One option for the disposal of Galileo is controlled impact on Jupiter or one of its satellites. Another option is to take advantage of the gravitational interactions between Galileo and Jupiter and its large satellites to engineer a controlled ejection into a heliocentric orbit. The latter possibility, though intriguing from a technical perspective, might mandate a nuclear-material, launch-safety review of the type Galileo underwent prior to leaving Earth in 1989. The reason for this is the very small, but nonzero, chance of eventual impact with Earth. The anticipated cost of such a review is so great —in excess of Galileo’s current annual operations budget of some $7 million—that NASA has no option but to dispose of the spacecraft within the jovian system.

Given below is COMPLEX’s assessment of the likely planetary protection implications of disposing of Galileo by having it collide with one of the Galilean satellites or with Jupiter itself. No consideration was given to disposing of the spacecraft by impact with one of Jupiter’s minor satellites.

  1. Io: The prospects for indigenous biological activity on or below Io’s surface are slight due to its incessant high-temperature volcanic activity, the absence of water on its surface, the absence of evidence for subsurface liquid water now or in the past, and the extreme surface radiation environment.3 Similarly, the prospects for the survival of terrestrial organisms deposited by Galileo on Io are bleak. Thus COMPLEX sees no planetary-protection objection to the disposal of Galileo by intentional or inadvertent impact with Io.

  2. Europa: The strong indirect evidence for a global ocean beneath this moon ’s icy surface makes it one of the places in the solar system with the greatest potential for the existence of life.4 Although any terrestrial organisms on Galileo have now been exposed to the vacuum of space and irradiated along with the spacecraft, it is impossible to be certain that none have survived. Nor is it possible to be certain that all surviving organisms will perish upon impact with Europa and not pose a biological threat to a hypothetical europan ocean.5 Thus, COMPLEX sees serious planetary-protection objections to the intentional or unintentional disposal of Galileo on Europa. Qualitative limits on acceptable probabilities of contamination are contained in the recently released report of the Task Group on the Forward Contamination of Europa.6

  3. Ganymede and Callisto: These bodies, two of the largest satellites in the solar system, are very different. Ganymede is fully differentiated, possesses a dynamo-driven magnetic field, and has a surface that displays evidence of substantial internal geologic activity in its early history.7 It is conceivable that hydrothermal processes may have been active near the boundary between its silicate mantle and surface ice, and that the chemical and/or biological products of this activity may have been transported to Ganymede’s surface via solid-state convection, cryovolcanism, or some similar process. As such, Ganymede’s biological potential cannot be shown to be zero, but it is certainly lower than that of Europa.8

    On the other hand, Callisto’s surface is heavily cratered and shows little or no evidence of internal geologic activity.9 Nevertheless, Callisto displays magnetic characteristics indicative of a global

3  

Space Studies Board, National Research Council, Evaluating the Biological Potential in Samples Returned from Planetary Satellites and Small Solar System Bodies—Framework for Decision Making, National Academy Press, Washington, D.C., 1998, pages 31 and 77.

4  

Space Studies Board, National Research Council, A Science Strategy for the Exploration of Europa, National Academy Press, Washington, D.C., 1999, pages 3, 22-23, 26-27, and 64.

5  

Space Studies Board, National Research Council, Preventing the Forward Contamination of Europa, National Academy Press, Washington, D.C., 2000.

6  

Space Studies Board, National Research Council, Preventing the Forward Contamination of Europa, National Academy Press, Washington, D.C., 2000.

7  

A.P. Showman and R. Malhotra, “The Galilean Satellites,” Science 286: 77, 1999.

8  

Space Studies Board, National Research Council, Evaluating the Biological Potential in Samples Returned from Planetary Satellites and Small Solar System Bodies—Framework for Decision Making, National Academy Press, Washington, D.C., 1998, page 34.

9  

A.P. Showman and R. Malhotra, “The Galilean Satellites,” Science 286: 77, 1999.



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