A Scientific Rationale for Mobility in Planetary Environments (1999)

The most important conclusion from this study is that mobility is not just important for solar system exploration—it is essential. Many of the most significant and exciting goals spelled out in numerous NASA and NRC documents simply cannot be met without mobile platforms of some type. To what degree is this basic conclusion dependent on the selection of the six case studies? To gauge this, COMPLEX considered an independent set of case studies, the portrait missions addressing the campaigns described in NASA's Mission to the Solar System: Exploration and Discovery¹ (Table 4.1). Achievement of four of the five campaigns contained in the NASA report depended critically on the use of mobility in the form of rovers, balloons, and robotic arms. Given the results of this independent check, COMPLEX is confident in the robustness of the conclusion that the use of some form of mobility is an essential feature of future solar system exploration missions.

A second conclusion is that the diversity of planetary environments that must be explored to address priority scientific questions requires more than one type of mobile platform. Thus, the simultaneous development of some combination of wheeled rovers, aerobots, aircraft, touch-and-go orbiters, and cryobots is not only justified but is also necessary, as long as there is a scientific justification for the development of each mobile platform. Technology development funds are likely to be scarce and so should be allocated only after a vigorous peer review of the proposed mobility device's technical feasibility and the scientific applications for which it will be used. As the Space Studies Board has previously recommended, technology development activities should be undertaken by the best-qualified individuals and teams within NASA, industry, and academia, as determined by peer review.²

With some exceptions, the current technical development efforts are appropriate and well focused. However, it is instructive to compare the tenor of recommendations in science-oriented presentations and of science-centered working groups with the thrust of technical development efforts. The science sources emphasize the need for very capable mobile platforms with the following characteristics:

• Synergy of instruments, that is, a suite of mutually complementary instruments rather than either a small number of instruments or many instruments that are independently conceived and developed;
• Extensive range and long lifetime; and
• One or more manipulative devices, such as claws, drills, and the like, some of which are likely to be complex and difficult to develop.

These characteristics define a mobile platform that is fairly large and potentially rather complex. In contrast,

the main thrusts of technical development, especially of rovers, are directed at reducing their size and increasing their autonomy. If size reduction also results in a corresponding reduction in range or other capabilities, it will, potentially, have a significant scientific impact. This is so because it creates a capability to make scientific measurements on a scale size that is not necessarily optimal for addressing the scientific questions to be answered.

The pattern of planetary exploration to date has been to make basic observations of planetary surfaces from orbiters and to establish hypotheses for interpreting these observations. These hypotheses are then tested by more directed observations and measurements. Because the hypotheses are based on orbital images with a relatively low characteristic resolution, this suggests that long-range traverses are required to test the relevant hypotheses. However, the focus of technical developments appears to be to create mobility systems capable of producing very detailed, but limited, data sets about very small areas. Thus, we run the danger of creating a technical capability to address scientific issues that might not, necessarily, relate to the framework of scientific questions and issues

developed as a result of prior studies. Reconciling these apparently contradictory priorities and minimizing their impact on the scientific productivity of mobility missions will require close cooperation between engineers and scientists.

Most science objectives defined for future solar system missions call for mobile platforms, manipulative devices, and instruments with significant capabilities. Attaining this level of capability will require reducing the total mass of mobile platforms while maintaining acceptable functional capabilities. The size of a mobile platform needs to be considered as part of a systems optimization based on scientific needs and mission constraints. Although very small mobile systems, such as the micro- and nanorovers currently under development, involve a significant reduction in mass, their payload capacity may be too limited for widespread application unless particular attention is paid to the development of appropriate micro- and nano-instrumentation.

Long-range mobility, whether with rovers, aerobots, or other devices, poses significant navigational challenges. This is in part due to the constraints imposed by long, two-way communication times and in part to the limited data downlink capacity available. The more time and downlink capacity are used for navigation, the less available they will be for returning scientific data. Lessons learned in the Marsokhod field tests and during the operation of Sojourner suggest that descent imagers should be included on lander and rover missions to provide critical information on the context of the landing site for use in rover navigation and science-operations planning. Navigational tools for long-range mobility should be available in as near real time as feasible. The hardware and software for intelligent autonomous operation and efficient operational planning should be actively developed.

Many planned and possible future missions will require spacecraft and mobility devices to operate in hostile environments. An environment can be hostile because of the high levels of radiation (e.g., the surface of Europa), high pressure (e.g., the atmospheres of the giant planets), high temperatures (e.g., the lower atmosphere of Venus), low temperatures (e.g., the surface of Titan), and very low gravity (e.g., the surfaces of comets and asteroids). Such environments place unusual constraints on spacecraft and instruments, indicating the need for long-range advanced planning and development.

These conclusions suggest two fundamental recommendations:

• Technological development of mobile platforms must be science driven. Available funds will never be adequate to develop all possible types and variants of platforms, and these scarce funds should not be wasted on devices of limited scientific utility no matter how technologically intriguing they may be. Thus, there should be science input into technology development from the very beginning.
• Mobile platforms, ancillary devices, instruments, and operational procedures must be thoroughly tested on Earth. This involves laboratory tests of instruments, field trials of individual components of space missions, and field trials of complete systems (mobile platform and instruments) and all relevant personnel (operators, design engineers, and scientists). To be fully effective, such field trials require thorough testing and calibration of instruments in the laboratory before they are mounted on a mobile platform, extensive field testing of mobile platforms both with and without instruments aboard, and full operational field testing of total systems. Proposals to conduct field tests should be peer reviewed in advance, and the test results should be promptly published in peer-reviewed journals.

In addition, several more-specific recommendations derive from the six case studies:

• Data downlink rates must be significantly increased, perhaps through the use of new technologies, such as the ongoing efforts to upgrade the Deep Space Network to operate in the Ka band or an eventual transition to optical communications. This is a problem that is not unique to mobile platforms.
• A means to control aerobot motion, both vertically and horizontally, needs to be developed.
• The capability to obtain descent images should be included on all lander and rover missions to provide critical context for navigation and science.
• Navigation tools and operational plans should be developed so that the impact of navigational needs on science return can be minimized.

In summary, the various disciplines interested in solar system exploration and research have many common needs for mobility and, thus, generally need not consider themselves as competitors for payload mass. For example, a rover carrying a suite of instruments designed to carry out a predominantly exobiology mission will differ very little from one designed to carry out a geology/geochemistry mission. Likewise, an aircraft or balloon mission designed to measure important atmospheric parameters at various altitudes can also collect surface spectral data important to geologists, geochemists, and exobiologists. Obviously, not all missions will satisfy all persons, but it seems clear that differences in mobile platform type and design are linked more to the target of the mission than to the interests of the scientists involved.

1. Roadmap Development Team, National Aeronautics and Space Administration, Mission to the Solar System: Exploration and Discovery—A Mission and Technology Roadmap, Version B, Jet Propulsion Laboratory, Pasadena, Calif., 1996.

2. Space Studies Board, National Research Council, Managing the Space Sciences, National Academy Press, Washington, D.C., 1995, p. 68.