When you put a robot on a planet like Mars, you can plan for the terrain pretty well, with sturdy treads that can handle a large number of planetary obstacles in an environment with gravity. Asteroids and comets? Not so much. They're smaller and lumpier, and their gravity is negligible. A rover like Curiosity would fling itself off the surface and fall over.
Cue the Hedgehog, a robot under development by researchers at NASA's Jet Propulsion Laboratory, Stanford University and the Massachusetts Institute of Technology. It's a little robotic explorer that can tumble along the surface of a low-gravity environment, unhindered by rough terrain.
The robot has evolved from the concepts seen two years ago. The Hedgehog is now a cube covered in spikes, which act as both feet and protection for the robot's body against the terrain. Inside its chassis, internal flywheels and brakes spin to move the robot in a sort of hopping, tumbling motion.
"The spikes could also house instruments such as thermal probes to take the temperature of the surface as the robot tumbles," said Issa Nesnas, leader of the JPL team.
It doesn't have a top or bottom, either. No matter which way up it lands, the Hedgehog robot is designed to continue working perfectly.
"The geometry of the Hedgehog spikes has a great influence on its hopping trajectory. We have experimented with several spike configurations and found that a cube shape provides the best hopping performance. The cube structure is also easier to manufacture and package within a spacecraft," explained Benjamin Hockman, lead engineer of the Stanford team.
Two prototype robots, one from JPL and one from Stanford, have now been tested in microgravity in NASA's C-9 aircraft. As the plane flies along a parabolic path, it creates a simulated microgravity environment inside. In June 2015, the plane flew 180 parabolas over the course of four flights, while the Hedgehog prototypes were put through their paces on different surfaces.
In a special enclosure, the robots successfully tumbled, hopped, turned in place (a move called a "yaw") and even performed a fast spinning movement to launch itself from a surface -- which could be useful if the Hedgehogs ever became stuck in place.
And, of course, they're much cheaper to manufacture than a rover, or even a lander like Philae, which touched down on comet 67P/C-G, then stayed fixed in one place to sample the comet's composition and send data back to Earth.
JPL's Hedgehog has eight spikes, and moves with the aid of three internal flywheels. It comes in at 5kg (2 pounds). The Stanford Hedgehog is a little smaller and lighter, with shorter spikes, and also contains three flywheels. Where the two differ is in the braking mechanism: The JPL robot uses disc brakes, while Stanford's uses friction belts.
"By controlling how you brake the flywheels, you can adjust Hedgehog's hopping angle. The idea was to test the two braking systems and understand their advantages and disadvantages," said Stanford team leader Marco Pavone.
The next step in the research, now that the robot's manoeuvres have been successfully demonstrated, is to increase the autonomy of the robots. The idea is that the Hedgehogs would be deployed to a comet or asteroid by a mothership, which would remain nearby to send and receive signals, acting as a relay station between the robot and Earth, much like how NASA's Mars rover program operates.