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Assessment of Solar System Exploration Programs 1991 (Chapter 4) Assessment of Solar System Exploration Programs 1991 4 Exploration of Primitive Solar-System Bodies In its report Strategy for the Exploration of Primitive Solar-System Bodies—Asteroids, Comets, and Meteoroids: 1980-1990 (SSB, 1980), COMPLEX established the scientific goals and objectives and related program requirements. SCIENCE OBJECTIVES COMPLEX recommends that the primary goal of investigation of asteroids, comets, and dust, during approximately the next decade, be to REPORT MENU determine their composition and structure and to deduce their history in order to NOTICE increase our knowledge of the chemical and isotopic composition and physical MEMBERSHIP state of the primitive solar nebula and to further our understanding of the FOREWORD condensation, accretion, and evolutionary processes that occurred in various SUMMARY CHAPTER 1 parts of the solar system before and during planet formation. CHAPTER 2 CHAPTER 3 The 1980 report defined three additional goals: CHAPTER 4 CHAPTER 5 CHAPTER 6 A goal in the study of primitive bodies is to determine their diversity of CHAPTER 7 composition and structure. CHAPTER 8 CHAPTER 9 A goal for investigation of the minor bodies is to understand the role REFERENCES played by accretion of these bodies in the evolution of the crustal and atmospheric composition and the crustal structure of the terrestrial planets. A goal in the study of minor bodies is the understanding of the dynamical processes responsible for the production, maintenance, and behavior of the gas, dust, and plasma envelopes of active comets. file:///C|/SSB_old_web/ssep91ch4.htm (1 of 5) [6/18/2004 1:57:55 PM]

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Assessment of Solar System Exploration Programs 1991 (Chapter 4) CURRENT STATUS OF NASA'S EXPLORATION OF PRIMITIVE BODIES This section addresses the progress made in achieving the scientific goals and objectives of exploring comets; asteroids, meteorites, and interplanetary dust, and notes areas that have significant deficiencies. Comets The primary objectives for the exploration of comets, in order of priority, are as follows: 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 the cometary atmosphere. Progress in the study of comets has been substantial owing to (1) the retargeting of the International Sun-Earth Explorer (ISEE-3), to study the Giacobini-Zinner and Halley's comets; (2) augmented ground-based and Earth- orbit telescopic observations carried out during the apparition of Halley's comet; (3) European, Soviet, and Japanese spacecraft missions to Halley's comet; and (4) a new start given to the NASA Comet Rendezvous/Asteroid Flyby (CRAF) mission in FY 1990 to rendezvous with a short-period comet. According to current mission plans for CRAF, the specific measurement requirements are to study the composition and structure of a comet nucleus and to map the nucleus in terms of composition, structure, and temperature. These are expected to be met with data returned from the mission starting at the turn of the century. Similarly, data already received from Giotto and those to be obtained by CRAF will address the measurement requirements of cometary dust and molecular species in the coma. Solar wind interactions were studied in the 1980s by ISEE-3, Giotto, and Vega. The diversity of comets is being studied primarily from ground-based observations. In short, the committee finds that significant progress has been made toward achieving the goals established in the 1980 strategy and will continue to be made if the current plans for CRAF are realized and support of ground-based research is continued. The 1990 deselection of the CRAF penetrator experiment 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. file:///C|/SSB_old_web/ssep91ch4.htm (2 of 5) [6/18/2004 1:57:55 PM]

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Assessment of Solar System Exploration Programs 1991 (Chapter 4) Asteroids The primary scientific objectives for the exploration of asteroids, in order of priority, are as follows: 1. To determine their composition and bulk density; 2. To investigate the surface morphology, including evidence for endogenic and exogenic processes and evidence concerning interiors of precursor bodies; and 3. To determine the internal properties, including states of magnetization of several carefully chosen asteroids selected on the basis of their diversity. Progress in achieving the above goals has been minimal in the past decade. There are no plans to measure the abundance of the major elements of an asteroid. The current capabilities of studying asteroid mineralogy-reflectance spectroscopy and radar-are not capable of unambiguously making associations with all meteorite types. NASA currently has plans only for flybys of asteroids. The agency should consider more capable missions to meet the established scientific objectives. The committee anticipates that the planned flybys of asteroids Gaspra and Ida by the Galileo spacecraft will provide some information on the size, shape, and bulk density of these objects, and imaging by the Galileo cameras should provide information on the nature of endogenic and exogenic processes. Flybys of asteroids are also expected to be made by CRAF and Cassini. In the past decade, ground-based studies have produced information that expands our understanding of the processes and composition of the asteroids as a whole population. Among the significant ground-based achievements are (1) demonstration of the compositional gradient across the main asteroid belt; (2) discovery of rare, Q-type asteroids, which are probably ordinary chondrite analogues; (3) collisional modeling studies explaining the size-frequency distribution of different taxonomic types; and (4) two-dimensional imaging of asteroids with radar. Continued support for ground-based studies of asteroids will enable additional advances in our understanding of asteroids in preparation for future flight missions. Meteorites, Interplanetary Dust, and Meteors Because meteorite studies are intimately related to studies of asteroids and comets, COMPLEX recommended in its 1980 report "that a vigorous file:///C|/SSB_old_web/ssep91ch4.htm (3 of 5) [6/18/2004 1:57:55 PM]

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Assessment of Solar System Exploration Programs 1991 (Chapter 4) program of laboratory and theoretical investigations of meteorites be maintained." Meteorite research is essential to understanding primitive solar system bodies and their origins, and progress over the past decade has been substantial. However, the committee is concerned that continued cuts in the research and analysis (R&A) budget, which is the source of funding for almost all meteorite research, will erode the scientific community's ability to do this valuable research. In this regard, the Origins of Solar Systems Program could be instrumental in deciphering the origins of primitive bodies. COMPLEX also recommended that, "to realize the full promise of meteorite research it is necessary to maintain laboratory capabilities at the highest level of evolving technology and to encourage the development of even more sophisticated analytical methods." Although several new techniques have been applied to meteorite research during the past several years, the lack of growth in the R&A budget has inhibited instrument development and has prevented significant upgrading of existing instruments. Considerable progress has been made in the study of interplanetary dust. In 1980, COMPLEX recommended "that the development of techniques to isolate, manipulate, and analyze small samples of extraterrestrial matter be vigorously supported." Advances have been made in the collection of stratospheric dust, in the curation and manipulation of the samples, and in techniques used to analyze them. The Infrared Astronomical Satellite (IRAS) provided information about the distribution of interplanetary dust through its discovery of dust bands in the asteroid belt. In discussing future research directions, COMPLEX has recommended that interplanetary dust experiments including collection, analysis, and orbit determination have high priority in the overall program of science conducted in Earth orbit. The committee now urges the implementation of such experiments for the space station. file:///C|/SSB_old_web/ssep91ch4.htm (4 of 5) [6/18/2004 1:57:55 PM]