruments in different directions without having to move the entire spacecraft. Although platforms increase the flexibility of the observations that can be done during a flyby (e.g., allowing the cameras to point at the planet while the antenna is pointed in a different direction back to Earth), they add weight, complexity, and cost. Voyager and other high-budget outer planet missions used scan platforms, but at one-fifth the cost of Voyager, New Horizons simply could not afford this luxury. Not having a scan platform in turn meant that all the instruments aboard New Horizons would be “body mounted” on the spacecraft, so pointing instruments to view their targets would mean pointing the entire craft for each observation.

  Everyone involved in designing New Horizons knew that, unlike the first flybys of Venus and Mars and Jupiter, there were no plans to follow up its exploration of Pluto with orbiters or landers. The data they would collect would need to suffice for the foreseeable future as the complete body of knowledge about Pluto and its moons.

  But because New Horizons was built in the 2000s, technology had greatly advanced and it was able to bring advanced capabilities that the Mariner and Voyager teams of the twentieth century had not been able to include: new sensors, much faster data-collection capabilities, and much greater instrument sensitivity. All seven instruments New Horizons carried were more advanced than anything that had been brought to bear on a previous first planetary flyby. Next we’ll describe the instruments that New Horizons carried to study the Pluto system.

  We’ll begin with the “Alice” ultraviolet spectrometer. Picture a spectrum of the wavelengths of light that our eyes can see, going from red to blue. But what’s bluer than blue? Ultraviolet. Those wavelengths, beyond what humans can see, reveal the composition of atmospheric gases. The details of the Alice instrument give some sense of how far instrument technology advanced from the days of Voyager to New Horizons. The Voyagers had also carried ultraviolet spectrometers, which had two pixels, or picture elements, so they could observe two separate ultraviolet wavelengths at once. This meant that building up a useful spectral map was a slow and time-consuming process of sweeping those two pixels across all the necessary wavelengths needed to build up a spectrum, and then laboriously pointing the instrument boresight at location after location, repeating this process to map the spectrum across the disk of each flyby object. In contrast to Voyager’s old-style ultraviolet spectrometer, Alice contained thirty-two thousand pixels, so it could observe at 1,024 wavelengths at each of thirty-two adjacent locations—simultaneously, vastly speeding the process of obtaining ultraviolet data over what Voyager could accomplish.

  Closely married with Alice, is the “Ralph” instrument. Its name came from a silly joke referring to the old Honeymooners’ TV show characters, Ralph and Alice Kramden. Whereas Alice’s objective was primarily to study Pluto’s atmosphere, Ralph’s objective was to map and also determine the composition of Pluto’s surface. The size of a hat box, Ralph contains two black-and-white cameras, four color filter cameras, and an “infrared mapping spectrometer” to map surface compositions. Ralph can see colors that are redder than any red humans can see, at wavelengths which are called infrared, where minerals and ices have characteristic spectral features that can be used to reveal the surface materials at any given location in Ralph’s field of view. Ralph’s spectrometer splits infrared light up into 512 spectral channels from 1.25 to 2.5 microns. Again, a comparison to the historic standard for flyby exploration, Voyager, is illustrative. The Voyagers’ equivalent instrument, called “IRIS,” was about the same size as Ralph, but because it was designed and built with 1970s technology, it contained just one infrared pixel. By comparison, Ralph’s mapping spectrometer has sixty-four thousand pixels. So on the Voyagers, IRIS’s telescope had to be pointed at each place on a target body to get a spectrum of just that place, and then repointed to successive locations to slowly build up a spectral map. But Ralph obtains a spectrum at each of 64,000 locations all at the same time—blanketing a target to map all its locations simultaneously—something light-years ahead of what Voyager could do.

  Determining the temperature and pressure of Pluto’s atmosphere was another objective of New Horizons. To make these measurements, New Horizons carried REX, which stands for Radio EXperiment. REX was designed to function in essentially the opposite way from its more primitive counterpart on the Voyagers. The Voyager radio experiment worked by sending radio X-band (4-cm wavelength) waves through planetary atmospheres it flew by from the spacecraft toward Earth. These waves were picked up by groundbased antennae of NASA’s Deep Space Network. By measuring the ways in which that radio signal was altered by passing through the atmosphere of the vari
ous planets and moons on Voyager’s itinerary, scientists could determine temperatures and pressures in those atmospheres. Because Pluto’s atmospheric pressure was much lower, this technique wouldn’t work, so REX solved this problem by performing the experiment the other way around: the Deep Space Network would blast a far more powerful signal than any instrument aboard a spacecraft could—with tens of kilowatts of radio power. Then REX would receive and record the signals sent from Earth that had passed through Pluto’s atmosphere.

  In order to measure atmospheric temperature and pressure, REX compares the frequency of the radio waves passing through the atmosphere to those of a reference standard. The shift that it measures is proportional to the bending of those radio waves caused by their passage through the atmosphere it is studying, which can in turn be used to compute the atmosphere’s pressure and temperature. In addition to determining atmospheric pressures and temperatures, REX is also capable of measuring the temperature of surfaces it stares at.

  Capping off the “remote sensing” instruments on New Horizons—the sensors that observe Pluto and its moons through optical and radio telescopes—was LORRI, which stands for the LOng Range Reconnaissance Imager. LORRI is essentially a magnifying telescope feeding a megapixel camera. Unlike Ralph’s color and spectroscopic capabilities, LORRI takes only black-and-white images. But because its telescope is of a much higher magnification than the one on Ralph, LORRI’s images have much higher resolution and show much greater detail. LORRI’s higher resolution also allowed New Horizons to see features on Pluto and its moons from much farther away than with Ralph. As a result, beginning about ten weeks before the flyby, LORRI would be capable of seeing more details on Pluto than the Hubble Space Telescope ever could. With this capability, LORRI also allowed New Horizons to map all of Pluto—even the parts they didn’t fly directly over on flyby day. Remember: Pluto rotates slowly, taking 6.4 Earth days to turn around once on its axis. This means that as New Horizons was approaching Pluto, the last time it was only able to see the “far side”—the side not flown over at closest approach—is 3.2 days before the flyby. New Horizons would be millions of miles away when it got its last look at that other hemisphere, but with LORRI’s telescope, New Horizons would be able to get good imaging resolution on those terrains.

  The next two instruments that New Horizons carries are so-called plasma instruments. Plasma is the term planetary scientists use for electrically charged particles. This is the domain of planetary science that experts like Fran Bagenal and Ralph McNutt study, and it is the hardest to describe for non-experts because it involves things people don’t normally encounter in our daily lives. On Pluto, plasma is created as sunlight ionizes the gases in Pluto’s atmosphere. As a result, by studying that plasma it’s possible to determine the rate that Pluto’s atmosphere is escaping and what the escaping gases are made of.

  The two New Horizons instruments designed to study plasma are SWAP (Solar Wind Around Pluto) and PEPSSI (Pluto Energetic Particle Spectrometer Science Investigation). PEPSSI measures very high-energy (megavolt) charged particles. PEPSSI can reveal the composition of material leaking out of Pluto’s atmosphere. Is it carbon? Is it oxygen? Is it nitrogen? Is it something else? SWAP’s job is to measure the escape rate of Pluto’s atmosphere, and it does that in an interesting way. As Pluto’s atmosphere escapes into space, there is a place in front of Pluto at which the outgoing, escaping gas reaches a pressure balance with the Sun’s incoming solar wind, and there is sort of a standoff where the two flows are balanced. The faster an atmosphere is escaping, the farther out into space it reaches that point of pressure balance with the incoming solar wind. So finding how many thousands of kilometers from Pluto that occurs allows one to determine the escape rate of gas from Pluto’s atmosphere. That’s SWAP’s job.

  There is one final instrument on New Horizons. It is called the SDC, or Student Dust Counter. SDC counts the impacts of interplanetary dust particles (tiny meteoroids) onto its detector surfaces. Every time there’s an impact on the face of the dust counter, SDC creates a little voltage jump in the instrument that reveals how massive the impacting particle is. SDC’s job is to measure interplanetary dust at much greater distances from the Sun than any other dust impact detector sent into space. The farthest any dust detector had operated from the Sun before New Horizons was just shy of the distance of the orbit of Uranus, or about half of the way to Pluto. SDC would provide a continuous trace of the solar system’s dust density all the way out from Earth to well beyond Pluto.

  SDC was also the first instrument built by students to be carried on any planetary mission. Getting it approved was not easy but Alan felt strongly from the start of the project that students should have opportunities to participate, so he used the idea of an educational training opportunity to convince NASA to add SDC to New Horizons. Today, thanks to that pioneering role of New Horizons in carrying a student-built instrument, most NASA planetary missions carry a student instrument, and they are now considered valuable tools for training the next generation of planetary explorers.

  ALLIGATORS IN THE WATER

  Throughout 2002 and 2003, the New Horizons team raced to design, build, assemble, and test the spacecraft and its payload, to pass numerous NASA technical and cost reviews, to work toward nuclear launch approval, to build a mission control, and to undertake all the other processes that had to dovetail in order to be ready to launch by the critical Jupiter launch window in early 2006. To complete the spacecraft required building literally hundreds of separate components for the guidance system, the communications system, the propulsion system, the ground systems, all seven instruments, and more. Every aspect of the spacecraft had to pass its own tests and then be shown to work perfectly together.

  By early 2004, perhaps inevitably, there came a time of crisis as some components failed testing, and others fell behind schedule. This isn’t unusual for spacecraft projects, but New Horizons really didn’t have much wiggle room to make up for these problems with schedule adjustments because of its one-shot January 2006 launch window.

  About that same time, NASA’s New Horizons program office at the Marshall Space Flight Center in Huntsville, Alabama, assigned oversight of the project to a brash and talented young manager named Todd May. Todd, an engineer by training, came from the human-spaceflight world and had little prior experience in robotic spaceflight. Alan and Glen were skeptical that Todd would be able to be much help, given his lack of experience in the many skill sets required for robotic planetary exploration.

  Immediately on taking the job, Todd told Alan that he wanted to come and visit SwRI Boulder, SwRI San Antonio, APL in Maryland, and other sites where work was being done on New Horizons so that he could meet all the principals and get the lay of the land for each facet of the project’s status. Alan:

  I remember our first conversation. Todd said to me in his thick Alabama accent, “I want to come out and get to know you better, so I’m coming to Boulder to talk. Let’s pick some dates.” I didn’t know who Todd May really was, and I thought he was just going to be some NASA manager who pushes a lot of paper and files a lot of reports. I kind of figured, “We’re in trouble on half a dozen aspects of the project, and I really don’t have the time to babysit this guy, but he’s the boss NASA assigned; he’s already been to APL (where I’d met him briefly) so I’ll have to make time for his Boulder visit.” When Todd came to visit me, he also visited Ball Aerospace, which is also located in Boulder, where they were building the Ralph instrument. Then Todd made four or five more trips in rapid succession, visiting APL, then other key project participants, getting himself immersed in New Horizons. And it didn’t take him very long, maybe a month, before he realized just how tough some of our developing problem areas were.

  For his part, Todd May didn’t know anything about Alan Stern either, or New Horizons, when he was given the role of managing the mission for NASA. Todd:

  One of my early trips on New Horizons was to APL. There was a monthly review going on,
and that’s where I met Alan and Glen. I sat through the review, and then Alan gave a little science overview. When he did that he did what he always does: he started with a PowerPoint of the stamp of Pluto that says NOT YET EXPLORED. I’ve got to tell you, that hit me where it counts. That’s the kind of exploration that really gets me excited. The whole idea of learning some truth or making some discovery or going somewhere that we haven’t been—to me that’s just a big part of who I am. You could say he had me at “not yet explored.”

  That day, I saw a project risk chart that the APL folks put together. They had about five or six risks that were all pretty high likelihood and that could cause the mission to fail. I told the team, “You guys don’t look like you’re on a trajectory to be successful.”

  About a week later, I got a call from Mike Griffin, who at that time had just taken over from Tom Krimigis as head of the APL space department. And Mike started by telling me, “You’re saying to people that we aren’t on a path to success. Are you trying to get this mission canceled? Are you trying to get me fired?” I said, “No, sir. I’m trying to make you successful, but I’m telling you, you’ve got a bunch of development risks, and I’m worried about that.”

  Todd was truly concerned, and called for a deeper review of the project—a look at every aspect of every spacecraft subsystem, every one of the seven instruments, the flight software, the ground system (mission control), the launch vehicle, the RTG, and the nuclear launch approval. He wanted to find out for himself where all the project areas of concern were, independent of what the SwRI and APL teams had identified. So he formed a team of experts in each area, who literally spent hundreds of hours on deep-dive cost, schedule, and technical reviews. In what Alan called a ninety-day-long “proctology exam,” Todd determined that most things were on track, but that there were deep problem areas as well.