more importantly, the orbiter could carry no useful scientific payload. Increasing the orbiter's mass to 100 kg to allow for a useful payload would have increased the flight duration to more than 20 years. Faced with these insuperable difficulties, work on an orbiter was dropped and all efforts were focused on the Pluto Fast Flyby concept.

Two years later, in 1994, the out-year funding profile for the space sciences had deteriorated even further. Even with plans for Russian participation (providing launch vehicles) in the Pluto Fast Flyby, the costs to the United States still hovered around $600 million. New start plans for a Pluto mission were put on hold as scientists and engineers worked on a way to reduce costs even further.

In parallel with efforts to develop a realistic Pluto mission, another concept, the Kuiper Express “sciencecraft,” was devised to investigate the feasibility of a mission to the Kuiper Belt. By removing the usual compartmentalization between spacecraft and instrument design to such a degree that the dividing line between spacecraft systems and scientific instruments becomes blurred, the sciencecraft reaps significant mass and, therefore, cost savings. Since a mission to a Kuiper Belt object would have many similarities to a Pluto mission, the sciencecraft concept was borrowed to form the basis of the Pluto Express mission detailed in Pluto Express: Report of the Science Definition Team (NASA, Washington, D.C., September 1995).

At first blush, the Pluto Express mission looks very similar to that proposed for the Pluto Fast Flyby. The Pluto (or Pluto-Kuiper) Express mission design does, however, include the option of an extended mission into the Kuiper Belt after the Pluto-Charon encounter, provided that no mission requirements are driven by this option. The Pluto-Kuiper Express concept envisages the launch in 2002, 2003, or 2004 of two spacecraft with four instruments apiece on Delta II launch vehicles. Use of the smaller Delta launch vehicle affects both the cost and the duration of the cruise to Pluto. For the ~$77 million cost of launch on a Delta II with an upper stage—as opposed to the Titan IV's cost of around $350 million—cruise time is lengthened from 7 to between 10 and 13 years depending on the exact launch date. An option to utilize solar-electric propulsion in place of a chemical upper stage could yield flight times under 10 years. In addition, the innovative engineering of the sciencecraft concept reduces development expenditures to $145 million to $200 million, depending on whether the mission flies one or two spacecraft. The mass of each spacecraft currently stands at ~100 kg.

While NASA includes discussion of the Pluto-Kuiper Express in its recent report Mission to the Solar System: Exploration and Discovery—A Mission and Technology Roadmap (NASA, Washington, D.C., 1996), where it is given a nominal launch date of 2001 to 2003, the only representation the mission currently has in NASA's budget is implicit; an “Outer Planets/Solar Probe” line item, slated for a new start in FY 2000, is included as an element of the Origins Initiative approved as a part of NASA's FY 1998 budget.

9. Science Applications International Corporation, Measure-Jupiter Mission Design Book: Report to NASA's Outer Planet Science Working Group, Washington, D.C., 1994.

10. R.L. Staehle et al., “Exploration of Pluto: Search for Applicable Satellite Technology,” presentation at 6th Annual American Institute of Aeronautics and Astronautics/Utah State University Conference on Small Satellites, Logan, Utah, 1992.

11. R.L. Staehle et al., “Exploration of Pluto,” IAF-92-0558, presentation at 43rd Congress of the International Astronautical Federation, Washington, D.C. , 1992.

12. H.W. Price et al., “Pluto Express Sciencecraft System Design,” IAA-L-0603, presented at the Second International Academy of Astronautics International Conference on Low-Cost Planetary Missions, Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, April, 1996.

13. Pluto Express Science Definition Team, Pluto Express: Report of the Science Definition Team, NASA, Washington, D.C., 1995.

14. Space Studies Board, National Research Council, Review of NASA's Planned Mars Program, National Academy Press, Washington, D.C., 1996, p. 25.

15. Solar System Roadmap Development Team, Mission to the Solar System: Exploration and Discovery—A Mission and Technology Roadmap (Version B), Jet Propulsion Laboratory, Pasadena, California, September 20, 1996.



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