Assessment of the CRAF and Cassini Science Missions Letter Report (1988) / Chapter Skim
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In two previous letter reports, dated May 31, 1985, and May 27, 1987, COMPLEX has commented on the mission plans and the strawman and selected instrument payloads for the CRAF mission, and evaluated these plans and instrumental capabilities in the light of results from investigation of Comet Halley. The present report again reviews the CRAF mission, this time in the context of the combined CRAF/Cassini initiative.
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European fabrication of the Titan probe and sponsorship of central scientific instruments and teams represent an unprecedented and effective international sharing of the science payload, data analysis, mission operation, and fiscal responsibilities of a planetary exploration mission. In the body of this report, we comment specifically on certain aspects of Cassini science planning, and emphasize that attainment of mission objectives ultimately depends on adequate support for development of capable science instruments.
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For this reason we endorsed continued development of this experiment in our 1985 and 1987 letter reports, while cautioning that substantial development work remained to be performed during the accommodation study to demonstrate the reliability of penetrator deployment and instrument function. We are informed that specific tests have now been carried out to demonstrate the ability of the penetrator to implant successfully in a range of materials representative of cometary nucleus models, and that the project has switched from a solid to a liquid fueled penetrator rocket to allow adjustment of penetration velocity based on assessment of nucleus material properties derived by remote sensing from the main spacecraft.
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From available data, the properties of both P/Wild 2 and P/Kopff appear to be appropriate for achieving the scientific objectives of the CRAF mission. The identified asteroid encounter for the prime mission, with Eunomia -- the largest S-type object in the belt, with unusual metal-rich and metal poor surface exposures according to Earth-based observation -- appears admirably suited for asteroid reconnaissance studies.
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For missions of lesser scope -- a category that includes Cassini since it does not carry the probe required to address the first order objectives of Saturn atmosphere investigations -- COMPLEX emphasized that the interrelated nature of the system must be kept in mind in defining mission objectives, and that the first such mission should include a reconnaissance of Titan's surface for guidance of future exploration as well as for its scientific importance. Assessment of the Cassini Mission The committee's preliminary assessment, based on expected Mariner Mark II design and performance characteristics, the proposed mission profile, and the objectives and measurement capabilities of the strawman science payload instruments on both the Titan probe and the orbiter, is that the Cassini mission as currently planned (1)
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With the exception of the possible addition of a 3-axis accelerometer package to measure impact signature and assess surface properties, the current surface science plan utilizes instruments already on the prove for atmospheric measurements during descent, and is therefore achievable with minimal incremental cost. The planned surface investigations -- mass spectrometer and gas chromatograph analyses of surface material, in addition to the accelerometer measurements -- could provide much more definitive information on surface composition than that inferred from orbiter or probe-descent measurements.
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The selected instrument complement for CRAF and the strawman payload for Cassini are comprehensive and well balanced, and can be expected to implement the science objectives for both missions as set out in the relevant COMPLEX exploration strategies. The committee urges that appropriate levels of funding be made available to support development of these science instruments for flight.
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