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Review of NASA's Planned Mars Program
4
Key Issues for NASA's Mars Exploration Program
In the course of COMPLEX's examination of NASA's program for the exploration of
Mars, several issues arose. They are as follows:
The completion of global mapping,
q
Enhancing mobility,
q
The adaptability of the program,
q
The role of international partners,
q
The development of instrumentation, and
q
Sample return.
q
These issues are discussed in the following sections.
COMPLETE GLOBAL MAPPING
Various science advisory groups are consistent in emphasizing the need to
complete the global mapping originally planned for Mars Observer. Such mapping
will place our current martian data in a global context and should lead to major
revisions in our understanding of how the planet has evolved. It will accordingly
provide a general framework for planning future exploration. A prerequisite for
future studies of Mars is the reflight of those Mars Observer instruments not
scheduled to be carried by Mars Global Surveyor, at the earliest possible date (i.e.,
PMIRR in 1998 and GRS in 2001).
Completion of the objectives of Mars Observer does not necessarily imply that no
more remote sensing will be necessary. Discoveries by the program in the next 5
years may indicate the need for further orbital observations at a later time. At
present we have limited knowledge of the chemical, mineralogic, and lithologic
variety present at the martian surface, the scales over which such differences
occur, and the remote-sensing techniques best suited for their detection.
Hydrothermal deposits, for example, are of special interest for exobiology
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may be so localized as to not be detectable unambiguously by the 3-km resolution
of the TES instrument to be flown in 1996. Consequently, high-resolution remote
sensing data for selected areas and ground-truth measurements will be important
for understanding the global data sets. The medium-range strategy should
therefore be sufficiently flexible that it can adapt to new knowledge, and be
amenable, for example, to schedule additional remote-sensing missions should
they prove necessary.
ENHANCE MOBILITY
To address the main scientific goals of the Mars Surveyor program, mobility is
necessary at the landing sites. While three components of the planet are available
for measurement (the atmosphere, the loose material at the surface, and the solid
planet), just the first two are expected to be accessible at most sites. Thus only
some components of Mars can be effectively characterized without significant
mobility. The atmosphere and soils are expected to furnish important information
on the climatic history of the planet. The dynamics of the present atmosphere
supplies clues about the behavior of past atmospheres, while the atmosphere's
chemistry provides information on its original composition, losses from the upper
atmosphere, and interactions with the surface. Similarly, the soil may suggest
weathering conditions in the past and indicate how near-surface volatiles migrate.
Unfortunately, the atmosphere and the soil have serious drawbacks in terms of
contributing to understanding of biology and long-term climate. Since the records
contained in the atmosphere and soil are cumulative, with each sample
representing a single point on an evolutionary path, many paths could lead to
today's conditions. Thus any reconstruction of planetary history from such records
is ambiguous.
The rock record, in contrast, displays discrete events whose time sequence can be
reconstructed. Each rock unit chronicles a specific occurrence such as a volcanic
eruption, a flood, a lake's evaporation, or a large impact; from the ordering of the
units the events can be placed in a time sequence. A rock's lithology, mineralogy,
chemistry, and other characteristics give evidence of the prevailing conditions
under which the rock was deposited. Rocks are commonly preserved by deposition
and burial. Through this process, past atmospheres, organics, or hydrothermal
mineral assemblages may be insulated from subsequent surface conditions, thus
preserving them so that they may be available for scrutiny.
The solid rock record has the highest potential, therefore, for providing unequivocal
evidence for past life: the most favored targets are lacustrine sediments, ancient
hydrothermal deposits, and evaporites. Rocks can also indicate past climates
through fluvial deposits, trapped atmospheres, and weathering horizons. Finally,
the rock record supplies the best documentation for how the solid planet has
evolved. In order to address the scientific goals of the Mars Surveyor program,
therefore, the rock record must be accessed and, to accomplish that, mobility is
essential. Unlike the atmosphere and soil, the rock record is very heterogeneous
and scattered. The rock types of greatest interest for studies of water, climate, and
life are likely to be present at only a few locations.
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The remote-sensing data will suggest where best to go, but landers will need to
carry components that are able to move about and search areas of interest, rather
than be restricted to examining whatever happens to be close to the lander upon
its arrival. In addition, some sites of high interest for the study of environments
conducive to life, such as lacustrine/fluvial sediments and mineralized zones
around hydrothermal vents, are very localized and virtually impossible to target
with the existing navigational capabilities. Thus, in order to have a reasonable
chance of successfully addressing questions of climatological and biological
interest, the landers need to be equipped with rovers that move about, explore, and
sample candidate sites.
The distances required to move cannot be stated precisely without more
information on Mars, but in view of the heterogeneity of the martian surface, the
landing errors anticipated, and the prospects for future rover development,
mobilities of tens of kilometers appear necessary (see Table 1 for the
specifications of Sojourner, Marsokhod, and a strawman advanced rover). Such
rovers must carry appropriate instruments since the purpose of mobility is to make
measurements at different places away from the immediate landing site.
Development of a roving capability is, therefore, intimately connected with the need
for miniaturization of scientific instruments, discussed below. These mobility
requirements may be mitigated partly by improving the accuracy of landing;
nevertheless, even with a capability to do a pinpoint landing, some mobility will be
essential to sample a variety of sites.
A major issue for the Mars Surveyor program is that the small landers planned for
the next decade are likely to have very limited mobility. The 1998 lander, for
example, has a total payload mass of 22 kg. Owing to the need to accommodate
science instruments and devices for the manipulation of samples, little mass is left
to enable significant movement around or from the landing site. COMPLEX
considers this inadequate to meet the stated objective of accessing the information
contained in the martian rock record in a variety of terrains. 2
Because of the essential information provided by detailed analyses of rock
fragments at different sites, NASA must aggressively pursue ways to enhance the
mobility of landers and other vehicles in order to allow measurements to be made
on a variety of rocks and terrains. This development can evolve as lander designs
and technologies advance. International participation-for example through the use
of Russian launch vehicles and Marsokhod minirovers, and French balloons-would
also improve the opportunities for mobility on, or close to, the martian surface.
ASSURE AN ADAPTABLE PROGRAM
At present the Mars Surveyor program is highly structured, with guidelines calling
for pairs of launches at each launch opportunity, and for all U.S. launches to be on
Med-Lite vehicles. Because of launch energy constraints, any lander planned for
the 2001 launch opportunity is likely to be a downscaled version of the 1998
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lander, and the 2003 landers could, perhaps, be even smaller. The main
opportunities for adaptation to scientific findings and new concepts are in the
choice of landing sites and, possibly, improved instruments. Nevertheless, within
the guidelines various mission types can be accomplished, including small landers
as planned, balloon missions, and networks of miniature meteorology stations.
NASA's experience with Discovery proposals demonstrated that the space science
and engineering communities are extremely resourceful in designing a wide variety
of highly innovative and cost-competitive mission concepts. COMPLEX has
stressed the virtues of flexibility in small mission programs. 3 Given the relatively
long life of the Mars Surveyor program, advances in both technology and our
knowledge of the martian surface and atmosphere should permit revolutionary
changes in how program goals might be best achieved. In view of today's
incomplete understanding of Mars, plus the complex and dynamic nature of the
planet, it is highly desirable to maintain maximum flexibility in the program to allow
for adaptation to any new discoveries and unpredicted opportunities. The program
should be sufficiently flexible to incorporate such changes, in addition to the
incremental improvements envisaged for the small landers as they evolve from
1998 through 2003.
COMPLEX is concerned that strict adherence to the guidelines of multiple
launches at all opportunities may be too constraining. The planetary science
community accepts the necessity to stay within the mandated cost cap, but also
recognizes that different strategies may be followed within the cost constraint. The
current plan of having multiple flights during each launch window has the
advantage of distributing risk, thereby reducing the chance of complete failure at
any launch opportunity. However, in some circumstances, this benefit may be
offset by the scientific advantages of placing a single larger capability at Mars, as
mentioned in the preceding section, where mobility and scientific capability are
both seen to be required for effective landers. Under the present guidelines, it may
not be possible to place a long-range, well-instrumented rover on the surface of
Mars, particularly one equipped with, for example, sophisticated exobiologic
instruments. Sample return is another example: despite major advances in
determining how this long-time goal of Mars exploration might be accomplished at
modest cost, sample return still appears to need larger payloads at Mars than can
be delivered currently with Med-Lite launch vehicles.
How can the Mars Surveyor program retain the adaptability to take advantage of
scientific and technological opportunities as they become available? Ideally,
mission profiles should not be fixed earlier than two launch opportunities prior so
that advisory groups can monitor progress and propose necessary program
changes. NASA should investigate the realism of this suggestion and also study
the use of different permutations in the number of launches and launch vehicles.
ENGAGE INTERNATIONAL PARTNERS
At the present time NASA has three potential international partners for joint
missions to Mars: Japan, Russia, and the European Space Agency. In addition,
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many other partners are available for cooperation at lesser levels, such as
provision of science instruments. Russia is supplying some components of PMIRR
to be flown in 1998 and a lidar system for the 1998 polar lander. NASA is building
the neutral mass spectrometer for the Japanese mission Planet-B to be flown in
1998. Talks are under way concerning the possibility of a joint U.S.-Russian
mission that might involve the use of a Russian launch vehicle and a Marsokhod
minirover. ESA and NASA conducted a joint study of the Intermarsnet mission, for
which NASA would supply three or four small landers whose data would be relayed
to Earth via an ESA orbiter. These landers would form a small seismology and
meteorology network, as well as make geochemical measurements. The
meteorology network could be supplemented by a U.S.-only launch of very small
meteorology landers.
These cooperative missions provide an important means for substantially
improving the overall science return of the Mars Surveyor program and, more
importantly, for filling in important scientific gaps (e.g., global seismology and
aeronomy) in NASA's program. COMPLEX commends NASA for aggressively
pursuing these opportunities and encourages additional efforts to engage
international partners, both at the missions level and in mutual provision of other
capabilities such as relay links and science instruments.
DEVELOP MICROINSTRUMENTS
The success of the Mars Surveyor program beyond completion of the Mars
Observer goals will depend largely on the capability of the landers deployed on the
martian surface. In this regard, the program is seriously constrained by the
performance of the proposed Med-Lite vehicle. Expectations are that the useful
mass of NASA's Mars landers will decline substantially between 1996 and 2003. It
is possible, however, that reductions in spacecraft mass may be counterbalanced
by developments outside the Mars Surveyor program. In addition to Mars Surveyor
1998's pair of microlanders, the New Millennium program will, 4 for example,
develop and test new materials, avionics, and spacecraft designs that could lead to
significantly smaller spacecraft. Nevertheless, the ability to reduce the mass
needed to achieve a successful landing on Mars will have limited value to the Mars
Surveyor program unless accompanied by a parallel development of
microinstruments, that is, instruments significantly smaller than those currently
available. It serves little purpose to deliver small spacecraft to the surface of Mars
unless they have useful analytical capabilities.
Sophisticated microinstruments will be increasingly required as the Mars Surveyor
program matures. Not only will lander masses likely decline, but the relatively easy,
exploratory observations also will have been performed. Later landers are likely to
be smaller and yet be required to conduct more sophisticated observations. As
emphasized in the section above on mobility, the rock record must be examined to
enable progress toward understanding the climatic and exobiologic history of the
planet, and this effort will require significant mobility. In addition, to do anything
other than remote sensing, eventually we will need devices to acquire and prepare
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samples. All these capabilities must be included within the payload allocation.
Landers will need to measure the mineralogy of primary silicates and low-
temperature volatiles, and isotopic compositions of atmospheric gases as well as
of stable and radiogenic species in secondary minerals such as clays and
carbonates; to detect and characterize organic matter; and to determine various
trace elements. It will also be necessary to examine microscopically both in situ
and prepared samples, and to carry out various kinds of mineralogical and
chemical analyses under the microscope. Many analytical techniques require
sample preparation. The development of sample-acquisition and sample-
preparation techniques must therefore be a part of NASA's instrumentation
program.
Funding within the Mars Surveyor program itself is too limited to foster significant
advances in instrument designs. Although NASA does have a relevant program for
this, the Planetary Instrument Definition and Development Program (PIDDP), it is
relatively modest and concentrates on instruments that are likely to be chosen for
flight in the near term. Previous reports by COMPLEX 5 and other National
Research Council committees 6six have recommended that NASA devote more
attention to the provision of instruments for small-spacecraft missions. The long-
term development of microinstrumentation could be a part of, or independent of,
the New Millennium program.
MAINTAIN GOAL OF SAMPLE RETURN
Sample return has long been a goal of Mars exploration. 7 Many critical
measurements are too complicated and interactive to perform remotely on a distant
planet in the foreseeable future. Determinations of absolute ages, for example,
strain the capabilities of terrestrial laboratories yet are essential for understanding
how a planet has evolved. Only very recently have techniques been developed for
determination of D/H and oxygen isotopes on different temperature extracts from
Shergotty-Nakhla-Chassigny meteorites that are believed to be from Mars; these
measurements have proved invaluable in improving our understanding of the
evolution of water on the planet. Because trace elements are so strongly
fractionated in geologic processes, they are excellent markers of past processes.
Yet many of the determinations are very difficult measurements to make even in
sophisticated terrestrial laboratories. Microscopic techniques for detection of
Archean life on Earth have been developed only in the last two decades and would
be extraordinarily difficult to employ remotely at Mars. The challenge presented in
conducting these critically needed analyses on a remote planet is a primary reason
for requiring returned samples. Furthermore, the diverse nature of Mars argues
that samples will need to be returned from many sites, presumably necessitating
several flights or long-range rovers before a fairly complete inventory will have
been taken.
Sample return can also be justified for operational reasons. Spacecraft to Mars will
be able to carry only a limited number of instruments and do a restricted set of
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measurements. It is not possible, a priori, to identify the most critical
measurements to make. Having samples on Earth enables a wide range of
measurements to be made and permits adjustment of the measurement strategy in
response to previous analytical results. New techniques can be developed in
response to results obtained from the samples themselves.
Experience with the Moon emphasizes the enormous value of returned samples
when placed in the context of global data. The lunar sample data provide the basis
for almost all our ideas about how the Moon evolved. With lunar samples in hand,
the analytical approach became wide-ranging and flexible, such that the emphasis
could shift as the meaning of each set of results became clear. These advantages
should be even greater on Mars, given the more complex geology and the
possibility of past life.
The above ideas are not new. The Committee on Human Exploration of the Space
Studies Board recently stated that "to take best advantage of human capabilities in
scientific exploration, it will be desirable, some argue essential, to return
reconnaissance samples from Mars prior to human exploration." 8 The reasons for
this are those above-an improved knowledge of martian processes and history,
which "will permit a more informed choice of the landing sites . . . and the types of
investigations [to conduct]" 9nine-as well as concerns about planetary quarantine
and soil toxicity.
The projected costs of sample return and the launch masses required have both
declined dramatically in the last decade. Despite this, it is still not clear whether
sample return can be achieved within the mass and cost constraints placed on the
Mars Surveyor program. This is especially true if rock samples, as argued above,
are to be returned. COMPLEX is encouraged that NASA continues to retain
sample return as a goal and that the agency has recently indicated that the number
of launches and launch vehicle constraints originally placed on the Mars Surveyor
program may be relaxed provided that there are compelling reasons. COMPLEX re-
emphasizes the importance of returning samples of martian soil, atmosphere, and,
especially, rocks and commends NASA for retaining sample return as part of the
Mars Surveyor program. Taking this option, however, places the onus on NASA to
explore ways that will allow samples (including rocks) to be returned within the
constraints of the Mars Surveyor program. If studies show that this is not possible,
then Mars sample return will need to be considered as a stand-alone, high-priority
scientific program that must compete against NASA's other scientific goals.
REFERENCES
1. Exobiology Program Office, Exobiological Strategy for Mars Exploration, NASA,
Washington, D.C., 1995.
2. Space Studies Board, National Research Council, 1990 Update to Strategy for
Exploration of the Inner Planets, National Academy Press, Washington, D.C.,
1990, p. 22.
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3. Space Studies Board, National Research Council, The Role of Small Missions in
Planetary and Lunar Exploration, National Academy Press, Washington, D.C.,
1995.
4. E. Kane Casani, Exploration for the 21st Century: The New Millennium, Jet
Propulsion Laboratory, Pasadena, Calif., January 1995.
5. See, for example, Space Studies Board, National Research Council, The Role of
Small Missions in Planetary and Lunar Exploration, National Academy Press,
Washington, D.C., 1995, pp. 20 and 28.
6. See, for example, Space Studies Board, National Research Council, Managing
the Space Sciences, National Academy Press, Washington, D.C., 1995, pp. 3, 65-
66, and 75.
7. See, for example, Space Science Board, National Research Council, Strategy
for Exploration of the Inner Planets: 1977-1987, National Academy of Sciences,
Washington, D.C., 1978, and Space Studies Board, National Research Council, An
Integrated Strategy for the Planetary Sciences: 1995-2010, National Academy
Press, Washington, D.C., 1994.
8. Space Studies Board, National Research Council, Scientific Opportunities in the
Human Exploration of Space, National Academy Press, Washington, D.C., 1994, p.
13.
9. Space Studies Board, National Research Council, Scientific Opportunities in the
Human Exploration of Space, National Academy Press, Washington, D.C., 1994, p.
13.
.
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