Part I of this article described a new kind of cube-shaped probe that NASA’s JPL and Stanford University are designing specifically for exploring in microgravity conditions called the Hedgehog. It also explained that a standard probe design would be almost useless in this sort environment. So instead of using external wheels, like the Mars rovers, Hedgehog uses internal flywheels to move, typically taking large hops toward an area of interest, and then smaller tumbles as it gets closer to its target.
There’s a third kind of maneuver Hedgehog can perform if it gets itself into a tricky situation. Its designers refer to as a "tornado" maneuver. In it, the cube aggressively spins and launches itself off the ground. The “tornado” maneuver could be particularly useful in the event that Hedgehog found itself in a sandy sinkhole it needed to escape from or some other situation in which the robot would otherwise be stuck. If the Hedgehog, for example, found itself in the kind of situation the Philae Lander found itself stuck in, Hedgehog could have simply performed a “tornado” maneuver, and carried on with its mission.
NASA’s JPL and Stanford each have their own design for the Hedgehog. The current JPL Hedgehog prototype has eight external spikes and three flywheels inside it. The basic probe weighs about 11 pounds (5 kilograms) by itself, but its designers say that, outfitted with instruments such as cameras and spectrometers, it could weigh more than 20 pounds (9 kilograms). Though this may seem like a lot of weight to move around just with internal flywheels, keep in mind that Hedgehog will be operating in microgravity, where a human being would weigh about as much a s paperclip weighs here on Earth. The Stanford prototype is slightly smaller and lighter, and it has shorter spikes.
Both prototypes maneuver by spinning and stopping three internal flywheels, but the braking mechanisms differ between the two prototypes. While JPL's version uses disc brakes, Stanford's prototype uses friction belts to stop the flywheels abruptly.
Marco Pavone, leader of the Stanford team explains, ”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.”
“The geometry of the Hedgehog spikes has a great influence on its hopping trajectory,” says Benjamin Hockman, lead engineer on the Stanford team. “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.”
Because Hedgehog will be operating in deep space, where sending each instruction and then waiting for telemetry to make it back to Earth could take significant periods of time, researchers are working on giving Hedgehog some autonomy, trying to increase the number of tasks the robots can do by themselves, without instructions from Earth. The idea is that Hedgehog would work in concert with an orbiting mothership, which would relay signals to and from the robot. The mothership would also help the Hedgehogs determine their positions and navigate.
Along with enabling it to operate in microgravity, the new design architecture would make the construction cost of a Hedgehog relatively low, especially compared to a traditional rover. Also, with lower cost and smaller payloads, several Hedgehogs could be packaged together for flight. So instead of sinking all of the resources for a mission into one rover, the mothership could release many robots at once or in stages, letting them spread out to cover much more area than a single, much more expensive rover could alone.