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Scientific Assessment of Options for the Disposal of the Galileo Spacecraft At its meeting March 29-31, 2000, the Space Studies Board’s Committee on Planetary and Lunar Exploration (COMPLEX) began work on an assessment of options for the orderly disposal of the Galileo spacecraft at the end of its mission. This assessment was made at the specific verbal and, subsequent, written request of John D. Rummel, NASA’s Planetary Protection Officer. COMPLEX was asked to provide “findings, conclusions, and recommendations” about the various end-of-mission options currently being considered by the Galileo Project. In addition, the committee was asked to comment on four subsidiary issues relating to the possibility of impacting Io and Europa and the biological contamination of Jupiter and Io. BACKGROUND Galileo entered orbit about Jupiter in December 1995 on a 2-year mission to conduct intensive observations of Jupiter’s atmosphere, rings, satellites, and radiation environment. In 1997, the mission was extended for an additional 2-year period to allow for additional studies of Europa and the first close-up observations of Io. In 1999, the mission was extended for another year to enable more studies of Io and Europa, and, in addition, concerted observations of Jupiter ’s magnetosphere with the Satum-bound Cassini spacecraft in December 2000. Galileo’s frequent passages through Jupiter’s intense radiation belts have exposed the spacecraft to a radiation dose some three times larger than that specified by its design. Nevertheless, the radiation-induced problems experienced so far have been limited to intermittent interference with spacecraft operations, and no catastrophic failures of subsystems and/or total-radiation-dose effects have been observed to date. Moreover, except for stuck gratings in the ultraviolet spectrometer and near-infrared mapping spectrometer (NIMS)1, Galileo’s instrument complement remains fully operational. PLANETARY-PROTECTION CONSIDERATIONS Despite Galileo’s general spaceworthiness, it is unrealistic to assume that it will remain both controllable and scientifically useful for the indefinite future. It is, therefore, prudent to begin planning for the most scientifically productive use of the spacecraft’s remaining life and to make provision for its safe disposal. The latter issue arises because of NASA planetary-protection policy. Obligations imposed by the United Nations’ Outer Space Treaty2 mandate that spacecraft missions be conducted in such a way as to minimize the inadvertent transfer of living organisms from one planetary body to another. Given the complex interplay of the gravitational fields of Jupiter and its four large satellites, the stability of Galileo’s orbit cannot be guaranteed indefinitely. Monte Carlo simulations of the spacecraft’s orbit indicate that Galileo has a relatively high probability of eventually colliding with one of Jupiter’s satellites unless some action is taken to achieve an alternative result. Thus, Galileo must be disposed of in a controlled fashion and in a manner that does not compromise the scientific integrity of any planetary body likely to be of interest for future biological studies. 1 The ultraviolet spectrometer is no longer operational, but NIMS can still be used in a fixed-grating mode providing 14 spectral channels in the 1- to 5-µm band. The extreme ultraviolet spectrometer remains operational. 2 United Nations, Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space. Including the Moon and Other Celestial Bodies, U.N. Document No. 6347, January 1967.
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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. 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. 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 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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ocean of salt water—the same characteristics displayed by Europa.10 Callisto’s interior is only partially differentiated and,11 thus, the absence of a distinct rocky or rocky-metallic core implies that volcanism and hydrothermal activity are unlikely. Therefore, even if water exists, a biologically useful energy source may be absent.12 Given these considerations, COMPLEX sees the planetary-protection implications of the intentional or unintentional impact of Galileo on Ganymede or Callisto as being intermediate in the broad range between disposal on Io and disposal on Europa. Given the limited scientific basis for judging the biological potential of these bodies, COMPLEX was not able to quantify the exact locations of Ganymede and Callisto on the Io-Europa spectrum of planetary-protection concerns. For this reason, prudence dictates a preference for end-of-mission scenarios that involve a minor risk of impact with either Ganymede or Callisto. Jupiter: Assuming any terrestrial organisms survive the destruction of Galileo during entry into Jupiter’s atmosphere, the only environment in which they can conceivably survive is in the atmosphere itself. But any free-floating organism that finds itself in a benign region of the atmosphere will be rapidly convected into a less favorable region and, thus, the chances of survival are essentially nil.13 In addition, the committee notes that no special planetary-protection procedures (e.g., bioload reduction or sterilization) were applied to the Galileo Probe, which was specifically designed to survive penetration to the 10-bar level in Jupiter’s atmosphere. Therefore, COMPLEX sees no objection based on planetary-protection considerations for the disposal of Galileo by impact with Jupiter. Thus, COMPLEX concludes that collision with either Io or Jupiter is the most appropriate planetary-protection strategy for the disposal of Galileo. Based on Dr, Johnson’s presentation, the committee understands that operational considerations point to Jupiter as NASA’s preferred option. COMPLEX concurs with this decision. SCIENTIFIC CONSIDERATIONS Dr. Johnson told COMPLEX that NASA has considered four different options for placing Galileo on a collision course with Jupiter (see Figure 1). The most conservative option involves an orbital maneuver in the summer of 2000 and a flyby of Ganymede (G2914) in December 2000, an option that places Galileo on a ballistic trajectory guaranteed to impact Jupiter in December 2002. This trajectory would permit a flyby of the small inner moon Amalthea (A30) in August 2001. The least conservative options involve a different orbital maneuver in mid 2000, followed by flybys of Ganymede (G29) and Callisto (C30), and multiple flybys of Io (I31, I32, and I33). This sequence then leads to three additional options. An orbital maneuver at the apoapsis following I32 can be used to fine-tune the exact circumstances of I33 to place Galileo on a ballistic trajectory designed to impact Jupiter in either December 2002, or September 2003, or January 2004. Two of these ballistic trajectories permit flyby of Amalthea (A34) in either September or November 2002. 10 M.G. Kivelson et al., “Europa and Callisto: Induced or Intrinsic Fields in a Periodically Varying Plasma Environment,” Journal of Geophysical Research 104: 4609, 1999. 11 A.P. Showman and R. Malhotra, “The Galilean Satellites,” Science 286: 77, 1999. 12 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 77. 13 Space Science Board, National Research Council, Recommendations on Planetary Quarantine Policy for Mars, Jupiter, Saturn, Uranus, Neptune, and Titan, National Academy of Sciences, Washington, D.C., 1978, pages 14-15. 14 The nomenclature indicates an encounter with Ganymede on Galileo ’s 29th orbit about Jupiter since December 1995.
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FIGURE 1 A timeline of possible trajectory options leading to the disposal of the Galileo spacecraft by collision with Jupiter. The most interesting scientific opportunities presented by these options are as follows: A relatively close passage by Amalthea, one of Jupiter’s innermost known satellites. This should yield an estimate of the mass and, correspondingly, the bulk density for the satellite. This estimate is important because Amalthea may be a fragment of an object that formed closer to Jupiter than the Galilean satellites, where temperatures in the circumjovian nebula would have been higher. Given that Amalthea’s volume is currently known to an accuracy of about 10%, a mass accurate to even 20% may allow conclusions to be drawn about conditions in the jovian nebula and the satellite-formation processes, in general. Mass determination requires no functional instruments, only tracking of the spacecraft ’s trajectory through monitoring of its downlinked radio signals. If remote-sensing observations are possible during the flyby, then monochromatic (clear filter) imaging would allow several secondary goals to be pursued. These include high-resolution images of streaks and crater interiors, searches for evidence of layering, and accurate crater counts. One or more relatively close passes over Io’s north and/or south pole to complete the survey of this satellite ’s magnetic properties. Polar flybys are needed to establish the presence and identify the nature of possible internal sources of the magnetic field measured during Galileo’s earlier encounters with Io. This field may, according to some computer simulations, be generated by deep internal flows driven by the nonuniform tidal heating of Io’s mantle. If present, this type of dynamo action would provide constraints on the nature of Io’s core that would, in turn, contribute information central to theories of planetary evolution. In addition, inductive currents, responding to the time-varying component of Jupiter’s field at Io ’s location, might produce an induced magnetic moment whose amplitude and phase would characterize the nature of near-surface conducting layers. Polar passes will also provide critical information on the plasma flow characteristics and ion pickup in the polar regions for comparison with previous observations made during low-latitude flybys. Such a comparison may help identify the mechanism that produces field-aligned beams of energetic electrons previously observed in Io’s plasma wake, and provide information about Io’s ionospheric structure and composition.
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Measurements in the polar regions were attempted earlier in the mission but were not successful because a transient fault placed the spacecraft in a safe mode during the I25 flyby. Moreover, valuable remote-sensing opportunities exist during polar flybys for Galileo’s imaging, NIMS, and photopolarimeter-radiometer instruments, following up on the array of discoveries made during Galileo’s previous close flybys of Io.15, 16, 17 If a choice must be made between flybys of Amalthea and Io,18 COMPLEX believes that scientific priority should be given to the latter because it has the greatest potential for providing important results. This is the case because the Io encounter or encounters will directly address the processes responsible for the active generation of planetary magnetic fields, a key question outlined in COMPLEX’s Integrated Strategy.19 This prioritization is also in accord with the committee’s general preference for formulating exploration programs that attempt to systematically address key physical and chemical processes rather than cataloging and classifying planetary environments.20 INTERPLAY OF SCIENTIFIC AND PLANETARY-PROTECTION ISSUES COMPLEX’s preference for an Io flyby requires the selection of one of the less-conservative trajectory options. Such a selection raises the immediate question of whether the scientific potential of this option justifies the risk associated with delaying from December 2000 to January 2002 the decision to place the spacecraft on a ballistic trajectory designed to intercept Jupiter. Would an additional year during which control over Galileo may be lost make it impossible to place the spacecraft on a Jupiter-bound trajectory? In attempting to make such a determination, COMPLEX is at a disadvantage because it was not given quantitative estimates of the probability of failure as a function of time. Nor was the committee given estimates of the likelihood of impact with the Galilean satellites associated with the various trajectory options. Moreover, the committee is not qualified to make its own estimates of such eventualities. As a result, COMPLEX was unable to address the subsidiary question concerning the likelihood of impact with Europa. The question concerning the likelihood of collision with Io is moot since such an event has no obvious planetary-protection consequences. Given this lack of information, COMPLEX recommends that the Galileo Project perform the calculations required to determine the spacecraft ’s risk of impact with Europa should control over the spacecraft be lost after the G29 flyby. These results should then be used to estimate the probability of the inadvertent contamination of a europan ocean by terrestrial microorganisms from Galileo, using the procedure outlined in the recently released report of the Task Group on the Forward Contamination of Europa.21 Comparison of the resulting probabilities with the contamination limit set in the task group’s report will provide an appropriate planetary-protection basis for determining options concerning Galileo ’s future trajectory. 15 A.S. McEwen et al., “Galileo at Io: Results from High-Resolution Imaging,” Science 288: 1193, 2000. 16 J.R. Spencer et al., “Io’s Thermal Emission from the Galileo Photopolarimeter-Radiometer,” Science 288: 1198, 2000. 17 R. Lopes-Gautier et al., “A Close-Up Look at Io from Galileo’s Near-Infrared Mapping, Spectrometer,” Science 288: 1201, 2000. 18 COMPLEX was told that the trajectory options allowing flybys of both Io and Amalthea may be inconsistent with the mission’s current financial resources. 19 Space Studies Board, National Research Council, An Integrated Strategy for the Planetary Sciences: 1995-2010, National Academy Press, Washington, D.C., 1994, page 92. 20 Space Studies Board, National Research Council, An Integrated Strategy for the Planetary Sciences: 1995-2010, National Academy Press, Washington, D.C., 1994, pages 33-34. 21 Space Studies Board, National Research Council, Preventing the Forward Contamination of Europa, National Academy Press, Washington, D.C., 2000, Appendix.
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Given that the recommended calculations are complex and may take some time to perform, as an interim measure the committee performed its own non-quantitative assessment of the situation. The threat of loss of control of the spacecraft comes mostly from additional damage to its electronic systems that will be caused by continued charged-particle irradiation in the Jupiter system. Based on the information supplied by Dr. Johnson, the committee estimated that an extra year of operations will increase the burden of radiation absorbed by Galileo by only approximately 20%. This estimate, plus the fact that Galileo remains healthy—it still possesses full redundancy in all of its major subsystems, and the radiation damage incurred thus far has not handicapped control of the spacecraft—suggests to COMPLEX that the probability of total loss of control during this extra year is relatively small. Based on these considerations, COMPLEX reached a consensus that deferring the destruction of Galileo until after the completion of the Io polar flybys is an appropriate course of action, pending the completion of a quantitative assessment of the risk of contaminating the putative europan ocean with terrestrial organisms hitchhiking aboard Galileo. Although this judgment falls short of being unequivocal, COMPLEX believes that it is appropriate. It is a relatively simple task for the Galileo Project to reassess the risk at each major juncture in the trajectory and plan accordingly. That is, there is sufficient time between each satellite flyby (i.e., G29, C30, I31, and so on) for the Galileo Project to assess the health of the spacecraft and, if a significant degradation in performance is detected, to initiate the appropriate maneuver at the subsequent apoapsis to place the spacecraft on a ballistic trajectory into Jupiter. Thus, as an adjunct to its conclusion that Galileo undertake the Io flybys, COMPLEX suggests the following risk-mitigation strategy. The spacecraft’s health should be closely monitored, and the detection of loss in redundancy in any critical command and control subsystem should trigger the initiation of the appropriate maneuvers necessary to place Galileo on a ballistic trajectory designed so that the spacecraft will collide with Jupiter.
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