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Assessment of Solar System Exploration Programs 1991 (Chapter 2)
Assessment of Solar System Exploration Programs
1991
2
Exploration of the Inner Planets
The committee's Strategy far the Exploration of the Inner Planets: 1977-
1987 (SSB, 1978) established a comprehensive set of scientific goals and
objectives for the exploration of Mercury, Venus, Earth, the Moon, and Mars,
together with a number of related policy and program recommendations. In that
report, COMPLEX concluded that observation and measurement of the
morphologic, physical, and chemical character of Mars, Venus, Mercury, and the
Moon on a global scale have high general scientific importance and are basic to
all planetological studies. The committee recommended that the triad of terrestrial
planets—Venus, Earth, and Mars—should be the major focus in exploration of
the inner solar system for the succeeding decade.
REPORT MENU
NOTICE SCIENCE OBJECTIVES
MEMBERSHIP
FOREWORD
Specifically, the committee established the following scientific objectives
SUMMARY
in the 1978 report for the inner planets:
CHAPTER 1
CHAPTER 2
CHAPTER 3
CHAPTER 4
CHAPTER 5 Mercury
CHAPTER 6
CHAPTER 7
Determine the chemical composition of the planet's surface on both a
CHAPTER 8
global and regional scale.
CHAPTER 9
REFERENCES
Determine the structure and state of the planet's interior.
Extend the coverage [to the entire planet] and improve the resolution
of orbital imaging.
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Assessment of Solar System Exploration Programs 1991 (Chapter 2)
Venus
Obtain a global map of the topography and morphology of [the
planet's] surface at sufficient resolution to allow identification of the gross
processes that have shaped it. [Obtain some images] of a limited number of
selected regions at a substantially higher resolution.
Determine the major chemical and mineralogical composition of the
surface material.
Determine the concentrations of photochemically active gases in the
65-135 km altitude region.
Investigate the physical and chemical interactions of the surface with
the atmosphere and study the composition and formation of atmospheric
aerosols.
Earth
Resolve the following fundamental questions:
1. Why the Earth alone possesses vast water oceans, and why the
Earth's atmosphere has a markedly different mass composition and evolutionary
sequence than either Venus or Mars;
2. Why the Earth and Venus, with nearly identical mass and diameter,
and why Mars with smaller diameter and bulk density, each had markedly
different thermal and tectonic histories;
3. What are the rates and mechanism of transport of materials from the
deep interior of the planets to their exteriors, and what are the chemical and
physical processes of exchange of matter between the interior and the crusts of
the planets and their atmospheres;
4. Why the Earth, Venus, and Mars possess very different internal
magnetic fields;
5. Why particular conditions on Earth have led to the evolution of living
organisms but apparently not on Venus or Mars;
6. How and why the atmospheric circulation and the long-term climatic
variations on Earth differ from those of her nearest neighbors;
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Assessment of Solar System Exploration Programs 1991 (Chapter 2)
7. How man and his culture will influence and modify the biosphere and
the physical and chemical composition of the Earth on a global scale.
Moon
Determine the chemistry of the lunar surface on both a global and
regional scale.
Determine the surface heat flow on both a global and a regional scale.
Determine the nature of any central metallic core in the Moon.
Mars
[Study local areas intensively] (a) to establish the chemical,
mineralogical, and petrological character of different components of the surface
material, representative of the known diversity of the planet; (b) to establish the
nature and chronology of the major surface forming processes; (c) to determine
the distribution, abundance, and sources and sinks of volatile materials, including
an assessment of the biological potential of the Martian environment, now and
during past epochs; (d) to establish the interaction of the surface material with the
atmosphere and its radiation environment.
Explore the structure and general circulation of the Martian
atmosphere.
Explore the structure and dynamics of Mars's interior.
Establish the nature of the Martian magnetic field and the character of
the upper atmosphere and its interaction with the solar wind.
Establish the global chemical and physical characteristics of the
Martian surface.
Fields and Particles
[Determine] the strength and character of the internal magnetic fields
(both global fields and, where possible, small-scale remanent fields).
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Assessment of Solar System Exploration Programs 1991 (Chapter 2)
Both cruise and orbiting phases of planetary missions should be
utilized to conduct appropriate interplanetary and solar measurements.
[Characterize] each planet's interaction with the solar wind.
CURRENT STATUS OF NASA'S
EXPLORATION OF THE INNER PLANETS
The committee recently completed a review of the 1978 document, which
was published as the 1990 Update to the Strategy for Exploration of the Inner
Planets (SSB, 1990b). Most of the scientific goals and objectives established in
the original strategy were found to be still relevant and unfulfilled. Rather than
establishing an entirely new science strategy, therefore, the committee left the
1978 strategy intact with the following modifications:
Mercury
The report (SSB, 1978) concluded that insertion of an appropriately
instrumented planetological payload into a relatively low-altitude, circular orbit
around Mercury required development of a low-thrust propulsion system. Such a
mission is now deemed feasible with conventional rocket launches and gravity
assists at Venus and Mercury. Therefore, a Mercury mission is a possible near-
term activity, and justification of such a mission should rest on the important role
of Mercury in understanding the origin and evolution of all the terrestrial planets.
In the 1978 report, exploration of Mercury's magnetosphere was
relegated to a secondary objective. One of the major unsolved problems in
geophysics is understanding how the Earth's geodynamo works; characterization
of Mercury's magnetic field should provide crucial insights and constraints on
dynamo theories. Because of the direct connections between Mercury's magnetic
field and the size and physical state of its core, determination of the multipole
structure of the planet's magnetic field should be a primary science objective
along with surface chemistry, internal structure, and imaging.
Venus
Essentially all of the strategy developed for Venus in the 1978 report
remains valid. The highest priority objective to provide a global map of the
surface of Venus at high spatial resolution is now being accomplished by the
Magellan mission. Results from missions by NASA's Pioneer Venus and the
Soviet Union's Venera spacecraft series raised some significant questions
pertaining to middle and lower atmospheric composition, including time variability
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Assessment of Solar System Exploration Programs 1991 (Chapter 2)
and the combined effects of dynamics and chemistry. Therefore, COMPLEX in its
1990 update recommended that "characterization of the basic structure,
composition, and dynamics of the Venus atmosphere be a primary objective."
The committee also elevated studies of the Venus interior to primary
status, together with studies of the surface and the atmosphere, so that
acquisition of seismic data is now a primary objective. The committee therefore
restated the recommendation of the 1978 report to emphasize that "acquisition of
seismic data from Venus should be maintained as a highly desirable goal," and
that "serious study of instruments operating at Venusian surface temperatures
should be undertaken and preliminary studies should be conducted to determine
the technical feasibility of sample return from Venus."
In addition, the committee noted that a more sensitive search for any
intrinsic magnetic field of Venus is of primary significance for determining the
nature of the planet's interior.
The Moon
In its 1990 update, the committee again endorsed the recommendations
in the 1978 report, and stated: "Measurement of the Moon's global chemical
composition remains a high priority, but the committee recommends that global
mineralogical measurements at high spatial and spectral resolution also be given
a high priority."
Mars
As described in the 1978 report, the detailed analysis of surface materials
involved samples to be collected from a region within easy reach of a landed
vehicle. The current scientific consensus is that such samples are no longer
considered adequate to address this objective. Rather, samples need to be
obtained from a variety of locations spread out over perhaps hundreds to
thousands of kilometers, and they need to be identified with sufficient information
to provide the geologic context for each sample.
Only determination of the whole-planet and major-unit chemical
composition at low spatial resolution from orbit is called out specifically in the
1978 statement of measurement requirements. Remote sensing instruments
available now or under development, however, permitted the committee to extend
this objective to include regional- and intermediate-scale surveys of surface
mineralogy and physical properties (e.g., density and grain size)-a particularly
important consideration for Mars, with its chemical and physical diversity at
various scales.
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Assessment of Solar System Exploration Programs 1991 (Chapter 2)
In 1989 the Soviet Phobos probe provided data on the Martian
atmosphere and surface, primarily in the equatorial regions, and on the Martian
charged-particle environment and its interaction with the solar wind. Because of
the short mission lifetime and the limited geographic extent of observations,
however, none of the major goals outlined in the 1978 report was addressed
completely.
The U.S. Mars Observer mission is a near-polar orbiting spacecraft
scheduled for launch in 1992, with global mapping in late 1993. Its goal is to map
the surface and atmosphere for an entire Martian year. If Mars Observer is
successful, the original COMPLEX objectives of establishing the nature of the
magnetic field and characterizing the global distribution of chemical and physical
characteristics. of the surface should be partly accomplished. In addition,
contributions will be made to the objectives of exploring atmospheric structure
and circulation, establishing the distribution of volatiles, and constraining the
planet's interior structure by means of topography and gravity data. Mars
Observer will also provide a base of data to guide the selection of sampling sites
for a Mars sample return mission and to identify resources to be used in potential
future human exploration of the planet.
The principal components of the 1978 report's objectives that will remain
largely unaddressed after a successful Mars Observer mission are the prime
objective of in situ (or remotely sampled) elemental, mineralogical, and
petrological studies of selected areas; seismological and precessional studies of
the interior structure of the planet; direct measurements of winds; and the
dynamical and chemical properties of the upper atmosphere and its interactions
with the solar wind. Also, the question of past life on Mars will remain open. If life
developed in the more clement ages on Mars, it may have left chemical and fossil
evidence. COMPLEX therefore recommended in its 1990 update report "that the
geochemical, isotopic, and paleontological study of Martian surface material for
evidence of previous living material be a prime objective of future in situ and
sample return missions."
Space Exploration Initiative
The administration and NASA are planning a major program called the
Space Exploration Initiative (SEI), which involves human exploration and
permanent habitation of the Moon and then Mars. COMPLEX is currently
contributing to a Space Studies Board report on the interaction of science
programs with the human exploration of space. That study addresses the
scientific information required to enable safe and effective human activities on the
Moon and Mars, and the opportunities that arise for planetary science as a part of
any program that prepares for, and carries out, prolonged human space
missions. These opportunities include, of course, those enhanced by the
presence of humans. Many of the same objectives for understanding the
environment of Mars and the Moon are shared by the scientific strategy for
planetary exploration and the concern for astronaut safety. The design and
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Assessment of Solar System Exploration Programs 1991 (Chapter 2)
implementation of future spacecraft missions to meet these combined objectives
should be a major goal as NASA carries out the Space Exploration Initiative.
In its 1990 update, however, the committee also made a recommendation
with regard to program balance. It urged that "exploration of the inner planets in
the next two decades should include further exploration of Mercury and Venus
because a program of planetary exploration that includes only Mars and the
Moon is scientifically inadequate."
Last update 12/12/00 at 3:34 pm
Site managed by Anne Simmons, Space Studies Board
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