Last November the ESA’s Philae lander had a less than text-book touch down on the surface of comet 67P/Churyumov–Gerasimenko (67P). Due to the comet’s microgravity, Phiae bounced several times and came to rest on its side, in the shadow of a boulder. For a lander with a top and a bottom, a design scheme born out of an Earth gravity environment, this was a massive problem. After this episode, which nearly resulted in a total mission failure, engineers from NASA’s Jet Propulsion Laboratory and Stanford University came up with a totally new type of robot designed specifically for traversing the surfaces microgravity environments like comets and asteroids. It’s called Hedgehog.
Hedgehog doesn’t have wheels. It doesn’t have landing struts. It doesn’t even have a top or a bottom, "Hedgehog is a different kind of robot,” explains Issa Nesnas, leader of the JPL team, “that would hop and tumble on the surface instead of rolling on wheels. It is shaped like a cube and can operate no matter which side it lands on.”
The new cube-shaped design has spikes on its surface and moves by spinning and braking internal flywheels. The spikes act as feet while the cube is hopping and tumbling, and protect the robot's body from the terrain. "The spikes could also house instruments such as thermal probes to take the temperature of the surface as the robot tumbles," says Nesnas.
There are currently two Hedgehog prototypes, one from Stanford and one from JPL. This June both were tested aboard NASA's C-9 aircraft for microgravity research. This is a plane that achieves microgravity during the dive phase of a series of parabolic arcs the plane performs as it flies. Over the course of four flights, the two robots demonstrated various types of maneuvers that could prove quite useful in traversing the surface of small bodies with very low gravity. Researchers also had the prototypes perform maneuvers on different materials to mimic a wide range of surface conditions: soft and crumbly, sandy, rough and rocky, and slippery and icy.
Robert Reid, JPL’s lead engineer on the project says, “We demonstrated for the first time our Hedgehog prototypes performing controlled hopping and tumbling in comet-like environments.”
The Hedgehog's simplest maneuver is referred to by its designers as a "yaw," or a turn in place. Once Hedgehog “yaw”s itself into the right direction, using one or two of its spikes, it can either tumble short distances by rotating from one face to another, or hop long distances. Typically Hedgehog takes large hops toward an area of interest, and then uses smaller tumbles as it gets closer to its target.