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Assessment of Planned Scientific Content of the LGO, MAO, and NEAR Missions
Assessment of Planned Scientific
Content of the LGO, MAO,
and NEAR Missions
On May 14, 1986, Committee on Planetary and Lunar Exploration Chair
Robert O. Pepin sent the following letter to Dr. Geoffrey Briggs, director of
NASA's Solar System Exploration Division.
At its February, 1986 meeting, the Committee on Planetary and Lunar
Exploration (COMPLEX) made first assessments of three of the candidate
Planetary observer-class missions identified and recommended by NASA's Solar
System Exploration Committee (SSEC) in its 1983 report "Planetary Exploration
Through Year 2000: A Core Program": the Lunar Geoscience Observer (LGO),
the Mars Aeronomy Observer (MAO), and the Near Earth Asteroid Rendezvous
(NEAR) observer. These assessments, which include the science objectives, the
mission profiles, and engineering considerations, were made with the help of
presentations by representatives of the three mission Science Working Groups
and engineering study teams.
As you know, it is the practice of COMPLEX to assess the scientific
content of a mission as it nears proposal as a new start candidate, in order to
measure the response of the Agency, in a mission context, to the Committee's
science strategy. The conclusions of the assessment are a measure of the
support of the Committee and the Space Science Board for the proposed
planetary mission.
ASSESSMENT SUMMARY
The Committee finds that science planning at this stage of mission
strategy development for the three observer missions is proceeding within the
guidelines established by COMPLEX for inner planet exploration (LGO and MAO)
and for asteroidal reconnaissance and initial exploration (NEAR). The Committee
plans to make further assessments during the development periods of these
missions leading to launch, and at these times will consider the critical matter of
measurement capabilities of the selected instrumental packages in the science
payloads, as they relate to the measurement requirements specified in the
relevant COMPLEX strategy reports.
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Assessment of Planned Scientific Content of the LGO, MAO, and NEAR Missions
RECOMMENDATIONS OF THE COMPLEX STRATEGY REPORTS
Mars Aeronomy Observer (MAO) Mission
Lunar Geoscience Observer (LGO) Mission
The COMPLEX report "Strategy for Exploration of the Inner Planets: 1977-
1987" defines the measurement of the morphologic, physical, and chemical
character of Mars, Venus, Mercury, and the Moon on a global scale as a broad
exploration objective of high general scientific importance, and basic to all
planetological studies. Within this general objective, COMPLEX recommended
that the major thrust of inner solar system exploration in this decade should focus
on Earth, Mars, and Venus, with the goal of understanding the present state and
evolution of terrestrial planets with atmospheres. Mercury and the moon, both
atmosphere-free, were considered to be complementary bodies of high scientific
interest; in particular, the relative ease and economy of lunar investigations, and
their high scientific return, indicated that the moon must remain an important
object of exploration which should receive strong consideration during the
decade.
In this context of overall rationale and general goals, COMPLEX defined
the primary objectives for the continued exploration of Mars, in order of scientific
priority, to be:
1. The intensive study of local areas
a. to establish the chemical, mineralogical, and petrological
character of different components of 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;
2. To explore the structure and general circulation of the martian
atmosphere;
3. To explore the structure and dynamics of Mars's interior;
4. To establish the nature of the martian magnetic field and the character of
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Assessment of Planned Scientific Content of the LGO, MAO, and NEAR Missions
the upper atmosphere and its interactions with the solar wind; and
5. To establish the global chemical and physical characteristics of the
martian surface.
Considerations of exploration strategy led the Committee to conclude that
global and in situ studies of the planet and the return of martian material are
complementary components of an overall program of investigation, each of which
is separately necessary; and to recommend that detailed exploration, on both
global and local scales, of the diverse environments of Mars for purposes of
understanding surface, near-surface, and atmospheric processes is a worthy goal
in its own right and should be accomplished within the next decade.
The Committee recognizes, albeit with regret, that it has so far been
necessary to restrict the size and cost of proposed Mars missions. Both Mars
observer and Mars Aeronomy Observer, which fall in this class, will do excellent
science, but do not address the high priority scientific objectives for Mars
involving intensive study of local areas of the planet via in situ studies, and
detailed planning, at least, for the return of martian material. These will
necessarily continue to be the first-order science objectives in the next decadal
strategy for martian exploration.
For the Moon, COMPLEX defined the primary scientific objectives for
continued exploration by spacecraft in the period 1977-1987, in order of
importance, to be:
1. To determine the chemistry of the lunar surface on both global and
regional scales;
2. To determine the surface heat flow on both a global and a regional scale;
and
3. To determine the nature of any central metallic core in the Moon.
Additional secondary objectives of global lunar exploration were
recommended:
1. To map magnetic-field anomalies in the vicinity of the lunar surface, and
relate the anomalies to geological structure;
2. To measure gravity and altimetry for studies of isostasy and global
asymmetry of crustal structure; and
3. To search for possible volatiles frozen into cold traps at the lunar poles.
Near Earth Asteroid Rendezvous (HEAR) observer Mission
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Assessment of Planned Scientific Content of the LGO, MAO, and NEAR Missions
The COMPLEX report "Strategy for the Exploration of Primitive Solar-
system Bodies—Asteroids, Comets, and Meteoroids: 1980-1990" presents in
detail the science objectives, exploration strategy, and measurement
requirements for the first phases of primitive body investigations. To guide the
exploration of these bodies over an indefinite period, COMPLEX recommended
that the primary goal of their investigation be to determine their composition and
structure and to deduce their history in order to increase our knowledge of the
chemical and isotopic composition and physical state of the primitive solar
nebula, and to further our understanding of the condensation, accretion, and
evolutionary processes that occurred in various parts of the solar system before
and during planet formation. There were three additional goals: to determine the
diversity of composition and structure of primitive bodies; to understand the role
played by accretion of these bodies in the evolution of the crustal and
atmospheric composition and the crustal structure of the terrestrial planets; and,
unique to comets, to understand the dynamical processes responsible for the
production, maintenance, and behavior of the gas, dust, and plasma envelopes of
active comets. Within the framework of these goals, COMPLEX defined a
prioritized set of primary science objectives for asteroids and comets which, in
the opinion of the Committee, could be accomplished in a ten-year period of
exploration.
Asteroid science objectives, in priority order, are:
1. To determine composition and bulk density;
2. To investigate surface morphology and evidence for operation of
endogenic and exogenic processes; and
3. To determine internal properties of selected asteroids of diverse types.
COMPLEX noted that a full response to these science objectives requires
rendezvous-type investigations of several selected asteroids.
ASSESSMENT OF THE CANDIDATE OBSERVER MISSIONS
This section comments on the degree to which the recommendations of
the relevant COMPLEX strategy reports are followed by the more general
elements of the candidate Planetary observer missions—their rationales, science
objectives, general strategies, and timeliness. Until the science instrument
definition phase for each mission is completed, evaluation of the actual suite of
proposed measurements and their likely uncertainties cannot, be carried out with
confidence. The Committee intends to make and convey these assessments at
appropriate times, after briefings by the selected science teams.
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Assessment of Planned Scientific Content of the LGO, MAO, and NEAR Missions
Mars Aeronomy observer (MAO) Mission
The Committee finds that the scientific objectives and straw-man
instrumental payload to implement them are fully responsive to priority (4), and in
part to (2), in the prioritized list of primary science objectives recommended by
COMPLEX for continued exploration of Mars. The approved Mars Observer
mission, which focuses on investigation of the global surface composition of mars
and aspects of martian climatology, addresses priorities (5) and (2), and those
global and regional elements of priority (1) that can be fulfilled by measurements
from orbit using current instrumental technology. A timely follow-up of Mars
observer with the MAO mission offers the possibility of scientifically synergistic
overlap in mission measurements, leading to enhanced understanding of the
surface-atmosphere-upper atmosphere relationships.
Among the objectives of MAO, the understanding of the planet's
magnetosphere and interactions with the solar wind will fill the remaining gap
between mercury and, now, Uranus (including one or two comets) in this area of
study. mars may be unique in having, at the same time, a substantial atmosphere
and a magnetic field that is barely strong enough to stand off the solar wind. The
atmospheric measurements are well-conceived and should bring our
understanding to, or beyond, the level we now possess for Venus. Comparison of
these two very different C02 atmospheres should help us to understand their
similarities and differences, both with each other and with the Earth.
Lunar Geoscience observer (LGO) Mission
The Committee finds that the candidate LGO mission is in total accord
with the COMPLEX decadal strategy for continued lunar exploration. All of the
science objectives identified by COMPLEX in the 1977 strategy report-primary,
secondary, and at each level of priority—are fully addressed by the proposed
suite of multidisciplinary measurements, excepting only those aspects of local
chemical and mineralogical measurements (isotopic age determinations, high
precision petrochemical analysis, and petrographic study) that inherently require
return of material from unsampled areas of the moon, such as the far side.
The LGO science objectives are in general more sharply defined, and
address important scientific questions at higher levels of detail, than those set out
by COMPLEX ten years ago. The COMPLEX objectives were formulated from
consideration of broad and fundamental issues concerning lunar origin, evolution,
and present state. They remain valid as general recommendations for exploration
of the Moon, and will remain so until the scientific questions they intrinsically
address are answered. But the focus has sharpened during an intervening
decade of maturation of experimental and theoretical lunar research, and of
technological development in the types of measurement capabilities of spacecraft
instrumentation. An example is mineralogical mapping of the Moon. The
COMPLEX measurement requirement is for 100 km resolution, comparable to the
dimensions of major geologic features. For LGO, the spectroscopic and
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Assessment of Planned Scientific Content of the LGO, MAO, and NEAR Missions
multispectral mapping capabilities of the Visible and Infrared Mapping
Spectrometer (VIMS) will probably allow mineral species and the spacial extents
of distinct rock and soil types to be identified and mapped at 1/2 km surface
resolution. The surface mineralogy objective can thus be implemented not only
globally and regionally, but also at a level of local detail—e.g., crater wall rock
types—unenvisioned in 1977.
It is likely, judging from performance estimates for instruments in the LGO
straw-man manifest, that instruments selected for the science payload will be
capable of meeting or significantly exceeding most of the 1977 COMPLEX
measurement requirements. Even the most challenging of the primary science
objectives, determination of surface heat flow on global and regional scales to an
accuracy of 20%, may be possible using a suitably designed and calibrated
Microwave Radiometer (MRAD) experiment on LGO. The MRAD concept is
promising, but further instrumental development is needed to demonstrate its
capabilities. This measurement also requires the existence of a sufficient number
of suitable lunar surface regions, relatively smooth and with regolith depths
greater than 3-4 meters, for its implementation.
Because of current international interest in and planning for missions to
the Moon, the candidate LGO mission provides ample opportunity for
international coordination and cooperation. The Committee regards the benefits
derivable from such coordination to be mutually complementary and highly
positive elements of planetary exploration.
Near Earth Asteroid Rendezvous (NEAR) Observer Mission
Of the three primary science objectives for exploration of asteroids, the
Committee finds that the objectives for the NEAR mission address major aspects
of priorities (1) and (2) for a single asteroid, with some results possible for (3).
The planned long-term maneuvering and orbiting strategy of the mission will give
a much better measurement of density than flyby reconnaissance; it will allow
detailed mapping at few-meter resolution, with even more detailed imaging of
selected areas. This imaging capability is significantly better than the 50 m
mapping and 5 m footprint images recommended by COMPLEX. The spectral
mapper, gamma-ray, and X-ray instruments will yield good data on composition.
Future NEAR science payload studies will define more clearly the limits of
accuracy expected on elemental and mineral abundances: the COMPLEX
strategy calls for principal asteroidal elements to be measured ultimately to 0.5
atom percent accuracy, and major mineral concentrations to within a factor of 2.
A magnetometer, considered at present to be a possible valuable augmentation
of the straw-man NEAR payload, would be needed to meet the COMPLEX
recommendation of measuring global or local remanence to a level of a few
gammas.
The NEAR type observer-class mission fits well into the overall strategy
envisioned by COMPLEX for primitive body exploration. Several such missions,
or one with multiple rendezvous capabilities, would ultimately be required to
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Assessment of Planned Scientific Content of the LGO, MAO, and NEAR Missions
implement fully this initial phase of asteroidal science objectives. A single
rendezvous such as NEAR would be an important step in clarifying the nature of
the many objects that closely approach Earth. This group of several dozen
objects almost certainly includes "burnt-out" comets, as well as asteroids ejected
from the asteroid belt, and is currently being expanded by several objects per
year from present search programs. As more such bodies are discovered,
especially those in orbits With low energy requirements for mission exploration,
their importance as targets in unmanned and, eventually,manned spaceflight
programs is likely to rise. Ground-based observations have grouped asteroids
into distinct spectral classes, probably related to the several petrologic classes of
meteorites, but the cross-calibration between the two sets of objects remains
unknown. A timely asteroid rendezvous mission of the NEAR type to an object of
known spectral class would be a major advance in clarifying the nature of one
class and allowing a start on connecting meteorite and spectral classes. This
additional potential for enhancing and calibrating our gross ground-based
understanding is thus important in both scientific and programmatic terms, over
and above the general conformance of the NEAR science planning with the
recommended COMPLEX strategy and objectives for direct asteroid exploration.
The Committee would like to thank the representatives of the Science
Working Groups and engineering study teams for their presentations and
discussions with the Committee.
Please feel free to contact me with any questions you may have, or for
further discussion.
Last update 6/19/00 at 8:39 am
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