Spiky Hedgehog robots to hop around asteroids and comets

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The Hedgehog robot, designed to explore the surfaces of comets, asteroids and small moons(Credit: NASA/JPL-Caltech/Stanford)

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As demonstrated by the bumpy landing of ESA's Philae lander on comet 67P/Churyumov–Gerasimenko, exploring comets, asteroids, and small moons can be difficult due to their low gravity. Not only can landing on one be like trying to alight on a trampoline, but roving around their surfaces is next to impossible because the negligible gravity offers practically no traction. To overcome this, a team of engineers is developing Hedgehog, a completely symmetrical robot rover for low-gravity exploration that moves by hopping.

A joint project by NASA's Jet Propulsion Laboratory (JPL), Stanford University, and MIT, the Hedgehog robot gets around these limitations with an unusual form of locomotion that allows it to hop, tumble, skip, and even launch itself with artificial "tornadoes." Essentially a cube with horns or spikes on each corner, it has no right way up and every face is identical, so it doesn't matter how it lands. In addition, the cube shape makes it easy to pack economically in a spacecraft.

Inside the cube are a set of three flywheels. These are slowly spun up and then a brake is suddenly applied to transfer the momentum from the wheels to the cube, causing it to topple over, hop, or spin like a top, depending on how much force is built up and in which direction it's applied. The spikes act as bumpers to protect the robot and claws to give it purchase, while also potentially housing sensors and antennae.

It seems like a simple arrangement, but according to the team, it allows the Hedgehog to turn, leap distances, and make short tumbles until it reaches its destination.

The basic idea behind the Hedgehogs is to make them small and cheap, so a deep space mothership can carry and deploy a number of them. The mothership would drop them on targets of interest, then relay data between the Hedgehogs and mission control back on Earth in much the same way the Mars orbiters do for the Curiosity and Opportunity rovers.

So far, two prototypes of the Hedgehogs have been built by Stanford and JPL researchers. The JPL Hedgehog weighs roughly 11 lb ( 5 kg), but a future version is planned that weighs 20 lb (9 kg) and carries instruments. Meanwhile, the Stanford Hedgehog is smaller and lighter and was built with shorter spikes, so the effectiveness of different spike designs can be assessed. The other major difference is that where the JPL version uses disc brakes to stop the flywheels, the Stanford uses friction belts.

"By controlling how you brake the flywheels, you can adjust Hedgehog's hopping angle," says Marco Pavone, leader of the Stanford team. "The idea was to test the two braking systems and understand their advantages and disadvantages."

NASA's C-9 aircraft for microgravity research gave two Hedgehog prototypes a ride in June 2015 to test their maneuvers(Credit: NASA)

The Hedgehogs have flown on 180 parabolic trajectories during four flights in June aboard a NASA C-9 aircraft to briefly simulate zero gravity. The robots were housed inside a clear plastic case, where they could be safely tested on a variety of sandy, rough and rocky, slippery and icy, and soft and crumbly surfaces to simulate a comet-like environment. The prototypes performed as expected and even executed a spectacular tornado maneuver, where the machine freed itself from a sandhole by spinning violently.

JPL says that the Hedgehog is now in Phase II development through the NASA Innovative Advanced Concepts (NIAC) Program. The team is working to provide the robots with enough autonomy to allow them to operate without constant supervision from Earth.

The video below discusses the Hedgehog project.

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