The difficult birth of NASA’s Pluto mission Michael J. Neufeld A history of the patronage, design, and funding of New Horizons offers a glimpse of how US space-science policy was formulated before and after the turn of the 21st century. IMAGE COURTESY OF NASA/APL/SWRI Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP: 208.58.6.29 On: Fri, 01 Apr 2016 13:25:58 Michael Neufeld is a senior curator in the space history department of the Smithsonian National Air and Space Museum in Washington, DC. s spectacular pictures slowly trickle back mation about large albedo differences on Pluto’s from an enormously distant New Hori- surface and about its large if very thin atmosphere. Moreover, astronomers could discern stark differ- zons spacecraft, which shot through the ences in surface composition between the two bod- Pluto–Charon system on 14 July 2015, ies; those observations led to the theory that a giant A impact had knocked off a portion of Pluto’s icy it is easy to think that this mission was mantle to form Charon—analogous to the favored inevitable—the capstone to the US’s pioneering explo- theory of Earth–Moon formation (see the article by ration of the solar system. But it took 17 difficult years to Dave Stevenson, PHYSICS TODAY, November 2014, get from the first proposal to launch day, 19 January 2006. page 32). Pluto had thus turned, in the 1980s, from a little-understood planetary oddball into a scien- On the way, there were many changes of course and three tifically important subject for research. outright cancellations. Far from being inevitable, the Pluto An opportune intervention mission was saved from oblivion several times by dogged In 1989 that scientific context and Voyager 2’s Nep- advocacy, protests by scientists and the public, political tune encounter led NASA once again to consider a intervention by Senator Barbara Mikulski (D-MD), and a Pluto flyby. Alan Stern, shown in figure 1, initiated timely endorsement by the National Academies. the project as a graduate student finishing his dis- sertation at the University of Colorado Boulder. He The first discussions of sending a spacecraft to the then had worked on Pluto at the University of Texas at Austin in the ninth planet date back to the late 1960s. Scientists at NASA’s early 1980s and remained fascinated both by the science and by Jet Propulsion Laboratory (JPL) pointed out that in the late the prospect of exploring new frontiers. Cognizant of the up- 1970s and 1980s, the alignment of Jupiter, Saturn, Uranus, Nep- coming Neptune milestone, which marked the end of Voyager’s tune, and Pluto, relative to Earth, would allow launching two primary mission, on 4 May he visited Geoffrey Briggs, head of or more spacecraft around 1977 on a “grand tour” of the outer solar-system exploration at NASA headquarters in Washington, solar system. What came of that idea was the Voyager program, DC. Stern told Briggs: “I want to know why we don’t have a which spectacularly explored the first four above-named plan- mission to Pluto. There’s a bunch of us that would like to see ets between 1979 and 1989. But the opportunity to send one of that.” Briggs replied, “No one’s ever asked that question of me the two Voyager spacecraft to Pluto was dropped in 1979 be- before. I think we should study it.” During the American Geo- cause Saturn’s largest moon, Titan, the only satellite in the solar physical Union conference in Baltimore the next week, Stern system with a substantial atmosphere, was both a higher sci- and other young colleagues organized the Pluto Underground, entific priority and a lower-risk target. a group dedicated to advocating such a mission. In 1978, meanwhile, US Naval Observatory astronomers The Underground was scarcely alone. A flyby of Pluto would had discovered a major Pluto satellite, soon dubbed Charon “complete the reconnaissance of the solar system,” stated a June (pronounced “Shar-on” or “Kar-on”). From the moon’s orbital 1989 NASA planning document. But that mission conception characteristics, it became possible to calculate the two bodies’ rested on Pluto’s status as the ninth planet and the end of the masses. Pluto was even tinier than expected—about 0.2% of solar system. The Kuiper belt—a region of icy bodies beyond Earth’s mass. By an extraordinary stroke of astronomical luck, Neptune once proposed by Dutch American astronomer the system’s plane was such that only seven years later, Charon, Gerard Kuiper—had yet to be confirmed. When confirmation which has an orbital period of less than a week, began to pass came a few years later and astronomers discovered many ob- in front of and behind Pluto as viewed from Earth—something jects of significant size, the news would revolutionize the con- that happens only every 124 years. Those events allowed cept of the outer solar system and bring Pluto’s planetary status further refinements of the masses and diameters of what into question (see the article by Mike Brown, PHYSICS TODAY, amounted to a double planet, with the satellite’s diameter April 2004, page 49). But in 1989 all that lay in the future, and being half the 2400 km diameter of the primary. for many, the fact that the US had explored eight planets but The transits and occultations also revealed important infor- not the ninth was argument enough. © SMITHSONIAN INSTITUTION APRIL 2016 | PHYSICS TODAY 41 Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP: 208.58.6.29 On: Fri, 01 Apr 2016 13:25:58 NASA’S PLUTO MISSION FIGURE 1. ALAN STERN, NEW HORIZONS PRINCIPAL INVESTIGATOR, celebrates a successful mission with his team in July 2015 by marking out the words “not yet” from a poster-sized print of the US Post Office’s 1991 “Pluto: Not Yet Explored” stamp. The spacecraft carried the tiny stamp on its journey to Pluto. From left: Stern, Leslie Young (deputy project scientist), Ralph Semmel (Johns Hopkins Uni- versity Applied Physics Laboratory director), and Annette Tombaugh (daughter of Clyde Tombaugh, who discovered Pluto in 1930), with her husband behind her. Far right is New Horizons coinvestigator Will Grundy. (Courtesy of NASA and Bill Ingalls.) When Voyager 2 flew by Neptune on 25 August, it returned rate spacecraft in the proposed Mariner Mark II series, even some surprising data. The planet’s large moon, Triton, about larger than Voyager. It would fly an Earth–Jupiter gravity-assist Pluto’s size, turned out to be remarkably geologically active de- trajectory similar to Pluto-350’s but would require a Titan IV- spite being extremely cold. Geysers spouted from an icy sur- Centaur, a booster rocket that cost at least a quarter of a billion face 40 degrees above absolute zero. A likely captured Kuiper dollars. In August 1990 Wesley Huntress Jr, a distinguished belt object, Triton was thus a possible Pluto analogue, given the former JPL scientist, took over NASA’s solar system explo- recent observations of the planet’s dynamic atmosphere. Ac- ration division from Briggs and quickly settled on the Mariner cording to JPL astronomer and Underground member Richard Mark II proposal. Terrile, the Triton encounter “had a phenomenal impact.” A working group led by Stern debated whether Pluto or Another factor made Stern’s overture opportune: It occurred Neptune should be the next target. Pluto won priority because while Briggs was trying to get a small-spacecraft initiative started. of the forceful advocacy of Stern and others for its scientific im- A sense of crisis prevailed in solar-system exploration at the portance. Also influential were two arguments coming out of time. No robotic planetary spacecraft had been launched be- the new science: The atmosphere might “snow out” on the sur- tween 1978 and 1989, in part because of the shuttle Challenger face as Pluto moved away from the Sun (computer models sug- accident in 1986. Moreover, NASA was building large space- gested that the atmospheric collapse might happen as early as craft so expensive that it could only afford one every half decade the 2010s). And as the highly inclined north pole of the Pluto– or longer. The Mars Observer orbiter mission, an attempt to lower Charon system turned toward the Sun, an increasing propor- spacecraft costs, had become mired in overruns and delays that tion of the southern hemispheres of Pluto and Charon would led many to question whether cheap missions were even feasi- disappear into a night that would last decades. Neptune would ble. But helped by Stamatios “Tom” Krimigis, who energetically always be there, but the longer it took to get to Pluto, the less advocated small planetary spacecraft, Briggs was able to start there would be to observe. the Discovery Program that fall to solicit competitive proposals That mission, however, quickly collided with a stagnating from scientists. He appointed entrepreneurial orbital-mechanics NASA budget. The US economic downturn that began in 1991, expert Robert Farquhar (shown with Krimigis in figure 2) of combined with federal budget deficits, the end of the Cold War, NASA’s Goddard Space Flight Center to lead it. and several embarrassing NASA setbacks—notably the Hubble Space Telescope mirror problem—resulted in sudden cutbacks From Pluto-350 to Pluto Fast Flyby in the projected space-science budget. At the turn of 1991–92, Farquhar was excited by the idea of a Pluto mission. He initi- Huntress was forced to reduce the scope of the Mariner Mark ated a study led by Stern and Frances Bagenal, one of the few II program to just the Cassini mission to Saturn.