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Space Topics: New HorizonsThe Road to New Horizons:
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New Horizons Artist's conception of the New Horizons spacecraft at Pluto and Charon. Credit: JHUAPL / SwRI |
The decision was not unreasonable at the time: despite almost 50 years of observations, very little was known about our ninth planet at the time. It was a very small, icy world with a strange tilted orbit and a single moon, discovered only one year before. By contrast, Saturn, Uranus, and Neptune were giant complex worlds, major players in our planetary system. Furthermore, although Saturn had already been visited by Pioneer 10 and 11, Uranus and Neptune had never been explored. And so it was that Pluto remained the only planet in the solar system that had never been visited by a man-made craft.
In the decade that followed the Voyager decision, knowledge of Pluto and its environs grew by leaps and bounds. Planetary scientists came gradually to believe that the ninth planet was not really an aberration of our orderly solar system. Computer models of the evolution of the solar system seemed to suggest that Pluto and its moon Charon are likely survivors of a dense population of large objects that inhabited that region in the early days of the solar system. Out there in the cold farthest reaches of the solar system, these objects had remained largely unchanged for the past 4.5 billion years. The models also suggested that it was highly probable that Pluto was not the lone survivor from that age, but that there are many more similar objects orbiting the Sun at a comparable distance. All this coincided nicely with the view advanced in the early 1950s by astronomer Gerard P. Kuiper, that the orbits of short-period comets point to the existence of a band of space-rocks beyond the orbit of Neptune.
When, in 1992, the first trans-Neptunian object was finally imaged by astronomers David Jewitt and Jane Luu with the 2.2 meter telescope at Mauna Kea in Hawaii, it confirmed what many astronomers have come to believe: far beyond the orbit of Neptune, a large number of space rocks left over from the formation of the solar system orbit the Sun at a similar distance to that of Pluto. Collectively, they became known as the "Kuiper belt." Pluto, far from being an oddball, was in fact a representative of the most numerous class of object in the solar system: the "Kuiper belt objects" (KBOs).
Voyager Artist's conception of the Voyager spacecraft. The twin voyagers completed a grand tour of the outer planets with the exception of Pluto. Credit: NASA / JPL |
As scientists were reevaluating the status of Pluto in the solar system, another event took place that contributed enormously to the rising interest in the ninth planet. In the summer of 1989, Voyager 2 arrived at Neptune for the last encounter of its grand tour of the solar system. In addition to providing spectacular views of Neptune's deep blue atmosphere, Voyager 2 also surveyed Neptune's only large moon -- Triton. There it found a world that in size, appearance, and composition appeared remarkably like Pluto. Triton turned out to be a complex and volcanically active world, and its retrograde motion around its planet seemed to suggest that it might itself be a captured Kuiper belt object. Could it be that Pluto sported a similarly complex environment? And how was it similar or different from that of Triton? Scientists wanted to know, and there was no way to find out without going to Pluto.
II. The Pluto Underground
It was around this time, a few months before Voyager's August 1989 Neptune encounter, that a group of mostly young planetary scientists came together at an Italian restaurant in downtown Baltimore, Maryland. The group, which included Fran Bagenal, Rick Binzel, Marc Buie, Bob Marcialis, Bill McKinnon, Ralph McNutt, Bob Millis, Alan Stern, Ed Tedesco, Larry Trafton, Larry Wasserman, and Roger Yelle, was discussing how to motivate NASA to send a mission to Pluto. The group, which continued working informally together in the following years and whose membership fluctuated considerably, called itself "The Pluto Underground."
The timing seemed inauspicious. The late 1980s and early 1990s witnessed economic recession in the US and shrinking budgets for space exploration. Furthermore, the missions favored by NASA at this time were large and expensive spacecraft -- such as the $2 billion bus-sized Cassini -- which were sent for in-depth investigation of previously-visited planets. The chances that NASA would change course and agree to send a low-budget reconnaissance mission to Pluto seemed slim, while the prospects for a large and expensive Pluto mission were practically nonexistent.
 Alan Stern
Alan Stern, a member of the Pluto Underground and Principal Investigator of New Horizons, seen here in an F-18 equipped for Vulcanoid research. |
Nevertheless, the first steps of the Pluto Underground's campaign proved successful. Within weeks of Voyager 2's fly-by of Triton, NASA's director of solar system exploration, Geoff Briggs, organized a study of a possible Pluto mission called "Pluto 350." The goal was to study whether a spacecraft weighing no more than 350 kilograms (770 pounds) would be able to carry sufficient scientific payload to conduct a worthwhile reconnaissance of the ninth planet. If one considers that the Saturn orbiter Cassini weighed in at 5,712 kilograms (12,593 pounds), the audacity of the notion of a 350-kilogram spacecraft becomes clear. In fact, the Huygens probe alone, which Cassini dropped into the atmosphere of Titan in January 2005, weighed 320 kilograms (700 pounds) -- close to the limit allowed by Briggs for the entire Pluto mission.
Over the next few years, the members of the Pluto Underground, with NASA funding and support, worked with JPL to design a workable mission within these strict mass limitations. The number of instruments was reduced to the bare essentials, and these in turn were made smaller and lighter. In the meantime Robert Farquhar of NASA's Goddard Space Flight Center, who headed the Pluto 350 study, was looking for ways to save the expenses involved in using one of NASA's heavy boosters to launch a spacecraft towards Pluto. He first noticed that instead of sending the spacecraft on a direct course, it was easier to launch towards Jupiter and use a gravity assist to accelerate the spacecraft towards Pluto. Such a procedure would require only a medium-sized launch vehicle in place of the heavy lifter needed for a direct launch, while the total travel time would be reduced considerably. Later on Farquhar refined his trajectory even further, proposing a launch towards Venus and using several Earth-Venus flybys to send the spacecraft towards Jupiter and then Pluto. While such a journey would take 19 years(!), it required the use of only a small booster and was therefore inexpensive.
III. A Fast FlyBy
While this was going on, two JPL engineers were quietly working on their own even more radical design for a Pluto mission. Knowing nothing of the Pluto Underground and its campaign, Robert Staehle and Stacy Weinstein were developing another Pluto spacecraft, which they named the Pluto Fast Flyby (PFF). When fully loaded with fuel and scientific payload, PFF would weigh a miniscule 140 kilograms (310 pounds) -- less than half of the Pluto 350 requirements. As a result, if it were launched by a heavy booster, it could make the journey to Pluto directly in as little as 7 years, compared to the 13 to 15 years estimated for Pluto 350 with its complicated trajectory. PFF had another advantage: being so small and inexpensive, it was planned as a two-spacecraft mission, with the each of the two spacecraft passing on opposite sides of Pluto, and thus mapping its entire surface.
A spacecraft as small as PFF required an astoundingly small, 7-kilogram (15-pound), science payload. The challenge facing PFF proponents was to convince NASA that it was possible to build such lightweight instruments and to pack enough of them onto the spacecraft to make the trip to Pluto worthwhile. Not everyone believed this was possible. When, in 1992, NASA's Outer Planets Science Working Group (OPSWG), chaired by Alan Stern, was required to choose between Pluto 350 and PFF, the debate was intense, and the vote was close. But when NASA Administrator Dan Goldin hinted that as far as NASA was concerned it was PFF or nothing, the choice was clear: OPSWG selected PFF for further study.
Over the next few years, teams of engineers funded by NASA worked to develop miniaturized instruments that would fit onto PFF's miniscule platform. In a radical departure from traditional practices, some teams proposed integrated systems, in which several instruments would make use of the same components, and the whole package would be integrated into the spacecraft itself. The goal of providing PFF with a capable instrument package appeared within reach. When, in 1994, it appeared that the cost of using large boosters to send the two craft on their way to Pluto would be prohibitively expensive, Alan Stern traveled to Moscow to look into using a contributed Russian heavy-lifter. By the time negotiations were completed, not only were the PFFs scheduled to launch on a Russian booster, but they would also be equipped with Russian-built descending probes to be dropped into Pluto's atmosphere.
Pluto Kuiper Express An artist's conception of the Pluto Kuiper Express flying by Pluto. Credit: NASA / JPL |
But PFF was not to be. In 1995 NASA Administrator Dan Goldin, a leading supporter of the Pluto mission, demanded that the cost of the spacecraft be further reduced and that it make use of the latest cutting edge technology. Staehle embarked on a redesign of the mission, which was now rechristened the Pluto Express. When, due to increasing scientific interest in the Plutonian region, the Kuiper belt was added to the mission's destination, it became the Pluto Kuiper Express (PKE).
IV. "Over, Cancelled, Dead"
By the year 2000, budget pressures had reduced PKE from a double to a single spacecraft mission. Nevertheless, it was still a highly capable spacecraft and NASA seemed set to make the final decision to build it. But PKE, originally designed as a low-budget mission, now had a price tag of more than $1 billion. In an era of tight budgets, NASA balked. In the fall of 2000, the mission was cancelled, and no successor mission was proposed. The mission is "over, cancelled, dead," said Ed Weiler, NASA administrator for space science.
But it wasn't. The announcement of PKE's cancellation came as a shock to the scientific community, where the study of Pluto and the Kuiper belt had moved from the margins to the very center of the field of planetary science. Within weeks of Weiler's announcement, commentaries and editorials appeared in leading news outlets condemning NASA's decision and calling for a Pluto mission. Every NASA advisory committee involved in planetary exploration seemed to take on the cause of the Pluto mission and protest NASA's decision. A high school student from rural Pennsylvania named Ted Nichols started his own "Save the Pluto Mission" website, collecting thousands of signatures for his online petition urging NASA to reconsider. And The Planetary Society got involved, collecting more than 10,000 letters in support of a Pluto mission, which it presented in person to senators and members of Congress on Capitol Hill. For the next few years the Pluto mission became a leading cause for The Planetary Society, and its repeated calls to the public to support the mission were crucial in paving the road leading to the 2006 launch.
V. Starting Over
Faced with an avalanche of criticism and demands to reconsider their position, NASA officials responded quickly. In December 2000, Weiler announced a call for proposals for a new Pluto mission, which would meet a predetermined set of scientific goals and cost no more than $500 million. The Pluto mission, it seemed, was again on track.
Within 3 months of Weiler's call, five groups of scientists and industry had submitted proposals, and by the spring of 2001, NASA had selected 2 finalists: The Pluto and Outer Solar System Explorer (POSSE), led by Larry Esposito of the University of Colorado with NASA's Jet Propulsion Laboratory (JPL) as the leading spacecraft builder; and New Horizons, led by Alan Stern of the Southwest Research Institute, with Johns Hopkins University's Applied Physics Laboratory (APL) as the main spacecraft builder.
At the end of November, NASA made its final choice: the spacecraft to visit the last unexplored planet in the solar system was to be New Horizons.
New Horizons launch
The Atlas V rocket carrying the New Horizons Pluto probe lifted off at 19:00:00 UTC on January 19, 2006 from Complex 41 on Cape Canaveral Air Force Station. New Horizons is the fastest spacecraft launched to date, reaching the altitude of the Moon's orbit in 9 hours and Jupiter in 13 months. Credit: NASA / Ken Thornsley |
New Horizons is a larger and more capable spacecraft than PKE was planned to be. With seven scientific instruments, as against PKE's three, it will conduct a more thorough and complete survey of the planet than was previously planned. In fact, it is in many ways similar in conception to Pluto 350. Unlike its predecessor, however, it be launched by NASA's most powerful booster, the Atlas V rocket. This will ensure that New Horizons will get to Pluto in only 9 years -- a journey time similar to the much lighter PFF.
But even as work began in earnest on New Horizons, the repeated cancellation threats that have plagued the Pluto mission since the beginning had not yet dissipated. When the 2003 NASA budget was submitted to Congress in 2002, funding for New Horizons was conspicuously absent. It took the professional intervention of the scientific community, led by the New Horizons team, and of the general public, led by The Planetary Society, to restore the funding. A crucial voice was added by the National Research Council, which ranked a mission to Pluto and the Kuiper Belt at the very top of its list of priorities for planetary missions. On several other occasions Congress threatened to cut some of the funding for New Horizons -- an action that would have delayed the launch for years and the encounter possibly for decades. In each case, the intervention of the New Horizons team, The Planetary Society, and their many allies, managed to restore the funding.
On January 19, 2006, 27 years after the decision to skip Pluto on the Voyagers' grand tour and 17 years after the Pluto Underground first met in a Baltimore restaurant, New Horizons soared into the sky on its way to Pluto. Its long and hazardous Earthly journey through the political and economic labyrinths of power is now done; its voyage of discovery to a new world at the farthest edge of our solar system is just beginning.
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Acknowledgement
Many thanks to New Horizons Principal Investigator Alan Stern for his help in preparing this article, and his permission to make use of his book Pluto and Charon: Ice Worlds on the Ragged Edge of the Solar System, co-authored with Jacqueline Mitton.
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