On the CRAF/Cassini Mission Letter Report (1992) / Chapter Skim
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In brief, the Board recognizes that the current and near-term national budget environment severely constrains the conduct of the nation's space research program. Therefore, the Board recommends that NASA carefully reevaluate the Cassini spacecraft and instrument complement with the objective of ensuring the mission's prospects for adequate and stable funding leading to the scheduled 1997 launch, while retaining the maximum science content possible.
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The United States, alone, has undertaken and completed the initial reconnaissance of the major planets of the outer solar system, visiting in turn Jupiter, Saturn, Uranus, and Neptune and obtaining revolutionary data about these planets and their atmospheres, moons and rings, and plasma environments. The Board believes that a vigorous program of outer solar system exploration is an essential part of a national space exploration agenda.
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COMPLEX views with dismay the proposal to cancel the currently approved CRAF mission. This mission is the outcome of many years of planning by numerous groups of distinguished scientists, NASA centers, and competitively selected scientific instrument teams.
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Cassini COMPLEX's 1986 report, A Strategy for Exploration of the Outer Planets: 1986 1996, states that the highest priority for outer planet exploration in the next decade is intensive study of Saturn -- the planet, satellites, rings, and magnetosphere -- as a system. Specifically, the recommended exploration and intensive study of the Saturn system include the following objectives: q Titan's atmosphere: Measure the composition, structure, and circulation of Titan's atmosphere, and characterize the atmosphere-surface interaction; q Titan's surface: Carry out a reconnaissance of the physical properties and geographic variability of Titan's surface; q Saturn's atmosphere: Determine the elemental composition, dynamics, and cloud composition and structure, to a level well below the H2O cloud base; q Saturn's rings: Measure particle composition and spatial distribution, determine the evolution of dynamic structures, and search for shepherding satellites; q Saturn's small satellites: Make comparative determinations of surface composition, density, geologic history, and geomorphological processes; q Saturn's magnetosphere: Specify the structure, dynamics, and processes, and the interactions of the magnetosphere with Saturn's atmosphere, rings, icy satellites, Titan, and the solar wind.
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Properties of the upper atmosphere will be measured during Titan flybys by the orbiter's ion and neutral mass spectrometer. The complementarity of obtaining orbiter data coincident with Huygens probe data is an important advantage of the Cassini mission as currently configured.
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Saturn's Atmosphere The Cassini orbiter will determine properties of Saturn's atmosphere at all latitudes and will monitor dynamical changes. Infrared spectra will yield composition and temperature throughout the stratosphere and upper troposphere and, when combined with near-infrared and imaging measurements of reflected sunlight, will determine the thermal energy balance both locally and globally.
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The Cassini payload and mission design appear to be fully capable of achieving the major scientific objectives of studying Saturn's magnetosphere. COMPLEX's overall conclusion is that the Cassini mission, as currently configured, is extremely responsive to the highest-order priority for exploring the outer planets, i.e., intensive study of Saturn as a system.
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As stated in the same letter, "deletion of the penetrator would severely compromise the ability of the CRAF mission to address the highest-priority goals identified by COMPLEX." PENL was the only experiment that would have sampled the comet nucleus in situ. As important as the loss of PENL was, however, COMPLEX continues to hold the view, stated in its 1980 report Strategy for the Exploration of Primitive Solar System Bodies -- Asteroids, Comets, and Meteoroids: 1980-1990, that comet "science objectives can be met during the next decade without undertaking to land on or penetrate a comet nucleus." This view is based on the fact that the descoped CRAF will still be able to contribute many things to comet science in response to COMPLEX's primary, near-term objectives.
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COMPLEX therefore believes that the descoped CRAF mission is still responsive to its highest-priority near-term goals for comet science. At the same time, COMPLEX reiterates its comments in Assessment of Solar System Exploration Programs: 1991: "The 1990 deselection of the CRAF penetrator requires continuing attention to alternative means of directly sampling a comet nucleus.
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Although reduction of power from the radioisotope thermoelectric generators is a concern, the rendezvous nature of this mission, with the spacecraft in prolonged close proximity to the comet, relaxes some of the constraints on power usage and makes power usage less a concern for CRAF than for a more rapid flyby mission. Therefore, COMPLEX concludes that the complications arising from the one-year launch delay for CRAF are more than offset by the enhanced opportunities resulting from NASA's redesign of the mission profile.
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