As part of the NASA Exploration Technology Development Program, the Jet Propulsion Laboratory is developing a vehicle called ATHLETE: the All-Terrain Hex-Limbed Extra-Terrestrial Explorer. 1 2 The vehicle concept is based on six wheels at the ends of six multidegree-of-freedom limbs. Because each limb has enough degrees of freedom for use as a general-purpose leg, the wheels can be locked and used as feet to walk out of excessively soft or other extreme terrain. Since the vehicle has this

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... ative mode of traversing through (or at least out of) extreme terrain, the wheels and wheel actuators can be sized only for nominal terrain. There are substantial mass savings in the wheels and wheel actuators associated with designing for nominal instead of extreme terrain. These mass savings are comparable-to or larger-than the extra mass associated with the articulated limbs. As a result, the entire mobility system, including wheels and limbs, can be about 25% lighter than a conventional mobility chassis for planetary exploration. A side benefit of this approach is that each limb has sufficient degrees-of-freedom for use as a general-purpose manipulator (hence the name "limb" instead of "leg"). Our prototype ATHLETE vehicles have quickdisconnect tool adapters on the limbs that allow tools to be drawn out of a "tool belt" and maneuvered by the limb. A rotating power-take-off from the wheel actuates the tools, so that they can take advantage of the 1+ horsepower motor in each wheel to enable drilling, gripping or other power-tool functions. Architectural studies have indicated that a useful role for ATHLETE in lunar exploration is to "walk" cargo off the payload deck of a lunar lander and transport it across the lunar surface. Current architectural approaches are mostly focused on the concept that the lunar lander descent stage will use liquid hydrogen as a propellant. This is the highestperformance chemical fuel, but is low density and hence requires large tanks. As a result, the cargo deck of the lander is very high off the ground (over 6 meters in current concepts [1]). An ATHLETE vehicle with limbs this long is able to directly step onto the ground, moving off the lander by stepping with its rear limbs only onto the widely-spaced "hard points" on the top deck of the lander space-frame structure. One major motivation for surface mobility is that any lander will spray ejecta from the plume of the descent 1 1 978-1-4244-2622-5/09/$25.00