  Caves and lava tubes on the Moon and Mars are sites of geological and
astrobiological interest but consist of terrain that is inaccessible with
traditional robot locomotion. To support the exploration of these sites, we
present ReachBot, a robot that uses extendable booms as appendages to
manipulate itself with respect to irregular rock surfaces. The booms terminate
in grippers equipped with microspines and provide ReachBot with a large
workspace, allowing it to achieve force closure in enclosed spaces such as the
walls of a lava tube. To propel ReachBot, we present a contact-before-motion
planner for non-gaited legged locomotion that utilizes internal force control,
similar to a multi-fingered hand, to keep its long, slender booms in tension.
Motion planning also depends on finding and executing secure grips on rock
features. We use a Monte Carlo simulation to inform gripper design and predict
grasp strength and variability. Additionally, we use a two-step perception
system to identify possible grasp locations. To validate our approach and
mechanisms under realistic conditions, we deployed a single ReachBot arm and
gripper in a lava tube in the Mojave Desert. The field test confirmed that
ReachBot will find many targets for secure grasps with the proposed kinematic
design.
