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3
Planetary and Lunar Exploration
BACKGROUND
Our solar system consists of nine known planets orbiting the
Sun, and a large number of other objects: moons, asteroids, plan-
etary rings, and comets. Among the mysteries that have preoc-
cupied human thought throughout history are the mechanisms by
which the solar system came into existence, the laws and physical
processes that shape the evolution and behavior of planets, and
the relationship of the solar system to the wider cosmos. The same
questions continue to preoccupy modern planetary science as well.
Planetary studies illuminate some of the deepest and longest-
standing scientific questions. Moreover, from a human perspective,
planetary studies have additional significance. Planets are likely
to be the only bodies in the universe capable of supporting ad-
vanced life. Among its other objectives, planetary science seeks
to understand the formation of life-supporting planets and the
conditions under which life arises and develops. The answers to
these questions will shape our perceptions about our origins and
our situation in the universe.
15
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GOALS OF PLANETARY EXP[O1lATION
The scientific goals motivating planetary exploration are:
.
To understand how the solar system originated;
. To understand how the planets evolved, including Earth
and the planetary satellites, and to understand their present states;
~ To learn what conditions led to the origin of life;
.
To learn how physical laws work In large systems.
Each of these goals is explored below.
To understand hour the solar system originated. Research aimed
at understanding the origin of the solar system focuses largely on
those objects thought to retain clues about the primordial con-
ditions and processes that attended the system's formation. The
most detailed clues come from investigations of comets, asteroids,
and meteorites—small primitive objects that have changed little
since their formation in the protoplanetary nebula.
The cold, volatile-rich matter of comets is thought to contain
the most faithfully preserved samples of condensecl protoplanetary
material remaining in the solar system. The asteroids form an or-
dered assemblage of protoplanetary fragments that seem to remain
near the locations of their original formation. They are thought
to reflect the radial variation of conditions in the protoplanetary
nebula. Laboratory analyses of meteorite fragments of asteroids
and comets show the importance of the information these objects
can provide. Detailed study of comets and asteroids is expected
to fundamentally advance our understanding of the solar system's
formation.
Planetary systems are believed to occur commonly in the uni-
verse as a result of the same processes that formed our own solar
system. Failure to find such systems would force a fundamental
revision of our theories about the origin of this planetary system
and about star formation. Studies of star-forming regions and the
discovery and study of other planetary systems will likely precipi-
tate important advances in our understanding of the formation of
the solar system, and in our understanding of planetary systems
as a class.
To understand how the planets evolved. Because we live on Earth,
a terrestrial planet, the evolution and environment of terrestrial
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planets Is of special interest. Substantial advances in understa~-
ing can be realized by investigating, as a class, the terrestrial plan-
ets Mercury, Venus, Earth, and Mars, and other close analogs. In
addition, studies of many of the outer-planet satellites and of the
largest asteroids should reveal important information about solid
planet evolution. Much of what we know of the terrestrial plan-
ets derives from ideas and concepts that originated in studies of
Earth. Conversely, planetary investigations of objects that evolved
under conditions far different from those on Earth may prod us to
seek a deeper grasp of natural terrestrial phenomena, as well as
a more complete understanding of Earth's history. By exposing
circumstances in which concepts based on terrestrial analogs fail,
planetary investigations help us define the limits of applicability
of these Earth-based ideas.
As the world population increases and stresses the ability of
our environment to accommodate it, terrestrial scientists will be
called upon to mode] environmental impacts and to help develop
tradeoffs between urgent resource needs and the consequences
of meeting those needs. Models that can predict the properties
of the widely varying atmospheres of the terrestrial planets will
make this job much easier and enhance the credibility of scientists'
pronouncements about this planet.
To learn what conditions ted to the origin of life. Earth remains
the only realm in which we know life has arisen. Our search to
understand the origin of life involves several planetary questions:
what are the physical conditions under which life arose, and have
living organisms, either incipient or well-developed, arisen in other
places where they can be studied? Presumably, living organisms
arose out of an organic, prebiotic medium and were preceded by
an intervad of chemical evolution, which led more or less contir~-
uously into biological evolution. By understanding the formation
of the planets, we will come to know the circumstances under
which life arose on Earth. Many objects In the solar system seem
not to have undergone substantial evolution since their formation.
Some Saturn's moon Titan, for example probably carry impor-
tant clues about the early material in which life arose. These
and other objects in the solar system including Mars- may have
had prebiotic chemical species or harbored forms of incipient life,
leaving evidence we can still collect.
-
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Investigations of the composition of cosmic matter and pr~m-
itive solar system matter show that the basic building blocks of
terrestrial life, including amino acids, occur naturally, at least in
trace amounts. One of the greatest challenges in understanding the
origin and distribution of life is to determine just how widespread
biological evolution may be in the cosmos. An important aspect of
this question is the degree to which special terrestrial conditions
were involved In preb~otic chem~c~ evolution. Detailed chemical
assays of comets, asteroids, and other primitive objects will reveal
the extent to which life could have arisen directly from preplane-
tazy matter without an interval of special processing to condition
the chem~c~ mix. This will provide important clues as to the
possible ubiquity of biological evolution. (For a discussion on the
origin of life from an exobiology perspective, see Chapter 7.)
To learn how physical laws work in large systems. Various phi
nomena are the unique result of the large scale of natural systems
or arise from the very long times over which slow processes work.
Investigation of large-scale physical processes involves virtually
all of the objects in the solar system. The giant planets provide
clues about properties of matter under high pressures; planetary
interiors and magnetospheres demonstrate the curious behaviors
of magnetized fluids and plasmas; Ad planetary atmospheres and
surfaces present puzzles about the long-term evolution of complex
interacting systems that constitute planetary environments and
interiors.
Because these phenomena do not occur under normal labo-
ratory conditions, it Is only through direct observations In the
solar system that we can understand them. Our current theories
of planetary tectonism and cosrn~c plasma processes, for example,
have developed in this manner. Since there is little prospect that in
the foreseeable future we will make In situ measurements In other
planetary systems, detailed investigations withm our own solar
system will continue to be the foundation upon which we build
much of our understanding of natural phenomena throughout the
universe.
AClill§Vl:MENTS OF PLANETARY EXPLORATION
In the past 20 years or so most of the planets have been visited
and several have been explored In some detail. A few highlights
will be mentioned here.
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The same physical processes operate on all the planets, but
different starting conditions have led to a remarkable diversity of
present states. This diversity is also influenced by violent colli-
sions. The first recognized effect was cratering, prominent on the
Moon, Mercury, and parts of Mars, where the largest basins and
craters represent effects produced until accretion ended about 3.7
billion years ago. Craters are prominent on many of the moons
of the outer planets, and a few of these moons seem to have been
shattered by even larger impacts and then reformed. On Earth,
geological processes have obliterated Al but a few relatively recent
craters, including one from an impact that may have caused mass
extinctions at the end of the Cretaceous era some 65 minion years
ago. Recent theories explain the anomalous densities of Moon
(Iow) and Mercury (high) in terms of enormous collisions with
molten protoplanets ~ which the iron had already sunk to the
center.
Great climatic change has been inferred for Mars, where abun-
dant water once flowed, and for Venus, which may once have had
the equivalent of a terrestrial ocean. Abundances of noble gases
are remarkably different on Venus, Earth, and Mars, and differ-
ent again on the parent bodies of meteorites. Ring systems are
now known around all four of the giant planets, Jupiter, Saturn,
Uranus, and Neptune; each ring system is totally different, and ev-
idence is accumulating that some, at least, are transient. Jupiter's
moon To ~ the seat of many simultaneous volcanoes that shoot
sulfur dioxide far above the surface. The sulfur and oxygen ream
pear as ions in a plasma torus enveloping To's orbit. There they
emit enormous amounts of ultraviolet radiation. More energetic
ions populate the entire Jovian magnetosphere and dominate much
of its behavior. Saturn's large moon Titan is a terrestrial planet
in many ways, but made of materiab characteristic of the outer
solar system. The dense nitrogen atmosphere contains methane
clouds and a dark organic haze. A global ocean of liquid ethane
is predicted. This variety of organic matter gives an environment
analogous to what may have existed on a prebiotic Earth.
There is little doubt that more surprises and new concepts are
still awaiting us and that the future of solar system exploration
will be as rich as its past.
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FUTURE PLAN1:TARY EXPI`O"TION A BALANCED
PLANETARY PROGRAM
Progress toward realizing these goals requires a balanced pro-
gram of basic science and exploration that includes studies of all
planets, the Moon, and select asteroids and comets in our solar
system. Concurrently, astronomical observations of star-forming
regions and other planetary systems should be made. It is not
safe to exclude parts of the system from study; experience has
taught us to expect the unexpected. The mitial exploration of
Al the planets, except Pluto, wiD be complete when Voyager flies
by Neptune in August 1989. The next step is a more detailed
examination of the planets to dissect the processes at work there.
So far, we have intensively studied only the Moon, Venus,
Mars, and Jupiter. Beyond 1995, planetary exploration will shift
increasingly toward orbiters, atmospheric probes, landers, sample
returns, and perhaps manned exploration the type of research
required for a more complete understanding of the solar system.
To complement these in situ investigations we will require labora-
tory experunentation and theoretical analysis as well. Data from
spacecraft are largely responsible for the rapid advance In our view
of the solar system. The steering group envisions that spacecraft
investigations will continue to play this primary role.
Prospective pr~l995 Missions
Several projects that are now ready for launch or under d~
velopment will set the stage for the vigorous solar system science
and exploration program in the early years of the twenty-first cen-
fury. Magelian will carry a radar system to map almost all the
surface structure of Venus at a resolution of 300 m. Resolution
of this quality wiD provide information key to comprehending the
variety of evolutionary histories and processes undergone by the
terrestrial planets. The Mars Observer mission will carry out the
first global geocherrucal analysis of the martian surface and will
investigate some properties of the planet's atmosphere. The Lunar
Geoscience Orbiter will carry out a similar survey of the Moon.
The Galileo probe will carry instruments deep into Jupiter's
atmosphere to measure its composition and physical structure.
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The orbiter will also perform synoptic observations of the atmo-
spheric dynamics, conduct a detailed investigation of the magneto-
sphere, and obtain detailed unages and spectra of numerous Jovian
satellites. The Comet Rendezvous Asteroid Flyby wall conduct de-
tailed In situ investigations of a comet nucleus and will probe the
physics of the comet-solar wmd interaction. The detailed comet
nucleus measurement win, hopefully, provide information about
the conditions under which protoplanetary matter accumulated in
the solar nebula.
The Soviet Union is expected to carry out major investigations
of Mars and its satellite Phobos. This ambitious project is to
include a new generation of analytical instruments for analysis
of the surface composition of Phobos. In addition, the USSR is
expected to carry out an asteroid rendezvous mission, involving a
flyby of either Mars or Venus (probably the former). The Soviets
are also considering a lunar polar orbiter mission, which would
investigate the Moon's global chemical and mineral composition,
magnetic fields, and temperatures.
Recommended Program: Post-1995
Over the Midyear interval from 1995 to 2015, the recommended
program encompasses investigations of all of the major planetary
bodies in the solar system along with selected satellites and prim-
itive objects.
Terrestrial Planets
Lanclers, rovers, selected sample returns, and networks of au-
tomated observation stations on planetary surfaces wiD be the
primary systems used to study the terrestrial planets. Specialized
surface Angers and rovers would allow the exploration of varied
terrains and the surface material analyses that are necessary to
ascertain the evolutionary histories of the planets. Analysis of
selected samples returned to Earth will help us to deterrn~ne both
the character and the absolute dates of many of the major evolu-
tionary events on the terrestrial planets.
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Recommended Missions
1. Mercury:
2. Venus:
a. Orbiter
b. Surface Landers/Sensor Network
a. Atmospheric Probe
b. Surface Landers/Sensor Network
c. Sample Return
3. Moon:
a. Surface Landers/Sensor Network
b. Scientific Rover
c. Sample Return
4. Mars:
a. Surface Landers/Sensor Network
b. Scientific Rovers
c. Sample Return
Outer Planets and Satellites
Each of the major outer planets Jupiter, Saturn, Uranus,
and Neptune presents a complex, ordered system including the
planets themselves, a magnetosphere, and a family of satellites
and rings. Studies of the outer planets should include orbiting
spacecraft to investigate all of these aspects. Atmospheric entry
probes will reveal Formation critical to determining the compm
sition and evolution of those planets. ~ situ studies of selected
satellites will collect Formation pertaining to the primordial state
of the volatile and organic matter in the solar system, and may
yield clues about prebiotic chemical evolution.
Recommended Missions
I. Jupiter:
a. Magnetospheric Polar Orbiter
b. Deep Atmospheric Probe
c. lo Lander
2. Saturn:
a. Orbiter and Atmospheric Probe
b. Deep Atmospheric Probe
c. Ring Rendezvous
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d. Titan Orbiter and Probe
e. Titan Lander (or floater)
3. Uranus:
a. Orbiter and Atmospheric Probe
b. Deep Atmospheric Probe
4. Neptune:
a. Orbiter and Atmospheric Probe
5. Pluto:
a. Orbiter
Primitive Bodies
~ situ studies of the prirn~tive bodies began with the missions
to HaIley's Comet. We wall need rendezvous missions to other
comets and asteroids to select the objects and the ~nstrumenta-
tion to be used in later detailed studies. Investigations following
rendezvous of a selected set of asteroids will allow us to determine
their compositions and structures, as wed as the variation of these
properties in the main asteroid belt. We wild thus obtain insights
into processes in the protoplanetary nebula and explore early evo-
lutionary mechanisms. ~ situ studies of comets and small outer
solar system objects will permit analysis of the most complete an<]
best-preserved samples of primitive matter, yielding clues to the
origin of the solar system and of life. Return of samples from se-
lected primitive bodies wiD allow the in-depth laboratory analysis
possible only on Earth to contribute to this effort.
Recommended Missions
I. Comets:
a. Coma Sample Return
b. Nucleus Rendezvous and Sample Return
2. Asteroids:
a. Multiple Rendezvous
b. Sample Returns
Other Planetary Systems
Discovering and studying other planetary systems require the
use of advanced telescopes in space. Planets disturb the motion of
their central stars, and the evidence of these disturbances can be
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found by measuring the positions and motions of stars. In addition,
such measurements can give information about the masses and
orbits of the surrounding planets—information that can provide
critical tests of our ideas about the formation of planetary systems
and stars. The steering group recommends the development of
specialized telescopes capable of detecting planets at least as small
as Uranus and Neptune around a large number of nearby stars. The
technology on which these telescopes depend Is now within reach
for use in space; the use of such telescopes in association with the
Space Station would permit an observing program sufficiently long
to allow the search for and study of planetary systems around a
large number of nearby stars. Once other planetary systems have
been discovered, there will be strong incentive to develop more
sensitive instruments for further studies.
Recommended Missions
1. Space Astrometric Telescope
A Mars Focus
A Mars-focused program is recommended in parallel with the
general program outlined above. However, this Mars-based pro-
gram is not a substitute for a broader, balanced program of plan-
etary exploration. There is no reason to expect that studying one
or two planets in depth wall allow us to understand how the entire
solar system originated and evolved.
Planets and their environments exhibit behavior that, for fun-
damental reasons, cannot be predicted from first principles. The
complexity of planetary environments is such that ~ planet can,
in principle, exist in a large variety of states with the same con-
ditions imposed from outside. The possible presence of living
organisms further extends the variety of states in which a planet
can persist. Fmally, accidents of evolution can affect the state of
a planet profoundly. A major challenge of planetary science is to
trace the evolution of terrestrial planets, and enumerate the pow
sible varieties and causes of their diverse environments. Meeting
this challenge wiD require comparative studies of the terrestrial
planets, including detailed studies of the changes that individual
planetary environments undergo.
Of particular interest in the comparison of terrestrial planets Is
the puzzle posed by the triad of planets with atmospheres: Venus,
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Earth, and Mars. The differences ~ their present environments
and in their styles of evolution seem large in comparison with
the differences in their sizes, locations, and overall compositions.
Solving this puzzle is important to us because the differences
between these planets occur in those aspects of their environments
key to sustaining life.
Spacecraft investigations of Mars during the past 15 years re-
veal that the planet has undergone perplexing changes throughout
its history. Although its surface is now dry and cold, there is
clear evidence of a sustained, abundant flow of water during tunes
past. Such changes in the martian environment directly pertain
to long-term concerns about the behavior of Earth's environment.
The prior presence of water on Mans raises important questions
about its early, if temporary, suitability for life.
Images returned to Earth by Mariner and Viking spacecraft
reveal spectacular geographical formations and dee~cut relief. It
is evident that detailed study of the martian surface will yield in-
formation about the character of the planet's early environments,
their arrangement in time, and perhaps clues as to the influences
that produced such marked environmental change.
Of all the planets beyond Earth, Mars is the one most access
sible to detailed study. It is relatively easy to reach with scientific
spacecraft, and the surface is the most conducive to sustained oper-
ation of scientific instruments on mobile platforms. Furthermore,
Mars is the only planet outside of the Earth-Moon system that we
can currently consider for manned exploration and settlement.
Recommended Missions
1. Network o\f Geophysical Stations
2. Rover for Geology and Geochemistry
3. Sample Returns
4. Possible Human Exploration (The issue of the role of hu-
mans is discussed in a separate section of this report.)
CONCLUSION
The recommendations put forward here, if implemented, will
advance our understanding of the solar system on the broad front
that is needed to progress toward answering some of mankind's
long-standing questions about the cosmos. A recommended Mans
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he
~0
Ecus Cabin tab bro~-b~ed program w1D tartar our under
strung of the tersest planets] ~cludlug Earth, ~~ wD1 a~
dress preslug ~estlons about planetary exponents ad tbeL
salty. Tbe recommended investigations wO1 Ego prowls the 1~
airman needed tar proper plug of later waned exploratory
Salons to the Coon and planets.
Representative terms from entire chapter:
atmospheric probe
