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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.
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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.
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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
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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.
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