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Space Studies Board Search: Jump to Top NewsJump to Science in the Subscribe to our FREE e- Headlines newsletter! NATIONAL ACADEMY OF SCIENCES NATIONAL ACADEMY OF ENGINEERING INSTITUTE OF MEDICINE NATIONAL RESEARCH COUNCIL June 18, 2004 Current Operating Status "On the CRAF/Cassini Mission" On March 30, 1992, Dr. Louis J. Lanzerotti, chair of the Space Studies Board, sent the following letter to Dr. Lennard A. Fisk, associate administrator for NASA's Office of Space Science and Applications. At its meeting on February 18 and 19, 1992, a subpanel of the Space Studies Board's Committee on Planetary and Lunar Exploration (COMPLEX) chaired by Professor Peter H. Stone, of the Massachusetts Institute of Technology, carried out a detailed review of the CRAF (Comet Rendezvous - Asteroid Flyby) and Cassini (Titan Probe-Saturn Orbiter) missions. This review was part of COMPLEX's continuing advisory program to assess the responsiveness of NASA missions to science objectives given in COMPLEX's published strategies for exploring the solar system. The results of this review were presented to the Space Studies Board at its meeting on February 26-28 for consideration in the broad context of the status and outlook of the U.S. civil space research program. COMPLEX's review was planned well before the release of the President's budget message on January 29. That message proposed cancellation of CRAF and called for a reassessment of the technical and schedule risks in the Cassini program. Nevertheless, COMPLEX proceeded with its review to assist those who must respond to the President's proposal. We recognize that scientists, as others, are subject to the effects of large budget deficits and that research must compete with other national needs. We feel obligated, however, to articulate the consequences of budgetary decisions. This letter summarizes the Board's overall assessment of the two missions and is accompanied by a summary providing COMPLEX's detailed scientific evaluation. 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. This reevaluation should take into account cancellation of CRAF, if this mission is indeed canceled as proposed in the President's FY 93 budget message. Based on COMPLEX's http://www7.nationalacademies.org/ssb/crafcassini392.html (1 of 10) [6/18/2004 9:56:02 AM]
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Space Studies Board evaluation, it is the strong recommendation of the Board that a scientifically responsive Cassini mission, reconfigured if necessary, proceed to development and launch on the present schedule. The Board is dismayed by the proposed cancellation of the CRAF mission, which would be of great scientific merit even without the comet penetrator experiment. However, the Board recognizes that present and anticipated resources are not likely to be adequate to successfully undertake both missions and to meet the science objectives of both at this time. The U.S. program of outer solar system exploration has brilliantly demonstrated American vision and technical mastery. 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. Because of the very long travel times to the outer solar system, seven or more years, it is important not to interrupt development of our next mission or delay its launch. The Saturn system, with its complex interacting system of magnetic fields, plasmas, rings, and moons, is an ideal laboratory for many of the physical processes believed to be important in the formation and present-day dynamics of our solar system and of planetary systems of other stars. It is for these reasons that the Board believes that high priority within the broad civil space agenda should be attached to the ongoing U.S. Saturn exploration program. We further believe that the Cassini mission should proceed without delay in order to benefit from the extremely favorable orientation of Saturn's rings at the spacecraft's projected arrival in 2004. This review of the Cassini mission was COMPLEX's first since the initial selection of the instrument payload and has been completed prior to the final confirmation of these instruments. Thus, COMPLEX's conclusions are based on the current state of definition of the mission. Given the pending confirmation of the payload, and the programmatic changes that could result from the proposed cancellation of CRAF and technical reassessment of Cassini, COMPLEX plans to reexamine Cassini at a later time. Scientific Assessment of the CRAF And Cassini Missions March 30, 1992 Summary At its meeting on February 18 and 19, 1992, a subpanel of the Space Studies Board's Committee on Planetary and Lunar Exploration (COMPLEX) chaired by Professor Peter H. Stone, of the Massachusetts Institute of Technology, carried out a detailed review of the CRAF (Comet Rendezvous-Asteroid Flyby) and Cassini (Saturn Orbiter-Titan Probe) missions. This review was part of COMPLEX's http://www7.nationalacademies.org/ssb/crafcassini392.html (2 of 10) [6/18/2004 9:56:02 AM]
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Space Studies Board continuing advisory program to assess the responsiveness of NASA missions to science objectives given in COMPLEX's published strategies for exploring the solar system. It is COMPLEX's opinion that Cassini is highly responsive to the scientific priorities set out in its report, A Strategy for Exploration of the Outer Planets: 1986-1996. 1 The instrument payload that has been tentatively selected, the mission plan that has been outlined, and the spacecraft that is being developed together provide an excellent opportunity to advance our understanding of Saturn and its satellites, rings, and magnetosphere. The Saturn system is unique within the solar system because of the wide variety of interactions—electrodynamical, hydrodynamical, and gravitational—among the system's different components. Improving our understanding of these interactions is important for developing better theories of evolution of the early solar system and of planetary and satellite systems in general. In addition, study of Titan's atmosphere is of high priority because it has a composition and chemistry that may be similar to Earth's early atmosphere. The Cassini mission as currently configured is extremely responsive to the objective of studying the Saturn system as a whole. COMPLEX notes with concern that present budget constraints are jeopardizing all of the planetary program's large missions, including Cassini. The recent reconfiguration of the Earth Observing System into a series of small spacecraft might be thought to provide a guide for the achievement of science goals outside the context of large missions. Such an analogy is inappropriate for Cassini. The long travel times between Earth and the outer solar system require long-lived components, specialized power systems, and long-distance communications fundamentally different from those required for Earth-orbital missions. With current technology, any mission sent past the asteroid belt must be more than a Discovery- class mission. While intermediate-size missions (larger than Discovery class, but smaller than Cassini) could undoubtedly achieve some of COMPLEX's objectives for the Saturn system, they could not achieve many others. For example, studies of the interactions between the different components of the system, and concurrent coordinated observations of Titan's atmosphere by both the Huygens probe and by remote sensing instruments, require large suites of instruments that place heavy demands on the spacecraft's resources. Thus COMPLEX believes that the Cassini exploration of the Saturn system cannot be fully accomplished by reconfiguration into one or more small spacecraft. 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. In proposing and planning CRAF, the research community fully recognized the importance of assessing priorities in choosing to pursue this major endeavor. COMPLEX has long articulated the unique scientific opportunities provided by the in situ study of cometary nuclei, believed to be the best-preserved relics of the http://www7.nationalacademies.org/ssb/crafcassini392.html (3 of 10) [6/18/2004 9:56:02 AM]
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Space Studies Board earliest history of our solar system. The report Strategy for the Exploration of Primitive Solar System Bodies—Asteroids, Comets, and Meteoroids: 1980-1990, 2 assigned highest priority to reconnaissance and initial exploration of comets, with special emphasis on the rendezvous mode planned for CRAF. This mode is essential for studying the sequence of events that occurs as a comet approaches and recedes from the sun. The CRAF mission has been developed in full accordance with the science objectives and recommendations of COMPLEX. In addition, the mission incorporates an excellent set of asteroid flybys, another high- priority recommendation of COMPLEX. COMPLEX recognizes that budget constraints have forced significant changes in CRAF since its last review in July 1990. These changes were the deletion of the penetrator experiment (PENL) and of the Scanning Electron Microscope and Particle Analyzer (SEMPA) experiment, a launch delay, a change in the mission's targets, and an increase in the required lifetime of the mission. However, in COMPLEX's opinion, these changes do not invalidate its earlier judgments. CRAF remains a scientifically sound mission, responsive to COMPLEX's most important near-term priorities for the exploration of primitive solar system bodies. Cancellation of CRAF will not lessen the importance of these scientific objectives, which should be pursued at the earliest possible opportunity. 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: Titan's atmosphere: Measure the composition, structure, and circulation q of Titan's atmosphere, and characterize the atmosphere-surface interaction; Titan's surface: Carry out a reconnaissance of the physical properties q and geographic variability of Titan's surface; Saturn's atmosphere: Determine the elemental composition, dynamics, q and cloud composition and structure, to a level well below the H2O cloud base; Saturn's rings: Measure particle composition and spatial distribution, q determine the evolution of dynamic structures, and search for shepherding satellites; Saturn's small satellites: Make comparative determinations of surface q composition, density, geologic history, and geomorphological processes; Saturn's magnetosphere: Specify the structure, dynamics, and q processes, and the interactions of the magnetosphere with Saturn's atmosphere, rings, icy satellites, Titan, and the solar wind. These objectives can be met with an appropriately chosen mission profile and http://www7.nationalacademies.org/ssb/crafcassini392.html (4 of 10) [6/18/2004 9:56:02 AM]
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Space Studies Board complement of scientific instruments, mounted on a spacecraft with sufficient power and communications capabilities. COMPLEX is favorably impressed with the progress made by the Cassini Project in the design of such a spacecraft and mission. The spacecraft accommodates the Cassini science requirements, while being flexible to mission changes. It appears to be a robust and capable carrier for the Cassini investigations. The level of maturity in the design is high for the current phase of development. It is clear that a number of difficult problems have been solved while maintaining prudent engineering margins. (In addition to adequately meeting the Cassini requirements, the spacecraft will also serve the needs of CRAF.) The instrument payload selected for Cassini is highly responsive to most of the important science objectives for the Saturn system. Following is a summary of the information that the currently configured mission will be able to obtain for each of the major components of the system. Titan Many of the scientific goals for Titan will be addressed by the Huygens probe, the component of the Cassini mission supplied by the European Space Agency (ESA). Its current suite of instruments, complemented by spectroscopic and radar observations from the Cassini orbiter, will do an excellent job of fulfilling these goals by providing a first characterization of Titan's atmosphere and surface. The probe's instruments include gas and haze-particle analyzers, capable imaging and spectral radiometers, as well as atmosphere profilers to determine temperature and pressure. Doppler tracking of the descent will provide the first direct measurement of Titan's atmospheric circulation. Near the surface the probe instruments will measure the composition of the atmosphere, the shock of landing—different for a solid as opposed to a liquid surface—and the density and refractive index of the liquid surface, if present. Although not in orbit around Titan, the Cassini orbiter will repeatedly pass over Titan's surface and will directly measure the composition of the upper atmosphere. The orbiter's infrared spectrometer will determine temperature and composition globally and as functions of time, complementing the measurements made during the probe's descent. Orbiter imaging at visible and infrared wavelengths will determine haze structure and variability. 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. As the Cassini orbiter repeatedly passes over Titan, its radar will yield further information on the nature of the surface in high-resolution strip-scans. This will allow imaging of a significant fraction of the surface at a resolution of 1 km or better. The radar will provide information on the composition of the surface and the depth of hydrocarbon oceans or lakes, if they are present. The radar will also operate in a radiometer mode and map surface dielectric constant variations over http://www7.nationalacademies.org/ssb/crafcassini392.html (5 of 10) [6/18/2004 9:56:02 AM]
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Space Studies Board the entire surface. In sum, Cassini will represent a major step in achieving COMPLEX's objectives for Titan. 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. Cloud structure and horizontal atmospheric motions within the upper troposphere will be obtained from temporal imaging sequences. Temperature, pressure, and ammonia abundance will be determined with excellent vertical resolution to a depth corresponding to a pressure of approximately 1 bar by radio occultations. When combined with infrared spectra, radio occultations will also provide an improved determination of the helium abundance. The Cassini instruments, together with an orbital tour that includes high-latitude coverage, will address all the Saturn atmospheric objectives outlined by COMPLEX except those for inert gas composition and isotopic abundances. Rings Throughout the Cassini mission, the orbiter will take images of Saturn's rings at a full range of viewing angles in both reflected sunlight and the thermal infrared. These images will form the database for tracking dynamical effects, wave motions, and spoke kinematics. The orientation of the rings as seen from Earth is particularly favorable for the proposed orbital tour. According to current mission plans, the rings will occult the orbiter 25 times, providing excellent measurements of the rings' transmission and scattering properties at three distinct radio wavelengths. The radio science experiment and complementary ultraviolet stellar occultation data will determine the particle size and mass distributions in all of Saturn's rings as a function of their distance from the planet. Cassini's complement of selected instruments is sufficient to achieve all the objectives for Saturn ring science set down by COMPLEX. Saturn's Small Satellites Besides Titan, Saturn's satellite system includes several small icy bodies, each displaying a variety of surface landforms and evolutionary histories. They have been affected by internal activity, possibly including tectonism and ice volcanism, and external processes, such as impact. The morphology and stratigraphy of the satellites will be evident from the data provided by the imaging system. These data will advance the understanding of satellite surface processes and history, in http://www7.nationalacademies.org/ssb/crafcassini392.html (6 of 10) [6/18/2004 9:56:02 AM]
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Space Studies Board addition to addressing the thermal history and state of satellite interiors. The spectroscopic instruments will determine the chemical and mineralogical composition of satellite surfaces. The distribution of various compositional units defined by both spectroscopy and imaging will permit the three-dimensional reconstruction of the configuration of the outer crusts of the icy satellites. Cassini's current instrument payload and mission configuration will provide an unprecedented view of the composition, state, and geological evolution of the small icy satellites of Saturn. The proposed Cassini mission configuration is fully responsive to COMPLEX's goals for the exploration of Saturn's small satellites. Magnetosphere The particles-and-fields instruments will be able to measure particle fluxes with good coverage and good resolution of energy, spatial orientation, mass, and time. Measurements of plasma waves will enable the determination of the sources and sinks of magnetospheric plasma. The particles-and-fields instruments also have an excellent capability to characterize the interaction between Titan and Saturn's magnetosphere. Cassini's magnetometer will be able to determine the configuration of Saturn's nearly axially symmetric magnetic field. This, when combined with the directional capabilities of the radio receiver, will allow determination of the origin of the kilometric radio emission modulation and, hence, the characterization of the nonsymmetric components of Saturn's magnetic field. The temporal and spatial (both radial and latitudinal) coverage of Saturn's magnetosphere during the Cassini mission should clearly establish the nature and origin of temporal variations in the magnetosphere. 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. CRAF The primary objective of CRAF since its inception has been a comet rendezvous. Comets represent some of the least-altered material left from the formation of the solar system. Thus the study of comets yields important constraints on conditions in the early solar nebula. However, ground-based observations of comets are limited by interference from Earth's atmosphere and by the generally poor viewing geometry for comets when they are near the Sun. Furthermore, the presence of a cometary coma makes viewing the nucleus difficult. The first close observations of a comet were obtained in 1985 when the International Cometary Explorer encountered Comet Giacobini-Zinner. Later, in 1986, spacecraft from Europe, Japan, and the Soviet Union completed fast flybys of Comet Halley. COMPLEX http://www7.nationalacademies.org/ssb/crafcassini392.html (7 of 10) [6/18/2004 9:56:02 AM]
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Space Studies Board concluded in a letter report 3 that these encounters left COMPLEX's objectives for the exploration of comets largely unchanged. These objectives, given in COMPLEX's 1980 report, Strategy for the Exploration of Primitive Solar System Bodies—Asteroids, Comets, and Meteoroids: 1980-1990, are as follows (in order of priority): 1. To determine the composition and physical state of the nucleus (determination of the composition of both dust and gas is an important element of this objective); 2. To determine the processes that govern the composition and distribution of neutral and ionized species in the cometary atmosphere; and 3. To investigate the interaction between the solar wind and cometary atmosphere. COMPLEX has reviewed CRAF four times, and each time concluded 4 that the mission as configured at the time of the review was responsive to the above objectives. The present review considers whether the changes in the mission since the last review, in July 1990, invalidate earlier conclusions. The significant changes were the descoping of the instrument payload, in the fall of 1990, and the change in the mission profile, in the fall of 1991, which delays the date of launch. The descoping of the instrument payload in the fall of 1990 was forced by a new NASA assessment of the costs and risks involved with the development of the CRAF penetrator experiment (PENL). This assessment led to a programmatic decision to remove PENL and the Scanning Electron Microscope and Particle Analyzer (SEMPA) from the instrument payload. In its July 1990 review, COMPLEX had identified SEMPA as not being as effective as the Comet Ice and Dust Experiment (CIDEX) and the Cometary Matter Analyzer (COMA) in addressing the most important science objective of CRAF, namely, determining the composition and physical state of the nucleus. Thus at the time, COMPLEX stated5 that SEMPA had lower priority than PENL and that, in spite of the loss of SEMPA, CRAF would remain responsive to COMPLEX's previously stated science goals. The loss of the penetrator experiment is much more serious. 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. Following is a list of what CRAF would still be able to do: http://www7.nationalacademies.org/ssb/crafcassini392.html (8 of 10) [6/18/2004 9:56:02 AM]
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Space Studies Board Measure the shape and size of the nucleus; q Measure the mass to better than one percent; q Accurately compute the bulk density from the mass and volume; q Measure the mass distribution of the nucleus; q Map the active and dormant regions of the nucleus to determine why q they are different; Study the morphology and evolution of craters; q Characterize the surface energy budget; q Determine the surface composition; q Determine the dust composition; q Determine the gas composition—both neutral and ion species; q Gain information about the onset of activity in the comet and the q formation of the coma; Characterize jet features and the relation between dust and gas in jets; q Study the magnetic field in the coma; and q Study the tail, including the interaction with the solar wind. q Many of these anticipated results directly address the primary objective of characterizing the nucleus. 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. The completion of this goal may still require acquiring and returning a sample of a cometary nucleus in some future mission." 6 The change in the CRAF mission profile was forced by the congressional budget decisions for FY 92. The change caused the CRAF launch to be delayed from February 1996 to April 1997. This slippage delays the comet rendezvous from 2003 to 2006, but accommodated a cut in the proposed funding for the CRAF/Cassini program in FY 92. This change does not per se cause any loss in the mission's anticipated return for comet science. The launch slippage does, however, enhance significantly the results anticipated for asteroid science. The primary goals of asteroid exploration, set forth in COMPLEX's 1980 primitive bodies strategy, are to determine the composition, bulk density, and surface morphology of asteroids. Elucidation of the diversity of asteroids is an essential aspect of these goals. The earlier mission profile would have included flybys of only one or two small asteroids, whereas the new profile includes flybys of two large asteroids, 88 Thisbe and 19 Fortuna, and at least one small asteroid, 1084 Tamariwa. The large asteroids are particularly important since they are likely to be primitive, undifferentiated bodies that would provide information about processes of planet formation in the early solar system. In addition to the three asteroids, the baseline mission now includes a gravity assist from Mars. This will afford an important opportunity to augment knowledge about Mars, using excellent ultraviolet, visible, and near-infrared imaging http://www7.nationalacademies.org/ssb/crafcassini392.html (9 of 10) [6/18/2004 9:56:02 AM]
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