EXPLORATION OF PLUTO

Pluto is the last known planet in our Solar System awaiting spacecraft reconnaissance. In its eccentric orbit taking it 50 AU from the Sun, Pluto presently has a thin atmosphere containing methane, which is pro jected to ''collapse'' back to the icy planet's surface in about three decades following Pluto's 1989 perihelion pass at 30 AU. Based on gro und and Earth-orbit-based observing capabilities limited by Pluto's sm all size and extreme distance, present top-priority scientific questio ns for the first mission concern Pluto and Charon's surface geology, m orphology and composition, and Pluto's neutral atmosphere composition. Budgetary realities preclude a large, many-instrument flyby spacecraf t, while distance and launch energy requirements preclude any but the smallest orbiter using presently available launch vehicles and propuls ion techniques. A NASA-sponsored Pluto Mission Development activity be gan this year. Two alternative cost-constrained mission implementation s are described, based on which a primary implementation will be chose n. The Pluto Fast Flyby (PFF) mission utilizes an 83 kg (dry) spacecra ft launched in 1998 aboard a Titan IV(SRMU)/Centaur for an approximate ly 7 year direct trajectory to Pluto. Instruments described are an int egrated CCD-imaging/ultraviolet spectrometer, with a possible integrat ed infrared spectrometer. The larger Pluto-350 spacecraft, approximate ly 316 kg, carries a broader instrument set, greater redundancy, and r equires > 11 year flight time launching in 2001 aboard a Delta or Atla s, toward Earth and Jupiter swingbys to provide the energy to reach Pl uto. Launch by Proton is under consideration. Both mission implementat ions store data during the brief encounter, to be played back over sev eral months. Cost is the primary design driver of both alternatives, w ith major tradeoffs between spacecraft development, launch services, r adioisotope thermoelectric generator procurement and launch approval, and mission operations. Significant benefits are apparent from incorpo rating ''microspacecraft'' technologies from Earth orbiters.