ATHLETE: An Option for Mobile Lunar Landers

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. Each vehicle is based on six wheels at the ends of six multi-degree-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 alternative mode of traversing through or at least extracting itself out of extreme terrain, the wheels and wheel actuators can be sized for nominal terrain. There are substantial mass savings in the wheel and wheel actuators associated with designing for nominal instead of extreme terrain. These mass savings are comparable-to or larger-than the mass increase of the articulated limbs. As a result, the entire mobility system, including wheels and limbs, can be lighter than a conventional mobility chassis. In addition, each limb has sufficient degrees-of-freedom to use as a general-purpose manipulator. Our prototype ATHLETE vehicles have quick-disconnect tool adapters on the limbs that allow tools to be drawn out of a "tool belt" and positioned by the limb. A 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. One of the most attractive uses for ATHLETE limbs is as part of a "mobile lander". Lunar landers are traditionally conceived-of as static - remaining stationary after landing on pads that deploy to a large radius to reduce the likelihood of overturning and that incorporate energy absorption (e.g. crushable materials) to cushion whatever residual impact remains after the rocket propulsion system is shut down. Landers implicitly integrate all the subsystems required for a complete spacecraft-power, communications, computation, inertial sensing, etc. All these subsystems would need to be re-implemented on any rover that deploys from a lander. Instead, these subsystems can be used "as-is" (perhaps with additional qualification testing) if only the lander were equipped with post-landing mobility. This obvious advantage of mobile landers has been recognized for decades (see Section 3), but use of ATHLETE limbs as landing outriggers together with crushable materials under the primary structure of the lander combines the benefits of the reduced mobility mass and tool-use of the ATHLETEconcept with the intrinsic efficiency of the mobile lander concept.