A Scientific Rationale for Mobility in Planetary Environments (1999)

For the last several decades, the Committee on Planetary and Lunar Exploration (COMPLEX) has advocated a systematic approach to exploration of the solar system; that is, the information and understanding resulting from one mission provide the scientific foundations that motivate subsequent, more elaborate investigations. COMPLEX's 1994 report, An Integrated Strategy for the Planetary Sciences: 1995–2010,¹ advocated an approach to planetary studies emphasizing "hypothesizing and comprehending" rather than "cataloging and categorizing." More recently, NASA reports, including The Space Science Enterprise Strategic Plan² and, in particular, Mission to the Solar System: Exploration and Discovery—A Mission and Technology Roadmap,³ have outlined comprehensive plans for planetary exploration during the next several decades. The missions outlined in these plans are both generally consistent with the priorities outlined in the Integrated Strategy and other NRC reports,^4,5 and are replete with examples of devices embodying some degree of mobility in the form of rovers, robotic arms, and the like.

Because the change in focus of planetary studies called for in the Integrated Strategy appears to require an evolutionary change in the technical means by which solar system exploration missions are conducted, the Space Studies Board charged COMPLEX to review the science that can be uniquely addressed by mobility in planetary environments. In particular, COMPLEX was asked to address the following questions:

1. What are the practical methods for achieving mobility?
2. For surface missions, what are the associated needs for sample acquisition?
3. What is the state of technology for planetary mobility in the United States and elsewhere, and what are the key requirements for technology development?
4. What terrestrial field demonstrations are required prior to spaceflight missions?

for employing mobility is to enhance the return of valuable scientific data, this report is focused on scientific rather than technological issues. COMPLEX therefore restricted its attention to six case studies, representative of the goals, environments, disciplines, and technologies drawn from previous COMPLEX and NASA reports:

• What is the nature of the circulation in the lower atmosphere on Venus?
• What tectonic processes are responsible for the structural and topographic features present on Venus?
• Is there evidence for extinct or extant life on Mars?
• What is the physical and chemical heterogeneity within small bodies such as asteroid 4 Vesta?
• What drives the zonal winds in the jovian atmosphere?
• What is the internal structure of Europa?

These six case studies are discussed in Chapter 2.

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 National Research Council documents cannot be met without mobile platforms of some type.

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. Technology development activities should be undertaken by the best-qualified individuals and teams within NASA, industry, and academia, as determined by peer review.

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 these 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. These tendencies create a tension between a model-driven approach to mobility and a technology-driven approach. 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 of 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 chal-

lenges. 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 they will be available 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. Space Studies Board, National Research Council, An Integrated Strategy for the Planetary Sciences: 1995–2010, National Academy Press, Washington, D.C., 1994, p. 25.

2. National Aeronautics and Space Administration, The Space Science Enterprise Strategic Plan: Origin, Evolution, and Destiny of the Cosmos and Life, National Aeronautics and Space Administration, Washington, D.C., 1997.

3. 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.

4. Space Studies Board, National Research Council, ''Scientific Assessment of NASA's Solar System Exploration Roadmap," letter report to Jurgen Rahe, NASA, August 23, 1996.

5. Space Studies Board, National Research Council, letter report to Wesley T. Huntress, Jr., NASA, concerning the draft Office of Space Science strategic plan, August 27, 1997.