The first use of an unmanned rover on another solar system body occurred in the early 1970s when the former Soviet Union's Luna 17 and 21 missions landed the Lunokhod rovers on the surface of the Moon (see Box 3.1). Not until the successful deployment of Sojourner on the surface of Mars in 1997 was another unmanned rover used for solar system exploration (see Box 3.2). More attention has been paid to the development of rovers than any other form of mobility.3,4,5,6,7,8
The range of rover types and the technological problems associated with their deployment were well understood by the late 1980s.9 Many rover types have been discussed to date, ranging in size from nano- and microrovers with total masses of less than 1 kg to large vehicles with masses in excess of 400 kg (Box 3.3).10 Although many generic science payloads have been proposed for discussion purposes, the overall thrust of much of the work to date has been technological. For much of the 1990s, major development efforts have been directed toward reducing the total size of rovers and increasing their autonomy.11,12
To be most effective, rovers must be able to carry a set of complementary science instruments for a significant range. The extent of this range will depend on mission-specific factors. These include the scientific objectives to be met, the scientific payload carded by the rover, the size of the landing-error ellipse, nature of the site chosen, prelanding knowledge of the site, and the availability of planning materials (e.g., maps based on very high resolution orbital or descent imagery) to facilitate rover operations. Given these caveats, assessments of, for example, various martian landing sites offered by the planetary geoscience community suggest that minimum ranges of 1 to 10 km are required.13 Longer ranges will be necessary if a rover is to characterize and sample geological units on a more regional scale or if it is to visit more than one specific site. To adequately perform the scientific characterization of a site, a rover requires the following capabilities:14
Furthermore, the rover must carry an integrated set of science instruments. That is, in the words of a report on a recent workshop on surface instruments, the rover "should not just carry a collection of individual instruments each playing its own tune, but must be an orchestra."15 Many planetary researchers believe that single-instrument rovers are not likely to be very useful scientifically, because an array of measurements taken by different instruments commonly is necessary to answer even simple questions. For example, adequately defining a rock type requires at least one instrument capable of measuring elemental abundances, visual to infrared spectrometers to determine mineralogy, and a very high resolution (<1 mm/pixel) imager to determine grain size, structure, and texture.
Technological requirements for the command and control of conventional rovers can be stringent. As exemplified by Lunokhod, system and vehicle control can be largely Earth based for lunar rovers because the two-way communication time is short. However, even though Pathfinder was a highly successful mission, the speed and range of Sojourner were limited by the long two-way communication time and by its limited autonomy.
Future rovers designed to conduct long traverses (tens to hundreds of kilometers or more) or to operate on more distant bodies face a significant operational challenge. Such missions will require significant local autonomy, including the ability to perform local navigation, identify or sample sites of potential scientific interest, regulate on-board resources, and schedule activities. But mission scientists want to retain some control over the rover and its operations. If they do, in the words of the Mars 2001 Science Definition Team, "the rover would spend most of its time stationary waiting for instructions from home and so distances traveled would be greatly reduced as would the number of analyses, images . . . ."16