  Robotic spacecraft have helped expand our reach for many planetary
exploration missions. Most ground mobile planetary exploration robots use
wheeled or modified wheeled platforms. Although extraordinarily successful at
completing intended mission goals, because of the limitations of wheeled
locomotion, they have been largely limited to benign, solid terrain and avoided
extreme terrain with loose soil/sand and large rocks. Unfortunately, such
challenging terrain is often scientifically interesting for planetary geology.
Although many animals traverse such terrain at ease, robots have not matched
their performance and robustness. This is in major part due to a lack of
fundamental understanding of how effective locomotion can be generated from
controlled interaction with complex terrain on the same level of flight
aerodynamics and underwater vehicle hydrodynamics. Early fundamental
understanding of legged and limbless locomotor-ground interaction has already
enabled stable and efficient bio-inspired robot locomotion on relatively flat
ground with small obstacles. Recent progress in the new field of terradynamics
of locomotor-terrain interaction begins to reveal the principles of
bio-inspired locomotion on loose soil/sand and over large obstacles.
Multi-legged and limbless platforms using terradynamics insights hold the
promise for serving as robust alternative platforms for traversing extreme
extraterrestrial terrain and expanding our reach in planetary exploration.
