Posted September 18, 2015

The next comet explorer might be a hopping cube

Wouldn't it be fun to stand on the surface of a comet? Among many spectacular differences to life on Earth, you would weigh almost nothing. Strangely though, this would make getting around rather difficult. On a comet, you'd better be a cube full of flywheels. Nasa and Stanford are playing around with some.

Most readers will remember the sad fate  of comet lander Philae. Philae landed successfully on comet Churyumov-Gerasimenko last November. But instead of touching down and staying down, it bounced around a couple of times and ended up in some hole in the ground where it didn't receive enough sunlight to keep working.

Weightlessness may sound nice and interesting, but too little gravity isn't good. A little more weight and Philae might have anchored itself into the ground. Had that happened, the probe possibly would still be working today. If you were a human standing on this comet, you would weigh about the same as a paperclip on Earth. If you would try to walk, you would take off and it would take you some time to land again. In fact, a powerful jump would probably launch you into space forever.

Next time comet researchers may want to land a probe that can move around and investigate as much as possible of a comet's surface. But a wheeled robot, like the Mars rovers Curiosity or Sojourner, won't be of any use. It will too easily lose touch with the ground, and steering it will be next to impossible.

That's why Nasa and Stanford University have built Hedgehogs. Hedgehogs are cube-shaped robots that are designed to hop from one place to another. The Hedgehogs contain flywheels that speed up gently, not disturbing the robot itself. If movement is desired, a flywheel can be brought to a sudden stop by a simple braking mechanism. When that happens, the rotation of the wheel inside is transferred tot the robot as a whole. A Hedgehog has three flywheels oriented in three different directions. By choosing which of the wheels to brake and by carefully applying the right amount of braking power, the creators can make the Hedgehog roll in any direction. They can even make it spin suddenly, and in low-gravity circumstances this can make the Hedgehog screw itself out of the ground, as it were. With a move like this, perhaps Philae could have liberated itself from the ditch it's stuck in. Check out the video for an idea what this would look like.

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Since Philae's terrain is quite rugged, this combination of rolling and hopping could be a better way of getting around anyway. It will be simpler and cheaper as well. It remains to be seen what kind of payload this kind of robot could carry. And of course the payload should be designed in such a way that it can function in any orientation.

About Herbert Blankesteijn

Herbert Blankesteijn is a technology journalist from the Netherlands who has written for many prominent Dutch newspapers. He presented and directed television and radio programmes and has 10 books to his name. Herbert is interested in nascent fields such as 3D printing, drones, robotics and the private space business.