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In its deliberations, COMPLEX considered three separate issues: planetary-protection considerations affecting the disposal of Galileo in the Jupiter system; the unique scientific opportunities presented by the various end-of-mission scenarios being considered by the Galileo Project and their relative priorities; and general considerations arising from possible conflicts between planetary-protection requirements and scientific opportunities. Full details are contained in the attached assessment.
With respect to planetary-protection issues, COMPLEX reached the following conclusions:
There is no planetary-protection-related objection to the disposal of Galileo by intentional or unintentional impact with Io or Jupiter.
There are 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 recent report of the Task Group on the Forward ontamination of Europa 1.
The planetary-protection implications of the intentional or unintentional impact of Galileo with Ganymede or Callisto are intermediate in the broad range between those for disposal on Io and for disposal on Europa.
COMPLEX understands that operational considerations point to collision with Jupiter as NASA’s preferred means for disposing of Galileo. COMPLEX concurs with this decision.
With respect to scientific priorities they afford, COMPLEX believes that the most important of the various options being considered by the Galileo Project are a conservative trajectory leading to a close flyby of the small moon Almathea, and a series of less conservative trajectories leading to one or more polar flybys of Io, and, possibly a flyby of Almathea as well.2 Given this choice, COMPLEX believes that the Io flybys have the greatest potential for providing important scientific results because they directly address the processes responsible for the active generation of planetary magnetic fields, a key question outlined in COMPLEX ’s Integrated Strategy.3
With respect to issues arising from possible conflicts between planetary-protection requirements and scientific opportunities, COMPLEX recognizes that its preference for an Io flyby requires the selection of one of the less conservative trajectory options. That is, choosing the Io encounters postpones for approximately 1 year Galileo’s placement on a ballistic trajectory into Jupiter and, thus, increases the chance that the spacecraft may suffer a fatal failure and end up on an unintended trajectory.
Unfortunately, the committee was not given quantitative estimates of the probability of spacecraft failure as a function of time. Nor was the committee given estimates of the likelihood of impact with the Galilean satellites associated with failure during the various trajectory options. Moreover, the committee is not qualified to make its own estimates of such eventualities. As a result, COMPLEX is not able to address the subsidiary question concerning the likelihood of unintentional impact with Europa. The subsidiary question concerning the likelihood of unintentional impact with Io is moot since there is no planetary-protection objection to impact with Io.
Given this lack of information, COMPLEX recommends that the Galileo Project perform the necessary calculations to determine the probability of Galileo impacting Europa should control of the spacecraft be lost after the G29 flyby. These results can then be used to estimate the probability of contaminating the putative europan ocean with terrestrial microorganisms by following the procedure outlined in the report of the Task Group on the Forward Contamination of Europa.4 Only if such a quantitative analysis is undertaken can COMPLEX give an unequivocal recommendation about the
Space Studies Board, National Research Council, Preventing the Forward Contamination of Europa, National Academy Press, Washington, D.C., 2000.
The Io plus Amalthea option is not consistent with Galileo’s current budget plan.
Space Studies Board, National Research Council, An Integrated Strategy for the Planetary Sciences: 1995-2010, National Academy Press, Washington, D.C., 1994, page 92.
Space Studies Board, National Research Council, Preventing the Forward Contamination of Europa, National Academy Press, Washington, D.C., 2000, Appendix.