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OCR for page 16
Review of NASA's Planned Mars Program
3
Overview of Mars Surveyor and Other Mars Missions
In this chapter COMPLEX describes NASA's plans for the exploration of Mars by
its own spacecraft and by U.S. participation in international missions. Throughout
this description, COMPLEX points out how these planned missions contribute to
the scientific priorities for the study of Mars listed in COMPLEX's Integrated
Strategy. COMPLEX has already assessed the Mars Observer instruments
scheduled to be reflown on Mars Surveyor missions in 1996, 1998, and 2001. 1 It
has not quantitatively assessed the capabilities of the instrument payloads to fly on
later Mars Surveyor missions because, in general, they are not yet specified in
sufficient detail. Once these missions are well defined, instrument capabilities must
be assessed against previously stated requirements.
Mars Surveyor is a congressionally authorized program of Mars exploration that
will start with the 1996 launch of Mars Global Surveyor and that will last for at least
a decade. The program's funding is strictly capped at approximately $100 million
per year, with an additional annual sum of $20 million for operations and $36
million for launch vehicles. At the program's initiation, NASA placed a variety of
nonscientific constraints on Mars Surveyor 2 More recently these "constraints"
have become "guidelines." For example, NASA required that two launches must be
made at each orbital opportunity (which recur every 26 months) and must (after
1996) use the proposed Med-Lite launch vehicle, which will have approximately
half the capacity of the Deltas used to launch Mars Global Surveyor and Mars
Pathfinder. Now NASA states that "if there are compelling reasons, a single launch
... may be acceptable for particular opportunities." 3 In addition, other essential
components of the program include public outreach, support for education,
development of new technologies, and preparation of the way for eventual human
exploration. According to NASA, Mars Surveyor is to address three themes: life,
climate, and resources, where the third is currently understood to include the origin
and evolution of the solid planet. The role and history of water on Mars represent
the common thread that unites these topics.
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1996 LAUNCH OPPORTUNITY
NASA will send a pair of spacecraft to Mars during the 1996 launch opportunity
(Figure 1). The first, Mars Pathfinder, is not part of the Mars Surveyor program, but
rather one of the inaugural missions in NASA's Discovery series of low-cost,
focused science missions. The second mission, Mars Global Surveyor, is the initial
element of the Mars Surveyor program. Russia has also scheduled a flight to Mars
during this launch opportunity. Below COMPLEX describes each of these three
missions.
FIGURE 1 The relative sizes of past and future Mars orbiters and landers are
apparent in these sketches. Although the orbiters are all approximately the same
size, their masses are very different. Mars Observer's dry mass was approximately
twice that of Mars Global Surveyor and four times that of the Mars Surveyor
Program's (MSP's) 1998 orbiter. Similarly, the solar panels powering Mars
Pathfinder and the MSP '98 lander make them seem to be as large as the nuclear-
powered Viking. In reality, Viking's dry mass was approximately twice that of Mars
Pathfinder and the MSP '98 lander. Note that the orbiters are drawn at a different
scale from the landers. Illustration courtesy of the Jet Propulsion Laboratory.
Mars Pathfinder
Mars Pathfinder was originally conceived primarily as an engineering test to
develop an inexpensive entry, descent, and landing system for future Mars landers,
and to pioneer ways of doing planetary missions at significantly lower costs than
were typical of past endeavors. A secondary technical goal is to deliver and
operate the semi-autonomous, solar-powered minirover, Sojourner (see Box 2 and
Table 1 for definitions and specifications), to demonstrate the mobile deployment of
science instruments and to assess the effects of environmental conditions on the
minirover's performance. Despite its technological emphasis, Mars Pathfinder has
the potential to return significant, new scientific data.
TABLE 1 Characteristics of Sojourner, Marsokhod, and a Strawman Advanced
Minirover
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Marsokhod 1 Strawmaw/AdvancedMinirover 2
Soujourner
Characteristics
Wheel diameter 0.13 0.3 0.1
(m)
Range (m/day) <60 100s <1000
0.2 (<10 m from --
Position fixing ( 1000 (anywhere)
lander)
m)
Communications 3 (via lander) 10 to 20 (via orbiter) 10 (via orbiter)
(Mbits/day)
Rover landed 3 -- 3
mass fraction (%)
1 Based on information from NASA's Ames Research Center and on experiences
in field testing Russia's Marsokhod vehicles.
2 Based on specifications drawn up at NASA's Workshop on Mobility, held at Ames
Research Center, July 19-20, 1995.
The mission profile calls for Mars Pathfinder to be launched on a Delta II booster in
December 1996 and to follow a direct trajectory to Mars. Upon its arrival in July
1997, the spacecraft will decelerate through the upper atmosphere, followed by the
deployment of parachutes and the firing of retrorockets, for a soft landing. The
spacecraft will be protected within a cluster of airbags. The chosen landing site is
at the confluence of two large channels, Ares and Tiu Vallis, that drain from the
uplands south of the Chryse Planitia basin. After landing, the deflated airbags will
retract and the tetrahedral-shaped spacecraft will open to release its instruments,
solar panels, and Sojourner. Surface operations are scheduled to last a minimum
of 30 days, with a goal of 1 year. The lander, whose payload is only one-third as
massive as its Viking counterpart's, is equipped with a stereo camera (equipped
with 24 filters) and a meteorology station. In addition, Sojourner carries its own
imaging system and an alpha-proton-x-ray spectrometer (APXS) for chemical
analysis of rocks and soil.
Assessment of Mars Pathfinder
The science results will depend on what is visible at Pathfinder's landing site and
what is accessible for analysis. The imaging is likely to show details that will help to
elucidate erosion, transportation of rocky materials by large floods, and surface
modification by the wind. However, lack of descent imaging will hinder the analysis
of these data, especially for determining the geologic context of the site and for
providing a link to orbiter observations. The meteorology measurements will give
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an indication of year-to-year variability when compared with the Viking data, since
the proposed touchdown is fairly close to the Viking 1 landing site. Potentially the
most scientifically important result from Mars Pathfinder will be a better
understanding of the martian highlands (see immediately below), whose nature is
largely unknown.
The Ares and Tiu Vallis floods are likely to have deposited a mix of highland rocks
at the landing site. Crater ages indicate that the floods occurred in the first half of
Mars's history, after the end of heavy bombardment. Although their exact sources
will not be known, the highland rocks are probably an aggregation of primordial
crust (a product of global fractionation at the end of accretion), ancient volcanic
rocks, ancient sediments, and impact debris. Since the mixture is likely to have
been stirred to considerable depths owing to the high rates of impact eons ago
during the heavy bombardment phase, the rocks are anticipated to be
heterogeneous. Differences among the rocks are expected, and various rock types
may be present as clasts within the same rock. Chemical analyses of a few
samples derived from the highlands may therefore shed considerable light on a
variety of processes that were important during the early formative stages of the
planet. The one suspected highland rock that we have in our meteorite collection is
an orthopyroxenite, and this presents a puzzle with respect to the formation of the
highlands. We do not know, however, whether this rock's composition is typical of
the highlands or of the ancient crust as a whole. We also are unaware of the
precise provenance of this object, and this limits its usefulness.
In summary, Mars Pathfinder has the potential for improving our understanding of
the very early history of Mars, including global fractionation, possible late
acquisition of volatiles, and climatic conditions during heavy bombardment. It can
also provide new insights into the geomorphic and atmospheric processes
currently operating on the surface. However, the restricted instrument complement
and lack of significant mobility will limit Pathfinder's accomplishments.
Mars Global Surveyor
In November 1996, NASA will dispatch Mars Global Surveyor to the Red Planet to
recover much, but not all, of the science lost with the failure of Mars Observer. This
1050-kg spacecraft will be launched by a Delta II expendable booster and is
scheduled to arrive at Mars in September 1997. Unlike previous Mars orbiters,
Mars Global Surveyor will initially be placed in a highly elliptical orbit and then use
aerobraking over a 4-month period to modify gradually the orbit to one that is
nearly circular and Sun-synchronous (2:00 p.m., equator crossing time), with a
period of 118 minutes.
Mars Global Surveyor is only half as massive as Mars Observer and will carry
spare copies of five of its predecessor's seven instruments. These include the
Mars Orbiter Camera (MOC), Thermal Emission Spectrometer (TES), Mars Orbiter
Laser Altimeter (MOLA), Radio Science (RS) package, and the Magnetometer and
Electron Reflectometer (MAG/ER). In addition, Mars Global Surveyor will transport
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the so-called Mars Balloon Relay furnished by France's Centre National d'Études
Spatiales. The latter will provide a communications link between Earth and the
landers that are to be deployed by the Russian Mars '96 mission. The two Mars
Observer instruments not carried by Mars Global Surveyor are the Pressure
Modulator Infrared Radiometer (PMIRR) and the Gamma-Ray Spectrometer
(GRS); they are currently scheduled to be on later flights (see below). The original
Mars Observer instruments were designed so as to match or exceed COMPLEX's
measurement requirements. 4
Assessment of Mars Global Surveyor
The original scientific goals of Mars Observer were to understand better the
planet's climate, the nature of its interior, and the evolution of the martian surface.
These objectives, which are contained within the themes of the Mars Surveyor
program, were to be achieved by systematically monitoring the state of the
atmosphere; by global mapping of various properties (e.g., surface composition
and elevation), as well as the magnetic and gravitational fields; and by imaging the
surface at high resolution. These goals are central to the strategy for Mars
exploration espoused by COMPLEX, 5 and thus Mars Global Surveyor's 2-year-
long orbital mapping mission is crucial for the future exploration of Mars.
A detailed comparison of surface topography with the local gravitational and
magnetic fields will significantly improve understanding of crustal thickness
variations and thermal conditions in the upper mantle and how these conditions
have changed with time. Better knowledge of the global magnetic field will provide
information on the deep interior and the interactions of the planet with the solar
wind. Mapping of surface mineralogy will greatly enhance our knowledge not only
of primary processes such as global fractionation and volcanism, but also of the
secondary climate-sensitive mechanisms such as weathering, solution, and
possible carbonate deposition and evaporite formation. High-resolution imaging will
yield insights into surface phenomena (such as fluvial erosion, ice-abetted creep,
and sublimation) that leave an imprint at the meter scale. Mineralogic mapping and
imaging will be particularly relevant when choosing sites for future exploration on
the surface.
Russia's Mars '96
Russia has long had ambitious plans to explore Mars. At present its primary efforts
center on launching a large (2500-kg dry mass) spacecraft to Mars in November
1996. This project has been plagued by financial problems and political instability;
although the spacecraft bus has been constructed and instruments are being
delivered, it is still unclear whether this mission will fly.
The Mars '96 spacecraft has three main functions: to release two small (30-kg)
landers and two 45-kg penetrators to the surface, to act as a relay for data from the
landed vehicles, and to make science observations from orbit. The small landers
and penetrators carry instruments designed to image the surface, analyze the
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surface materials (using the NASA-supplied Mars Oxidant Experiment), and
monitor seismic and meteorologic activity. The orbiter will carry a wide array of
internationally furnished instruments, including two cameras, a near-infrared
mapping spectrometer, and a radar sounder.
Preliminary Assessment of Mars '96
The high-resolution camera on Mars '96 fills a resolution gap between the Viking
and Mars Global Surveyor (MGS) imaging, whereas the infrared spectrometer
observes a part of the spectrum not accessible to MGS; thus the Mars '96 remote-
sensing instruments are complementary to those of MGS and are important
components of the long-range exploration strategy. Many of the experiments on
Mars '96 measure plasma characteristics and the planet's aeronomy in order to
investigate the solar-wind interactions with Mars, an area not addressed by the
Surveyor program as currently planned.
The landers will provide surface data on rock and soil compositions for areas in
Mars's northern hemisphere, as well as providing seismic and meteorological data
for as many as four sites concurrently. However, the landing sites are not
appropriately arranged to fulfill the scientific goals of a meteorological or
geophysical network.
1998 LAUNCH OPPORTUNITY
Consistent with the desire to have multiple launches at each launch opportunity,
NASA plans to send two spacecraft to Mars during the 1998 launch opportunity, an
orbiter and a lander (see Figure 1). Both spacecraft will be approximately half as
massive as their counterparts from the 1996 window and will be launched using the
proposed Med-Lite booster. Japan has also scheduled a flight to Mars during this
opportunity. Below COMPLEX describes each of these three missions.
Mars Surveyor 1998 Orbiter and Lander
The 338-kg (dry mass) orbiter will be dispatched in December 1998 and will reach
Mars in September of the following year. This spacecraft will carry one of the two
Mars Observer instruments that remain to be flown, the Pressure Modulator
Infrared Radiometer (PMIRR). Some of the optical components of PMIRR will be
supplied by the Russian Space Agency. In addition to PMIRR, the spacecraft will
carry an integrated wide- and medium-angle imager (Mars Surveyor 1998 Orbiter
Color Camera) weighing 1 kg (for comparison, the cameras on Mars Observer and
Mars Global Surveyor are some 20 times more massive) and having a best
resolution of 40 m.
The lander component of Mars Surveyor 1998 will be launched in January 1999
and reach Mars some 11 months later. Some 10 days before the lander enters the
martian atmosphere, two 15-kg probes, so-called microlanders provided by
NASA's New Millennium program, will detach themselves and follow independent
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trajectories to Mars. Each of these microlanders will hard-land 1.5-kg penetrators
to a depth of approximately 0.2 to 2 m into the martian surface. Each penetrator
will carry accelerometers, temperature and pressure sensors, and a water-
detection experiment, and will have a lifetime of some 90 days.
Unlike Mars Pathfinder, Surveyor's main lander will not use airbags but, rather, a
descent propulsion system akin to that employed by Viking 1 and 2. The reason for
this change, its consequences for the Mars Surveyor program, and its impact on
important issues such as terminal hazard avoidance are beyond the scope of this
study. The 331-kg (dry mass) lander is equipped with a descent imager, a 1-kg
lidar system furnished by Russia's Space Research Institute, and an integrated
science payload designed to monitor the weather conditions and assess the nature
of the volatiles in the layered terrain on the northern edge of Mars's southern polar
cap. The surface science package, the Mars Volatile and Climate Surveyor,
consists of a mast-mounted stereo imager and meteorological instruments, a
thermal and evolved gas analyzer, and a 2-m-long robotic arm equipped with a
sampling device and a microscope camera. The lander is designed to survive for a
minimum of 86 days and, perhaps, an additional 60 before its batteries succumb to
the intense cold and incomplete illumination of its solar panels.
Preliminary Assessment of Mars Surveyor 1998
For a full martian year PMIRR will measure the atmosphere's temperature profile
and monitor its water vapor and dust content from the planet's surface to an
altitude of approximately 80 km. PMIRR's data will provide detailed information on
the distribution and exchange of water in the lower atmosphere and furnish
fundamental observations of the current meteorology and atmospheric dynamics.
For optimal utility, PMIRR and TES should fly on the same platform, but Mars
Observer's failure precluded this option. Even though the two instruments will not
now operate simultaneously, careful calibration may partially recover their
synergism. The wide-angle camera will provide useful information about diurnal
weather changes on Mars, and the medium-angle camera will record changes on
the surface resulting from atmospheric processes.
The lander will provide a variety of interesting new data because of both its
instrument complement and the nature of the proposed landing site. From an
altitude of approximately 8 km above the surface until touchdown, a descent
imager will supply continuous wide-angle views of the martian surface to provide a
context for the landforms visible from the landing site at the edge of the southern
polar cap. The terrain in this region is believed to consist of alternating layers of
dust and ice and is, thus, very different from that viewed by the Viking landers or
Mars Pathfinder's proposed landing site. Many have hypothesized that this layering
results from periodicities of Mars's climate; if this is true, the site is of great
scientific interest. Not only will this landing site afford the opportunity to obtain
measurements of the layered terrain, but the meteorology system will also provide
the first measurements of the atmosphere in both Mars's southern hemisphere and
its polar regions. The Russian lidar system will provide unique data on dust and
haze in the polar atmosphere.
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Japan's Planet-B
Japan's Planet-B spacecraft is also planned for launch to Mars in 1998. Scheduled
for liftoff in August 1998, this small, spin-stabilized spacecraft will spend 4 months
in Earth orbit before being placed into a trans-Mars orbit to arrive at Mars in
October 1999. It will carry an array of instruments designed to characterize the
upper atmosphere and the planet's interactions with the solar wind. A NASA-
supplied neutral mass spectrometer will be aboard. The elliptical orbit of the Planet-
B spacecraft is designed to allow in situ measurements of the compositions of
atmospheric ions and neutral species, and of magnetic fields and plasma waves, to
altitudes down to at least 150 km.
Preliminary Assessment of Planet-B
Planet-B's studies of Mars's upper atmosphere and its interaction with the solar
wind represent an important step toward filling a major gap in the Mars Surveyor
program as currently defined. In many respects this mission satisfies the stated
measurement and science goals of the Mars Aeronomy Observer (a candidate
follow-on to Mars Observer) mission. 6
2001 LAUNCH OPPORTUNITY
Missions during the 2001 launch opportunity are currently undefined and will
present a particular challenge because of the location of Mars along its elliptical
orbit. As a result, atypically high energies are required for orbit insertion.
Mars Surveyor 2001
Although their payloads may be smaller than those dispatched to Mars in 1996 and
1998, a suitably modified Med-Lite booster will be able to transport an updated
version of the Gamma-Ray Spectrometer (GRS), thereby completing the flight of all
of the Mars Observer instruments. The second launch during the 2001 launch
opportunity will likely be of a small lander, whose objectives are yet to be
determined. Another possibility is a cooperative mission with another partner such
as Russia, which has deferred the launching of a Marsokhod minirover (see Table
1 for specifications) and a balloon until at least this launch window.
Preliminary Assessment of Mars Surveyor 2001 Orbiter (with GRS)
The flight of GRS to Mars in 2001, if feasible, would be a significant achievement.
This instrument provides data that are essential to Mars Surveyor's goals of
studying life, climate, resources, and water. Its prime function is to determine the
chemical composition of the surface and investigate how that composition varies
from place to place. The bulk chemistry of the crust will yield important constraints
on how the crust, mantle, and core formed, while local and regional differences will
give clues about a wide variety of fractionation mechanisms, such as hydrothermal
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alteration and evaporation of lakes.
The GRS is also the only Mars Observer experiment capable of directly observing
water in the regolith and mapping its near-surface extent and temporal variations.
Accordingly, data from this experiment will also substantially improve our
understanding of the sources, sinks, and exchange processes of water in today's
climate. Because the GRS maps chemical variations across the surface, its data
are essential for choosing the landing sites of future missions.
2003 LAUNCH OPPORTUNITY
NASA currently has no defined mission plans for the 2003 launch opportunity. As
part of its Horizon 2000 Plus strategic plan, 7 the European Space Agency (ESA)
is examining the feasibility of using its new Ariane V launch vehicle to transport
three or four small landers and a communications relay orbiter to Mars in 2003.
The principal goal of this Intermarsnet mission is to establish a network of
simultaneously operating seismology stations. 8 Implementation of the mission is
contingent on NASA's providing the landers. ESA is to contribute the
communications orbiter and the launch vehicle. At the time of writing, the status of
Intermarsnet is in considerable doubt. Budgetary problems at ESA may force the
postponement of the mission (to later than 2003) or lead to its outright
nonselection. Moreover, the failure of Ariane V during its first test flight and the
consequent loss of its payload, the high-priority Cluster mission, have thrown ESA
planning into disarray. This launch window may be the best opportunity to deploy a
global network of simple meteorology stations with concurrent orbital sounding
because an orbiter is already included in the Intermarsnet program.
Preliminary assessments suggest that 15 to 20 microlanders could be
accommodated within the restrictions of a Med-Lite payload. It is not clear,
however, whether development costs of both microlanders for the NASA-only
mission and landers derived from the 1998 and 2001 designs for Intermarsnet can
be accommodated within the cost constraints of the Mars Surveyor program.
2005 LAUNCH OPPORTUNITY
Sample return has long been a major U.S. objective for Mars exploration.9 NASA
has designated it as a goal for the 2005 launch opportunity. Sample return
missions defined in the late-1980s called for multiple landers with masses of many
thousands of kilograms and requiring billion-dollar budgets. 10 However, despite
large reductions over the last several years in the expected costs and launch
masses required for sample return, sample return could be very difficult under the
present guidelines for the Mars Surveyor program (i.e., that spacecraft cost
approximately $100 million and have a mass of a few hundred kilograms or less).
Nevertheless, sample return's retention as a goal for a mission almost a decade
hence emphasizes its importance and the necessity of devising cheaper ways of
accomplishing it.
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If it turns out that indeed sample return cannot be accomplished within the Mars
Surveyor program, then it will be appropriate to reconsider the importance of
laboratory specimens for truly understanding Mars. If, as seems highly likely,
sample return continues to be of paramount interest, the nation will need to study
options for a larger program (e.g., $300 million to $400 million) that will return
samples.
Box 2 Rover Characteristics
Large rovers are highly sophisticated vehicles, powered by radioisotope
thermoelectric generators, that were considered in the context of various Mars
sample return and other mission concepts developed in the 1980s. 1-4 They are,
in general, quasi-autonomous and were conceived to be wide-ranging and
capable of operating independently of their landing vehicle.
Total mass5-400 to 1500 kg
q
Payload mass-35 to 150 kg
q
Range-0.1 to 10 km/day
q
Communications-Directly to Earth or via an orbiter
q
Lifetime-Hundreds of days to years
q
Minirovers are more modest, battery- and solar-powered vehicles developed in
the context of the austere Mars missions concepts that were devised in the early
1990s. The smaller vehicles in this category are, in general, dependent on their
landers for communications and cannot travel far from their landing site. Mars
Pathfinder's Sojourner and Russia's Marsokhod (see Table 1 for detailed
specifications) fall at, respectively, the lower and upper ends of this size category.
Total mass-10 to 70 kg
q
Payload mass-2 to 20 kg
q
Range-Tens of meters per day
q
Communications-Via lander or orbiter
q
Lifetime-Tens of days
q
Microrovers are vehicles smaller than Sojourner that were designed to be
compatible with the smaller landing vehicles baselined in the Mars Surveyor
program after Mars Pathfinder. Microrovers overlap with another family of
vehicles, the instrument deployment devices (IDDs). These are small
mechanisms designed to deploy a single instrument away from the parent lander.
While IDDs can be equipped with a primitive form of autonomous navigation, their
lifetime and ranges will be severely limited because they are so small that they
will have trouble surviving nighttime temperatures of ~180 K. Microrovers and
IDDs may be the sole size of vehicle permissible for a NASA-only Mars program
in the next decade.
Total mass-0.05 to 2 kg
q
Payload mass-0.01 to 0.5 kg
q
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Range-10 to 20 m
q
Communications-Via lander
q
Lifetime-<1 day
q
Nanorovers are the current technological frontier in rover design. Such devices
will probably be restricted to operating within view of a lander, and martian
temperature extremes will almost certainly limit their lifetimes to less than 1 day.
Total mass-<0.05 kg
q
1 National Aeronautics and Space Administration (NASA), Solar System
Exploration Committee, Planetary Exploration Through Year 2000: An
Augmented Program, NASA, Washington, D.C., 1986.
2 Science Applications International Corporation (SAIC), Planetary Missions
Performance Handbook, Volume IV: Mission Descriptions, SAIC-86/1853, SAIC,
Schaumburg, Illinois, August 1986.
3 John Niehoff, "Mars Rover/Sample Return Mission Overview," presentation to
Space Science Board Ad Hoc Study Group Feasibility Study of Joint Mars
Sample Return Mission, April 28, 1987.
4 European Space Agency (ESA), Mars Rover Mission: Interim Report of ESA
Science and Technology Definition Team, SCI(87)2, ESA, Paris, April 1987.
5 The boundaries of the various mass categories listed are, of course, arbitrary,
and the particular values shown were chosen because they reflect the masses of
actual or conceptual vehicles considered for Mars exploration in recent decades.
REFERENCES
1. Letter report regarding an assessment of the impact on integrated science return
from the 1992 Mars Observer mission, from the Committee on Planetary and Lunar
Exploration to Geoffrey A. Briggs (NASA), July 12, 1988.
2. Daniel J. McCleese, Mars Surveyor Program Scientist, presentation to
COMPLEX, February 9, 1995.
3. Office of Space Science, NASA Headquarters Guidelines: Mars Surveyor
Program, NASA, Washington, D.C., 1995.
4. Letter report regarding an assessment of the impact on integrated science return
from the 1992 Mars Observer mission, from the Committee on Planetary and Lunar
Exploration to Geoffrey A. Briggs (NASA), July 12, 1988.
5. Space Studies Board, National Research Council, An Integrated Strategy for the
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Planetary Sciences: 1995-2010, National Academy of Sciences, Washington, D.C.,
1994, pp. 103-105 and 191-193.
6. Mars Aeronomy Observer Science Working Team, Mars Aeronomy Observer:
Report of the Science Working Team, NASA Technical Memorandum 89202, Jet
Propulsion Laboratory, Pasadena, Calif., October 1, 1986.
7. European Space Agency (ESA), Horizon 2000 Plus: European Space Science in
the 21st Century, ESA SP-1180, ESTEC, Noordwijk, The Netherlands, November
1994.
8. European Space Agency (ESA), Intermarsnet: Report of Phase-A Study, ESA
Publication D/SCI(96)2, ESA, Paris, April 1996.
9. Space Science Board, National Research Council, Strategy for Exploration of
the Inner Planets: 1977-1987, National Academy of Sciences, Washington, D.C.,
1978.
10. See, for example, Science Applications International Corporation (SAIC),
Planetary Mission Performance Handbook, Volume IV: Mission Descriptions, SAIC-
86/1853, SAIC, Schamburg, Ill., August 1986, pp. 101-116.
.
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