understanding of the diversity of planetary objects, their evolutionary histories, and the fundamentals of how they work. Mobility will be required for this phase of planetary exploration.

What is Mobility?

A variety of recent planetary exploration missions either have demonstrated the advantages that derive from the ability to move instruments from one location to another in planetary environments or have indicated that such a capability is a logical approach to conducting future priority studies. A prime example of the former is Mars Pathfinder's deployment of the rover Sojourner on the martian surface in July 1997. The data returned from this mission about the elemental composition of martian soil and rocks was a direct consequence of Sojourner's ability to position an alpha proton x-ray spectrometer against a variety of materials across an area of several hundred square meters. A prime example of the latter is provided by the release of Galileo's probe into Jupiter's atmosphere in December 1995. Although it returned important data, the probe was only able to sample a limited portion of Jupiter's atmosphere for a few tens of minutes. The probe's results and inherent limitations suggest that a next logical step in the exploration of Jupiter's atmosphere is the deployment of a long-lived, balloon-borne instrument package.

None of this is new—the value of mobility has been recognized from the earliest days of lunar exploration. Yet, more than a quarter of a century separates the Apollo 17 astronauts' last traverse across the lunar surface in their rover and Sojourner's first tentative excursion on the surface of Mars. In this interval, profound advances have been made in robotics, and a variety of technologies have been developed that make it feasible to build mobile devices with both unprecedented capabilities and masses that are compatible with current launch vehicles. Developing these technologies has required and will continue to require the expenditure of substantial resources and, thus, it is imperative that the technologies developed be appropriate for scientific applications.

The purpose of this report is to develop the scientific rationale for mobility in planetary environments. The Committee on Planetary and Lunar Exploration (COMPLEX) attempts to do this in Chapter 2 by discussing a series of case studies that, though not all-inclusive, are representative of the range of scientific applications that may be addressed by mobility in the near- to mid-term future. As such, this report is different from most other COMPLEX reports. It does not develop a series of scientific priorities that might be addressed by future planetary missions. Rather, it advances a series of arguments to support the idea that investments in planetary-mobility technology should be determined on the basis of the scientific priority of the expected observations and not on the basis of technological expediency. In an era of limited resources, NASA cannot afford to develop technologies and then search for possible scientific applications.

COMPLEX defines mobility to include any means to move manipulative, sampling, imaging, or measuring platforms from one place to another both horizontally and vertically in the atmospheres or on the surfaces of solar system objects and to move and manipulate instruments and sample materials. This includes but may not be restricted to balloons, rovers, hoppers, aircraft, and so-called touch-and-go orbiters. Many of these must carry devices for instrument positioning, digging, drilling, and sample manipulation. Flybys and orbiters around large bodies are explicitly excluded. Human exploration can, in principle, provide a high degree of intelligent mobility but is beyond the scope of this document. Similarly, issues such as the methods for storing and transporting sample-return materials, power sources, and the specific characteristics of instruments and spacecraft are not within the purview of this report.

A key concept relating to the need for mobility in solar system exploration is the realization that planetary phenomena exist on a variety of spatial and temporal scales. The scales on which measurements must be made are functions of the complexity of the environment under study, the characteristic scale lengths of important physical processes, the scientific objectives of the study, and the specific types of measurements required to address these objectives. These scales need to be clearly defined and related to the overall objectives of each mission involving mobility.

Planetary atmospheres are good examples of this diversity of scales. The general circulation in Venus's atmosphere is dominated by global spin, whereas that of Earth is dominated by mid-latitude jets and large-scale eddies. Mars's atmosphere migrates from pole to pole in response to the planet's seasonal cycle and periodically

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