318 the end of the limb, attached to the wheel hub, allows  for interchangeable tools, such as a gripper or an  auger, to be used for manipulation. This unique vehicle design allows for significant weight savings over a  traditional planetary roving vehicle that must have la rge wheels to allow for low ground pressure as well  as high torque wheel actuators since the vehicle cannot walk in extremely soft or steep terrain. 

The ATHLETE project started in March of 2005 [1, 2]. The first generation vehicle was completed in  October of that same year. This 1000-kg, 2-m tall  vehicle was developed rapidly with the intent of  providing a hardware platform to aid in the developm ent of the robotic system’s software. This software  development vehicle ended up performing five field test s in natural terrain throughout the United States.  These field tests enabled the team to  test the vehicle’s capab ilities such as traversa bility over soft terrain,  walking, repelling, and manipulation in unstructured environments. From this first prototype several new capabilities were discovered and several vehicle deficiencies were revealed.     In October of 2008 the design of the second-generation vehicle was initiated. The intent of the second  vehicle was to maintain all the functionality of the first generation, as well as add the capability of generic cargo transport and offloading from a lunar lander. This  new requirement led to the development of two,  cooperative three legged vehicles know as Tri-ATHLETEs. These vehicles can function independently to traverse without cargo, but can also combine on eit her side of a structural pallet, using a generic hook  and pin interface, to form a single six legged system with one Tri-ATHELTE vehicle acting as the master  and the second vehicle as the slave. The pallet can be loaded with any type of cargo; Figure 1 shows a  mock habitat as the payload. Once docked, the vehicl e can transport the cargo to the desired location.  The Tri-ATHLETEs can then undock, as shown in Figure 2, leaving the pallet and payload behind, and drive to a new location as two, three legged robots to combine with a new pallet and payload for transport. 

Figure 2.  Two Tri-ATHLETE vehicles prior to docking to the mobile habitat / pallet   The requirement of cargo offloading from the deck of a 3.2m tall lander, shown in Figure 3, drove to an additional joint over the original six degree of freedom ATHLETE limb. Because of the extreme height of the lander deck compared to the limb length necessar y for walking and manipulation, the seventh degree  of freedom is implemented as a thigh pitch joint that simply acts to change the length of the limb from a long configuration for high cargo offloading to a more compact configuration. The resulting kinematic configuration is a yaw – pitch – pitch – pitch – roll – pitch – roll arrangement shown in Figure 4. While this is less than ideal from a workspace optimization perspe ctive, it allows for the upper thigh to extend and  then fold upon itself creating the desi red long and compact configurations.   The second generation ATHLETE system stands 4.1 m at full height and is approximately half scale of the conceived flight vehicle. This will allow for payload retrieval from the top of a 6.4-m tall Altair Lunar  NASA/CP-2010-216272