The New Horizons mission to Pluto (and beyond)

Barrie W. Jones is an astronomer whose new book, Pluto: Sentinel of the Outer Solar System, deals with the discovery and exploration of planetoid Pluto. In this excerpt, he describes New Horizons, a spacecraft that will visit Pluto in 2015.

We would clearly learn a lot more about Pluto, its three satellites, the E-K belt, and anything else beyond Pluto, were a spacecraft targeted to investigate this far flung region of the Solar System. As yet no such spacecraft has visited Pluto and beyond, but one is on its way, New Horizons.

The long path to new horizons

Table 8.1 (below) lists the spacecraft that have already visited the outer Solar System, and reached their targets. You can see that Pluto is the only one of the original nine planets that has not been visited by a spacecraft. Why? One reason is surely that when each of the missions in Table 8.1 was being selected for development, no other KBOs were known, the first to be discovered was the small body 1992 QB1 in 1992 (Section 6.2), and therefore Pluto and its satellite Charon was regarded as just a small, isolated system. Consequently it was of considerably less interest than it is now, with its numerous companions in the E-K belt. Attention was instead focused on the rich domain of the four terrestrial planets and the four giant planets plus their numerous satellites.

As our knowledge of Pluto grew, so did interest in sending a spacecraft there, such that in the late 1980s a small number of planetary astronomers began to campaign for a mission to Pluto. The campaign was aided by the 1989 flyby of Neptune by Voyager 2. In particular, Neptune's largest satellite Triton, with a diameter just a little larger than Pluto's, was found to have a rather similar surface and atmospheric composition to Pluto. Moreover, active geysers on Triton were observed, probably the result of the sub-surface sublimation of nitrogen. Could such geysers also be present on Pluto?

The New Horizons mission to Pluto (and beyond)

Adding to the growing interest in Pluto was the view that Triton was captured by Neptune and that before capture it was a freely orbiting twin of Pluto.

Among those pressing NASA for a mission to Pluto were The US astronomers Frances Bagenal and Alan Stern. Stern's visit to NASA headquarters in 1989 was instrumental in the positive outcome, and studies for a low-mass (350 kg) Pluto mission began that year. In spite of scepticism among many in NASA, the studies continued. The need for a mission was urgent, for three reasons.

First, after Pluto's perihelion in 1989 its distance from the Sun began to grow, making the journey to Pluto longer, and weakening the data-carrying radio signals that would be received on Earth.

Second, after perihelion the solar insolation (the solar radiation falling on Pluto) began to fall, which must ultimately result in the precipitation of nearly all of its atmosphere on to the surface, changing the surface as well as the atmosphere, and leaving little atmosphere to study. Pluto's perihelion and aphelion distances are, respectively, 29.7 AU and 49.6 AU, corresponding to a decrease in insolation of (29.7/49.6)2, which is 0.36, and so at aphelion the insolation is only 36% that at perihelion. For comparison, the insolation on Mars is about 43% that on the Earth, not much greater than the variation experienced by Pluto in its orbit. It was uncertain when, in the next few decades, Pluto's atmosphere would begin to precipitate, how quickly, and to what extent. Aphelion is not until 2113, but the rate at which Pluto recedes is a maximum around perihelion (Kepler's second law, Section 1.2). Pluto needed to be visited before much of its atmosphere had frozen.

Third, on Pluto, winter in its southern hemisphere was fast approaching, and with Pluto's large axial inclination of 57.5◦ (retrograde) with respect to its orbital plane, an increasingly large proportion of its southern hemisphere was being plunged into darkness: by 2015 almost half will be too dark for much to be discerned. The same is the case for Charon. The sooner the arrival of a spacecraft, the more of the globe would be sunlit at some time during Pluto's and Charon's day.

The first study of a Pluto mission was in 1990, by the Discovery Working Group led by Robert Farquhar (NASA Goddard Space Flight Center). Arguments brought to NASA yielded fruit in 1991 when NASA's Solar System Exploration Subcommittee placed a Pluto flyby in the highest priority category for new missions in the 1990s. Much discussion followed, with various proposals to NASA for spacecraft and launch vehicle coming and going. After all this, in the autumn of 2000 NASA's administrator for Space Science, Edward Weiler, announced that there would, after all, be no mission to Pluto. This led to a storm of protest, not just from ‘plutophiles' and other planetary astronomers, but from the broader public. There was much media coverage, overwhelmingly in favour of a Pluto mission.

Within a few weeks, in the week before Christmas 2000 Weiler invited proposals to NASA for a low budget mission ($500 million) to Pluto and the Edgeworth-Kuiper belt (a previous proposal for a Pluto- E-K belt mission had been unsuccessful). The deadline was March 2001, then put back to April. Five proposals were submitted. A shortlist of two was announced in June 2001, then in November 2001 NASA announced the selection of the New Horizons proposal, from Alan Stern et al., as being the better of the two for performing science at Pluto and in the E-K belt, and the more likely of the two to meet the launch target of January 2006, and do so within the budget.

Alas! In early 2002 the budget submitted to the US Congress by the US Office of Management and Budget showed New Horizons cancelled, on the basis of cost estimates that were for an earlier, expensive JPLmission that had already been cancelled! After pressure, again from astronomers and the public, Congress, late in 2002, sent a budget to the US President with New Horizons re-instated. The next NASA budget in February 2003 showed New Horizons to be fully funded, with a launch date in January 2006. On 19 January of that year the spacecraft was launched successfully from Cape Canaveral in Florida on an Atlas V rocket with a third stage added to increase the speed. Alan Stern is the principal scientific investigator.

Pluto: Sentinel of the Outer Solar System, by Barrie W. Jones, Copyright © 2010 B. W. Jones. Reprinted with the permission of Cambridge University Press.