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Cube robot could cope with microgravity conditions of asteroids

07 February 2016

A team of engineers has built a wheel-less, limbless cube-like rover for exploration in some of the most extreme conditions in space.

A depiction of Hedgehog on Mars' moon Phobos (illustration: Ben Hockman/Stanford Engineering)

Designed by Stanford’s Marco Pavone, assistant professor of aeronautics and astronautics, and Ben Hockman, a graduate student in mechanical engineering, the 'Hedgehog' space rover has no wheels for rolling, no arms for grabbing and no legs for stepping. It’s just a single overgrown die with the ability to hop and tumble itself across the unknown terrain of asteroids, comets and small moons.

And while Hedgehog is not yet part of any planned space missions, the rover’s research team recently cleared a significant hurdle along the way to turning their block robot into a viable space-exploration rover.

The twin challenges of extreme environments and low levels of gravity make landing on and exploring small bodies exceptionally difficult. And while there have been a few spacecraft that have made contact with asteroids and comets, such as the Japan Aerospace Exploration Agency’s Hayabusa mission and the European Space Agency’s Rosetta mission, they have had only partial success. At best, such probes have been confined to the spot where they landed.

The Hedgehog project is part of a collaboration between researchers at Stanford, MIT and the Jet Propulsion Laboratory (JPL), and is funded by the NASA Innovative Advanced Concepts (NIAC) program.

“By turning a disadvantage for mobility [low gravity] into a requirement for mobility, the proposed rover would be able to do things on small bodies that are simply impossible right now – and yet, Dr. Pavone’s proposed rover utilises almost no new technologies, just uses existing technologies in a new, creative way,” says Jason Derleth, NASA Innovative Advanced Concepts (NIAC) programme executive.

In a microgravity environment, the type of wheeled or tracked rovers used to explore the surface of Mars would almost certainly fail. In environments where you would weigh as much as a paperclip and if you jumped hard enough you would never come back, wheels and treads would struggle to gain traction, and one little unforeseen jostle could easily tip a rover over to flail helplessly on its back. 

Instead of rolling along, Hedgehog exploits the fact that objects aren’t so firmly anchored to the ground by hopping. Three internal flywheels are housed within a cube with corners that are strengthened to absorb the shock of landing.

Those flywheels move the cube through a process of internal actuation. By spinning and abruptly braking, the flywheels achieve varying amounts of torque, enabling the cube to hop controllably when it needs to cover distance, like a gymnast somersaulting across a mat. In the event that Hedgehog gets buried in a sandy sinkhole, it can even perform a 'tornado' manoeuvre that corkscrews upward to escape.

Hedgehog as been tested in the zero gravity environment created by NASA's 'Vomit Comet' aircraft when engaged in a parabolic dive. “It was an opportunity to see Hedgehog perform a hop or any other manoeuvre as it would on an asteroid or comet,” Hockman says. “We were quite pleased with how it performed. We’re still analysing all the data, but even just seeing Hedgehog hop in the direction we commanded it to hop is reassuring. We’ve successfully demonstrated a whole new paradigm of mobility in microgravity.”

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