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A Science Strategy for the Exploration of Europa
Imaging Europa in at least two colors to produce a global map with a resolution of better than 300 m/ pixel, and sampling all feature types at a resolution better than or equal to about 10 m/pixel.
The Science Definition Team suggested that the radar and Doppler tracking experiments should be facility instruments (i.e., provided by the mission) and that the others be selected through open competition. NASA issued a single Announcement of Opportunity in September 1999, soliciting scientific investigations for the Europa Orbiter, Pluto/Kuiper Express, and Solar Probe missions.
According to current plans, the Europa Orbiter would have a total mass of some 900 kg, including 20 kg of scientific payload and more than 500 kg of fuel for its orbital maneuvering engine. The spacecraft would be powered by a new-generation, radioisotope power source. The Europa Orbiter is tentatively scheduled for launch aboard the space shuttle in November 2003 and will follow a direct trajectory to Jupiter. Following entry into orbit about Jupiter in February 2007, the mission will follow three distinct operational phases. The Science Definition Team dubbed these the satellite tour, the end game, and the Europa orbit. These phases encompass the following activities:
Satellite tour. A Galileo-like ballistic cruise, lasting approximately 2 years, that utilizes multiple flybys of the Galilean satellites to circularize the spacecraft's initial, highly elliptical, orbit about Jupiter. Limited science operations will probably be conducted during this part of the mission, but their scope and extent have not yet been determined.
End game. The final series of maneuvers, lasting approximately 3 to 4 months, designed to modify the spacecraft's trajectory so that it can be captured into a polar orbit about Europa.
Europa orbit. The orbiter would conduct its observations of Europa from a precessing, circular polar orbit with an altitude of some 200 km and an orbital period of approximately 1.6 hours. The duration of this phase of the mission is limited by the total radiation dose the spacecraft can survive. With the spacecraft hardened to survive a radiation dose of 4 megarads (by comparison, Galileo was hardened to survive 150 kilorads), its expected orbital lifetime is approximately 1 month. Numerical simulations suggest that a spacecraft with an orbital inclination greater than some 45 degrees will impact Europa within a few months of its demise. Although the Europa Orbiter's expected lifetime is short, it is believed to be adequate to address the primary scientific objectives specified by the Science Definition Team.
C.F. Chyba, "Report of the Europa Orbiter Science Definition Team," letter to Dr. J. Bergstrahl, NASA Headquarters, May 18, 1998.
1. S.W. Squyres, R.T. Reynolds, P.M., Cassen, and S.J. Peale, "Liquid Water and Active Resurfacing on Europa," Nature 301: 225, 1983.
2. R.T. Pappalardo et al., "Does Europa Have a Subsurface Ocean? Evaluation of the Geological Evidence," Journal of Geological Research — Planets, 1999, in press.
3. A.C. Clarke, 2010: Odyssey II, Balantine Books, New York, 1982.
4. For a review of current understanding of Europa and the other galilean satellites, see, for example, A.P. Showman and R. Malhotra, "The Galilean Satellites," Science 286: 77, 1999.
5. Space Studies Board, National Research Council, An Integrated Strategy for the Planetary Sciences: 1995-2010, National Academy Press, Washington, D.C., 1994, page 60.
6. Space Studies Board, National Research Council, An Integrated Strategy for the Planetary Sciences: 1995-2010, National Academy Press, Washington, D.C., 1994, page 61.