Neil Armstrong and Buzz Aldrin, along with all of the other Apollo astronauts who walked on the Moon, were able to naturally adjust their gait for the low-gravity environment. But robots aren’t capable of adapting so easily. In large part, that’s because a robot will leave the ground completely as it hops along — something that robots generally aren’t designed to do. When the robot is in the air, it needs to be able to correct its attitude in order to land smoothly and continue on efficiently. To give SpaceBok that ability to correct, the team equipped it with a reaction wheel.
A reaction wheel is a weighted disc that is spun with a motor. When it spins, inertia takes over and creates an opposing force. That can be used to stabilize the robot in midair, similar to how an acrobat shifts their body to come out of a spin for a landing. To simulate a low-gravity environment to test SpaceBok in, the team built special test rigs. For example, one puts SpaceBok on a sled that slides along an inclined rail. That replicates the dynamics of gravity on the Moon. Another rig simply rolls on extremely flat ground in order to achieve a zero-gravity effect. In these tests, the team was able to prove that SpaceBok would be capable of achieving a bounding gait in space.