1
Introduction

Historical Perspective

Exploration has traditionally involved the ability to move from one place to another, making observations and collecting samples along the way. European voyages of discovery in the 13th to 15th centuries were conducted primarily by ship. These were followed by scientific expeditions on land and sea, such as Captain Cook' s voyages of the Endeavor in the Pacific during the 18th century. In many cases, these expeditions were followed by intensive investigations of local areas, spurred by economic forces.

The exploration of the American West serves to illustrate the role of mobility. Early expeditions were in part "feasibility studies" prompted by political and economic pressures. The pioneering trips of Alexander McKenzie across the Canadian Rockies in 1793, and of Meriwether Lewis and William Clark farther south in 1804–1805, served to define the geographical limits and accessibility of much of the West. The great scientific surveys conducted by John Wesley Powell down the Green and Colorado rivers through the Grand Canyon, and the extensive survey along the 40th parallel conducted in the 1860s and 1870s by Clarence King, served to map the West, survey its resources, and enable development. In these cases, "mobility" was provided by boat, horse, or foot, which enabled movement from one site to the next. A spectrum of observations was possible, from study of the horizon for the broad view of the terrain, to microscopic analyses of soils, rocks, and biota.

In the voyages of discovery and expeditions in the West, the ability to react to new discoveries along the way as new data were collected, analyzed, and synthesized was critical. In some cases, this involved staying at a scientifically rich site longer than originally planned. Sometimes, it meant rejecting specimens or samples in favor of better collections made later. In other cases, it meant changing the originally planned path because of unexpected hazards or to take advantage of new insight into the region.

In some respects, exploration of the solar system has followed a similar path to that of the exploration of our own planet. The first decades of planetary exploration have involved mostly spacecraft that have flown past, orbited, or probed the planets as initial reconnaissances. In some instances, limited mobility was provided by foot or rovers (manned and robotic) on the Moon, and on Mars by the short-range rover Sojourner (see Chapter 3, Box 3.1 and Box 3.2). With the exception of the Pluto-Charon system, every major planet and satellite has been visited by spacecraft, at least in a reconnaissance mode. The stage is now set to begin exploration in a mode analogous to the expeditions of the American West. This style of exploration has the potential to provide a new level of



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--> 1 Introduction Historical Perspective Exploration has traditionally involved the ability to move from one place to another, making observations and collecting samples along the way. European voyages of discovery in the 13th to 15th centuries were conducted primarily by ship. These were followed by scientific expeditions on land and sea, such as Captain Cook' s voyages of the Endeavor in the Pacific during the 18th century. In many cases, these expeditions were followed by intensive investigations of local areas, spurred by economic forces. The exploration of the American West serves to illustrate the role of mobility. Early expeditions were in part "feasibility studies" prompted by political and economic pressures. The pioneering trips of Alexander McKenzie across the Canadian Rockies in 1793, and of Meriwether Lewis and William Clark farther south in 1804–1805, served to define the geographical limits and accessibility of much of the West. The great scientific surveys conducted by John Wesley Powell down the Green and Colorado rivers through the Grand Canyon, and the extensive survey along the 40th parallel conducted in the 1860s and 1870s by Clarence King, served to map the West, survey its resources, and enable development. In these cases, "mobility" was provided by boat, horse, or foot, which enabled movement from one site to the next. A spectrum of observations was possible, from study of the horizon for the broad view of the terrain, to microscopic analyses of soils, rocks, and biota. In the voyages of discovery and expeditions in the West, the ability to react to new discoveries along the way as new data were collected, analyzed, and synthesized was critical. In some cases, this involved staying at a scientifically rich site longer than originally planned. Sometimes, it meant rejecting specimens or samples in favor of better collections made later. In other cases, it meant changing the originally planned path because of unexpected hazards or to take advantage of new insight into the region. In some respects, exploration of the solar system has followed a similar path to that of the exploration of our own planet. The first decades of planetary exploration have involved mostly spacecraft that have flown past, orbited, or probed the planets as initial reconnaissances. In some instances, limited mobility was provided by foot or rovers (manned and robotic) on the Moon, and on Mars by the short-range rover Sojourner (see Chapter 3, Box 3.1 and Box 3.2). With the exception of the Pluto-Charon system, every major planet and satellite has been visited by spacecraft, at least in a reconnaissance mode. The stage is now set to begin exploration in a mode analogous to the expeditions of the American West. This style of exploration has the potential to provide a new level of

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--> 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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--> exhibits global dust storms. The giant planets have distinct belts and zones and several, particularly Jupiter and Neptune, display a variety of long-lived vortices. These phenomena can only be addressed by measurements made on the spatial and temporal scales appropriate to the environment under study. In other words, the key issue is placing appropriate instruments in the right places and at the right times. In the context of a rover mission, for example, these characteristic scales will have a profound influence on the placement of observation and sampling sites and the timing of traverses. Certain types of observations (e.g., those concerning mineralogy and small-scale surface processes) are best addressed by very detailed sampling of a limited geographic region. Other questions (e.g., those concerning regional geology) are resolved only by samples collected from and observations made at widely separated sites. If the mission design philosophy does not acknowledge the interplay between scientific objectives and the relevant characteristic scales, the rover's capabilities and instrumentation may not be optimal to address the scientific goals of the mission. Alternatively, the rover capabilities may dictate that it address issues that are not of the highest scientific importance. The interplay between the characteristic scales associated with important physical processes and those scales defined by the mobility-system's performance relate to the important concepts of model- and technology-driven missions. In the former, the capabilities of the mobility system are defined by the requirements necessary to address a particular set of scientific questions. In the latter, the scientific issues to be addressed are defined by the capabilities of the mobility system. Since the primary reason for placing a mobile device in a planetary environment is to carry an instrument package that will return valuable scientific data, COMPLEX believes it is important that missions incorporating some form of mobility be designed to test explicit hypotheses, i.e., that they be model driven. Unless missions adopt this philosophy, there is a distinct danger that mobile systems will be designed without clear science goals in mind. The result could be a solar system exploration program driven by technology rather than by science. Mars Pathfinder is an example of a model-driven mission. That is, a depositional model, derived from studies of analogous terrains on Earth, was applied to images obtained by the Viking orbiters and used to select a landing site that would provide access to an abundance of diverse rock types. Likewise, current planning for future Mars Surveyor missions and advanced mobility devices, such as the Athena rover, recently deleted from the 2001 lander mission,* involves selecting candidate landing sites based on geomorphic and geological models likely to preserve evidence for past biological activity. Athena and its instruments are designed to address issues that require the rock types sought. As science goals become more specific in the future, it will be even more important that mobile systems be designed to test hypotheses. Given this philosophy, COMPLEX's approach to this report has been to identify science objectives that require mobility. Then it will be possible to determine whether the current state of mobility technology is sufficient and, if it is not, address in at least a preliminary way the development necessary to achieve the science objectives. There will be insufficient resources to pursue all possible variants of mobile spacecraft, and the decisions concerning which variants to develop and which to abandon should be guided by science priorities. These science priorities should then lead to technological priorities that, in detail, are beyond the scope of this report, although COMPLEX indicates some very general priorities where appropriate. Achieving useful mobility is not easy, as has been demonstrated in field tests on Earth and by the operation of the Sojourner rover on Mars. This also must be kept in mind when science-driven priorities for development are set. Organization This report includes the following: A brief review of fundamental science goals in solar system exploration (Chapter 1); A discussion of the observations required and the role of mobility in addressing six representative case studies designed to address questions derived from these fundamental goals (Chapter 2); and *   This report is written on the assumption that Athena will not be deployed by the 2001 lander, as originally planned, but that its scientific payload will eventually fly on a later Mars Surveyor mission.

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--> A discussion of technical capabilities for a variety of mobile devices (Chapter 3). Some of these systems embody well-established technologies with heritages from past planetary missions (e.g., balloons and rovers); others employ technologies that have yet to be exploited by planetary scientists (e.g., aircraft and cryobots). This chapter also discusses the development and field testing required if mobility devices are to meet science needs. This discussion is structured according to the specific questions in the charge to COMPLEX (see the preface). Chapter 4, "Conclusion and Recommendations," summarizes the importance of mobility for the successful completion of the diverse tasks required to address many of the key questions in solar system science and also summarizes development priorities for mobility technology with respect to these tasks. Scientific Goals for Solar System Exploration The science objectives to be addressed by mobility relate directly to the broad scientific goals for solar system exploration, as stated by the Space Studies Board. These objectives are the following:1 Understanding how physical and chemical processes determine the main characteristics of the planets, thereby illuminating the workings of Earth; Learning how planetary systems originate and evolve; Determining how life developed in the solar system and in what ways life modifies planetary environments; and Discovering how the simple, basic laws of physics and chemistry can lead to the diverse phenomena observed in complex systems. These broad scientific goals lead to some 35 primary objectives in eight subject areas (Table 1.1), ranging from protoplanetary disks to planetary atmospheres. These primary objectives, in turn, lead to a great many more specific objectives and questions. Specific Objectives and Case Studies The diversity of planetary environments found in the solar system is matched by the range of scientific disciplines needed to address them. Researchers with expertise in the geosciences and atmospheric sciences, together with those expert in exobiology and the study of particles and fields, have discovered a myriad of important topics to study on bodies as diverse as Sun-scorched Mercury and the frigid Kuiper Belt objects. Reviewing the science that can be uniquely addressed by mobility in exploring the atmospheres and surfaces of solar system objects is a daunting task. Indeed, conducting a comprehensive review was not feasible within the constraints of the current study. Rather than tackle the challenging task of performing a detailed analysis of the mobility required to meet each of the 35 primary objectives listed in An Integrated Strategy for the Planetary Sciences: 1995–2010,2 COMPLEX performed a preliminary examination, which indicates that mobility will be required to address a significant number of them, as listed in Table 1.1.

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--> TABLE 1.1 Mobility Needed to Meet the Primary Objectives Identified in COMPLEX's Integrated Strategy Subject Area Primary Objective Need for Mobility* Protoplanetary Disks   • Develop (through theoretical modeling) a detailed understanding of the aggregation of stellar and planetary systems, starting at the formation phase of dense molecular cloud cores. Low • Observe nearby star-forming regions to obtain data that can guide and constrain our understanding of protostellar formation. Low • Define the conditions and processes during the evolution of the solar nebula through laboratory analysis of meteorites and interplanetary dust particles and observations of primitive solar system objects, such as comets and asteroids. Low Planetary Systems   • Construct an internally consistent, quantitative theory of the formation of our entire planetary system that contains sufficient details to permit comparison with as much observational evidence as possible, including the meteoritic record. Low • Detect and determine the orbital properties of planetary systems circling enough nearby stars to yield a statistically significant estimate of the frequency of planetary systems. Low • Ascertain, as is technically feasible, the atmospheric temperatures and compositions of those extrasolar planets. Low Primitive Bodies   Describe the nature and provenance of carbonaceous materials in cometary nuclei, especially as they pertain to the origin of terrestrial life. Medium • Identify the sources of the extraterrestrial materials that are received on Earth. Low • Delineate how asteroids and comets are related and how they differ. Medium • Determine the elemental, molecular, isotopic, and mineralogic compositions for a variety of samples of primitive bodies. High • Characterize the internal structure, geophysical attributes, and surface geology of a few comets and asteroids. High • Understand the range of activity of comets, including the causes of its onset and its evolution. High • Ascertain the early thermal evolution of primitive bodies, which led to the geochemical differentiation of these bodies. Medium Life   • Define the inventory of organic compounds in the cores of molecular clouds, and improve our understanding of the prebiotic organic chemistry that took place in the solar nebula. Low • Improve knowledge of the processes that led to the emergence of life on Earth, and determine the extent to which prebiotic and/or protobiological evolution has progressed on other solar system objects, specifically Mars and Titan. High Surfaces and Interiors of Solid Bodies   • Understand the internal structure and dynamics of at least one solid body, other than Earth or the Moon, that is actively convicting. Medium • Determine the characteristics of the magnetic fields of Mercury and the outer planets to provide insight into the generation of planetary magnetic fields. Low • Specify the nature and sources of stress that are responsible for the global tectonics of Mars, Venus, and several icy satellites of the outer planets. High • Advance significantly our understanding of crust-mantle structure, geochemistry of surface units, morphological and stratigraphic relationships, and absolute ages for all solid planets. High • Elucidate the chemical and physical processes (impact cratering, surface weathering, and so on) that affect planetary surfaces. Medium • Characterize the surface chemistry of the outer solar system satellites, and determine the volatile inventories and interaction of the surface and atmosphere on Triton and Pluto. Medium • Establish the chronology of at least one other major body in the solar system. High

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--> Subject Area                                                                           Primary Objective Need for Mobility* Planetary Atmospheres   • Ascertain the key chemical balances and processes that maintain the current compositions of the atmospheres. Medium • Specify the processes that control dynamics on the outer planets, on Mars, and on Venus. High • Understand Mars's inventory of volatiles and its evolution and how these relate to historical climate changes. High • Determine reactive-gas isotopic ratios, rare-gas abundances, and isotopic abundances for all the planets with substantial atmospheres, to help understand atmospheric origin, history, and maintenance. Low Rings   • Measure the radial, azimuthal, and vertical structure of all the ring systems at sufficient spatial resolution and clarify whether the observed variability is spatial or temporal in nature. Low • Determine the composition and size distribution of the ring particles at a few places in several different systems. Low • Develop kinematic and dynamic models of ring processes and evolution that are consistent with the best ground- and space-based observations. Insofar as possible, connect these processes to ones that were active as the solar system originated. Low Magnetospheres   • Determine how, and the degree to which, plasma and electromagnetic environments affect planetary gas (including the atmosphere), dust, and solid surfaces. Low • Understand how solar wind and planetary variations drive magnetospheric dynamics, including substorms, for various magnetospheric conditions. Low • Determine the roles of microscopic plasma processes in the mass and energy budgets of planetary magnetospheres, and ascertain the energy conversion processes that yield auroral emissions. Low • Discover how differing plasma sources and sinks, energy sources, magnetic field configurations, and coupling processes determine the characteristics of both intrinsic and induced planetary magnetospheres. Low • Determine what studies of contemporary planetary magnetospheres tell us about processes involved in the formation of the solar system. Low • Characterize the plasma environments and the solar-wind interactions of Pluto-Charon and Mars. Low * Low, little or no mobility required; medium, robotic arms or other types of sample collection devices needed; and high, mobile platform equipped with sophisticated instrumentation required. COMPLEX then went on to consider a subset of the priority activities identified in past reports, in particular the Integrated Strategy, and performed a series of case studies designed to illustrate important issues relating to the use of mobility in planetary exploration. These case studies address the following important scientific questions: What is the nature of the circulation in the lower atmosphere on Venus?3 What tectonic processes are responsible for the structural and topographic features present on Venus?4 Is there evidence for extinct or extant life on Mars?5,6 What is the physical and chemical heterogeneity within small bodies such as asteroid 4 Vesta?7,8 What drives the zonal winds in the jovian atmosphere?9 What is the internal structure of Europa?10 COMPLEX emphasizes that the questions listed above do not necessarily represent the highest-priority issues to be addressed by planetary scientists in the near future. Nor does the ordering of the questions imply any particular priority. Rather, they are chosen to be representative of important issues, defined in past reports by COMPLEX, concerning a broad range of planetary environments (i.e., the terrestrial planets, giant planets, and primitive bodies), questions that also figure prominently in the five "campaigns" outlined in NASA's Mission to

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--> the Solar System: Exploration and Discovery—A Mission and Technology Roadmap.11 These questions involve studies spanning a broad range of scientific disciplines (i.e., the geosciences, atmospheric sciences, and exobiology), and, as Chapter 2 indicates, can be best addressed by a broad range of mobility techniques, including the use of traditional devices such as balloons and rovers (which understandably dominate the discussion since these technologies are the best developed), as well as by devices less familiar to planetary scientists, such as aircraft and cryobots. References 1. Space Studies Board, National Research Council, An Integrated Strategy for the Planetary Sciences: 1995–2010 , National Academy Press, Washington, D.C., 1994, p. 12. 2. Space Studies Board, National Research Council, An Integrated Strategy for the Planetary Sciences: 1995–2010, National Academy Press, Washington, D.C., 1994, pp. 3–6. 3. Space Studies Board, National Research Council, An Integrated Strategy for the Planetary Sciences: 1995–2010, National Academy Press, Washington, D.C., 1994, pp. 122, 125, 133–134. 4. Space Studies Board, National Research Council, An Integrated Strategy for the Planetary Sciences: 1995–2010, National Academy Press, Washington, D.C., 1994, pp. 93, 102. 5. Space Studies Board, National Research Council, An Integrated Strategy for the Planetary Sciences: 1995–2010, National Academy Press, Washington, D.C., 1994, p. 61. 6. Space Studies Board, National Research Council, The Search for Life's Origins, National Academy Press, Washington, D.C., 1990, p. 124. 7. Space Studies Board, National Research Council, An Integrated Strategy for the Planetary Sciences: 1995–2010, National Academy Press, Washington, D.C., 1994, pp. 58, 63–54. 8. Space Studies Board, National Research Council, The Search for Life's Origins, National Academy Press, Washington, D.C., 1990, pp. 124, 125. 9. Space Studies Board, National Research Council, An Integrated Strategy for the Planetary Sciences: 1995–2010, National Academy Press, Washington, D.C., 1994, pp. 118, 122, 125, 132, 133. 10. Space Studies Board, National Research Council, An Integrated Strategy for the Planetary Sciences: 1995–2010, National Academy Press, Washington, D.C., 1994, pp. 80, 90. 11. 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, pp. 10, 13–14, 21–22, 27–28, 34–35, 42–43.