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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.
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
Characteristics Soujourner Marsokhod 1 Strawmaw/AdvancedMinirover 2 Wheel diameter 0.13 0.3 0.1 (m) Range (m/day) <60 100s <1000 Position fixing ( 0.2 (<10 m from -- 1000 (anywhere) m) lander) 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
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
the so-called Mars Balloon Relay furnished by France's Centre National d'Ãtudes 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
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
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.
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
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.
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. q 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 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. q 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 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. q Total mass-0.05 to 2 kg q Payload mass-0.01 to 0.5 kg
q Range-10 to 20 m q Communications-Via lander q Lifetime-<1 day 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. q Total mass-<0.05 kg 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