On June 28, 2000, Space Studies Board Chair Claude Canizares and Committee on Planetary and Lunar Exploration Chair John Wood sent the following letter to Dr. John Rummel, NASA planetary protection officer.
In your letter of April 13, 2000, you reiterated a verbal request made in March for advice from the Space Studies Board's (SSB's) Committee on Planetary and Lunar Exploration (COMPLEX) on planetary protection concerns and other issues related to the final disposition of the Galileo spacecraft now in orbit about Jupiter. In particular, you asked that COMPLEX "evaluate the Galileo Project's favored alternatives for the spacecraft's planned trajectory during the remainder of the mission" and provide "a short statement of [COMPLEX's] findings, conclusions and recommendations relative to that plan." In addition, you indicated that it would be particularly useful if COMPLEX could address four subsidiary issues in terms of their implications for planetary protection. These issues concerned:
The planned trajectory's ability to avoid impact with Europa;
The likelihood of the spacecraft impacting Io during science operations or after the end of the mission;
The possibility of the biological contamination of Io; and
The eventual deposition of the spacecraft on Jupiter.
Work on this assessment began at COMPLEX's March 29-31, 2000, meeting at the Arnold and Mable Beckman Center in Irvine, California. Torrence Johnson (Jet Propulsion Laboratory), the Galileo Project Scientist, briefed the committee on the various options under consideration for the final disposal of the spacecraft and outlined the opportunities and risks associated with the last phase of Galileo's operational life.
In the discussion following Dr. Johnson's presentation, COMPLEX received additional input from Robert Pappalardo (Brown University), an affiliate member of the Galileo Imaging Team. In addition, individual committee members consulted with Damon Simonelli (Cornell University), an affiliate member of the Galileo Imaging Team, and with Margaret Kivelson (University of California, Los Angeles), Louis Frank (University of Iowa), and Donald Williams (Applied Physics Laboratory), the principal investigators of Galileo's magnetometer, plasma subsystem, and energetic particle detector, respectively. The committee also reviewed relevant reports issued by COMPLEX and other National Research Council (NRC) committees (e.g., Recommendations on Quarantine Policy for Mars, Jupiter, Saturn, Uranus, Neptune, and Titan [1978], An Integrated Strategy for the Planetary Sciences: 1995-2010 [1994], Evaluating the Biological Potential in Samples Returned from Planetary Satellites and Small Solar System Bodies [1998], A Science Strategy for the Exploration of Europa [1999], and Preventing the Forward Contamination of Europa [2000]) and held extensive discussions in closed session.
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 Contamination 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 degree to which the proposed trajectory options are consistent with the requirements necessary to avoid the forward contamination of Europa.
In the interim, COMPLEX performed its own qualitative analysis. Based on the information supplied to the committee, an extra year of operations can be expected to increase the burden of radiation absorbed by Galileo by only about 20%. This estimate, plus the fact that Galileo retains full redundancy in all essential systems and that the radiation effects sustained thus far have not handicapped control of the spacecraft, suggests to COMPLEX that the probability of total loss of control during this extra year is relatively small. Moreover, the chances of total failure can be mitigated by prudent monitoring of the spacecraft's health and by a commitment on the part of NASA to retarget Galileo onto a Jupiter-bound trajectory following the loss of redundancy in any major command and control subsystem.
In summary, COMPLEX concurs with NASA's decision that impact with Jupiter is the most appropriate means of disposing of Galileo. COMPLEX recommends that the Galileo Project perform qualitative risk assessment of the various trajectory options. Pending the completion of these calculations, the committee reached a consensus that an appropriate interim course of action is to defer the destruction of Galileo until after the completion of the Io polar flybys, in order to obtain as much science as possible from the mission.
4Space Studies Board, National Research Council, Preventing the Forward Contamination of Europa, National Academy Press, Washington, D.C., 2000, Appendix.
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