Box 4.1 History of NASA's Pluto Mission Concepts

The Space Science Board's 1986 report A Strategy for the Exploration of the Outer Planets: 1986–1996 (National Academy Press, Washington, D.C., 1986) identified the Pluto-Charon system as a likely and logical long-term candidate for a flyby reconnaissance mission. Since then, NASA plans for a Pluto mission have been repeatedly redesigned. Each iteration has been geared toward producing a mission architecture that could, in general, be accomplished either more rapidly or at lower cost than could its predecessors.

NASA's 1991 Solar System Exploration Division Strategic Plan called for a Pluto Flyby/Neptune Orbiter program, in which a pair of Mariner Mark II (Cassini-class) spacecraft would be launched in the period 2001 to 2003 by Titan IV/Centaurs on a 15- and 20-year cruise to Pluto and Neptune, respectively. Each 5,000- to 6,000-kg spacecraft would carry a payload of 14 instruments and cost in excess of $2 billion. By 1992, however, it was becoming clear that space science funds through the turn of the century would not support such an ambitious architecture. The project was officially descoped to a single Mariner Mark II Pluto Flyby or Neptune Orbiter mission and then quietly dropped.

At the same time, the idea for a Pluto Very Small or Pluto Very Small or Pluto Fast Flyby program was born out of technological feasibility studies at NASA's Jet Propulsion Laboratory. In this scenario, two spacecraft, each carrying four instruments, would be launched toward Pluto in the period 2001 to 2003. Even though each spacecraft would have a mass of approximately 150 kg, launch on a Titan IV/Centaur would be required to reduce the flight time to 7 to 8 years. A $400 million price tag included mission development and operations up until 30 days after launch, but excluded both the cost of the radioisotope thermoelectric generators necessary to power the spacecraft in the outer solar system and the cost of the two Titan IV/Centaurs. Actual mission costs would be around $1.2 billion plus expenditures for lifetime operations.

At about the same time the Pluto Fast Flyby concept was developed, a number of alternative missions were investigated. Prime among these was the so-called Pluto Flyby 350 concept developed for NASA's Discovery Program Science Working Group. It envisaged a somewhat larger, fully redundant spacecraft (~300 kg), carrying a greater range of instruments than the Pluto Fast Flyby concept. To limit total mission costs, Pluto Flyby 350 eliminated launch by a Titan IV and, instead, relied on Earth and Jupiter flybys to inject it on course to Pluto. If launched on an Atlas or Delta in 2001, Pluto Flyby 350 would have reached Pluto more than 11 years later. The missions savings on its launch vehicle costs were, however, negated by the need for a more elaborate spacecraft with a significantly longer operational lifetime.

At about the same time, consideration was also given to a Pluto orbiter mission. Studies indicated that a 35-kg spacecraft could be placed into orbit about Pluto by following a trajectory similar to that of Pluto Flyby 350. This concept was highly unattractive because the total flight time was more than 16 years and,


1. D.C. Jewitt, J.X. Luu, and J. Chen, “The Mauna Kea-Cerro Tololo (MKCT) Kuiper Belt and Centaur Survey,” Astronomical Journal 112:1225, 1996.

2. R.H. Brown et al., “Surface Composition of Kuiper Belt Object 1993SC,” Science 276: 937, 1997.

3. D.P. Cruikshank and M.W. Werner, “The Study of Planetary Systems with the Space Infrared Telescope Facility (SIRTF),” Planets Beyond the Solar System and the Next Generation of Space Missions, D.R. Soderblom, ed., ASP Conference Series 119:223, 1997.

4. For a review of the current status of infrared detector technology, see, for example, G.H. Rieke, Detection of Light: From the Ultraviolet to the Submillimeter, Cambridge University Press, Cambridge, U.K., 1994.

5. G.H. Rieke, Detection of Light: From the Ultraviolet to the Submillimeter, Cambridge University Press, Cambridge, U.K., 1994, p. 170.

6. A. Dressler, ed., Exploration and the Search for Origins: A Vision for Ultraviolet-Optical-Infrared Space Astronomy, report of the HST & Beyond Committee, Association of Universities for Research in Astronomy, Washington, D.C., 1996.

7. H.S. Stockman, ed., Next Generation Space Telescope: Visiting a Time When Galaxies Were Young, STScI M-9701, Space Telescope Science Institute, Baltimore, Maryland, 1997.

8. Science Applications International Corporation, Low-Cost Outer Planet Mission Definitions: Report to NASA Headquarters, Washington, D.C., 1995.

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