This past week we've been treated to some astonishing imagery from the surface of the asteroid 162173 Ryugu. Thanks to the Japan Aerospace Exploration Agency's (JAXA) Hayabusa 2 mission there are presently a pair of teeny 'rovers' somewhere on the surface of this ancient body.
These machines, MINERVA-II1A and MINERVA-II1B represent just parts of a truly remarkable (and slightly overwhelming) array of scientific exploration devices carried to Ryugu. There are 2 additional lander-rover-like-devices, there are infrared and visible like cameras, there is a sample return system and capsule for return to Earth, there is an impactor (to release samples from the asteroid's surface), there are 5 bean-bag like 'marker' objects to drop to the surface for navigational purposes, there's a laser altimeter...and more.
If Hayabusa 2 was an automobile you'd say that it was fully-loaded with all optional extras. Here's a chart of some of that gear:
The currently deployed rovers are small, cylindrical objects, that weigh about 2.4 lbs on Earth. Covered with solar panels and spiky pins to keep them from burying too deep in the asteroid's regolith, they can actually maneuver around. But on a body as small as Ryugu (barely a kilometer across) the surface gravity is extremely feeble - providing a downwards acceleration of about 0.0001 meters per second per second, or about 0.001% of Earth's pull.
The challenge of that weak pull is that conventional wheels or thrusters would likely throw the rovers off into space with the slightest turn or puff. The pebbly, dusty surface also raises the odds of simply getting stuck.
So the answer for the MINERVA rovers? An internal electric motor drives a 'torquer' - basically a small cylinder that rotates and stops, imparting an equal but opposite force to the rover. This sits on a turntable inside the rover, so that the torque for the hop can be applied in any direction. Once the rover hops it follows a ballistic trajectory (think high-school physics), a nice arcing path that brings it down again in the new location.
Of the other mobile elements of the mission, the extra rover MINERVA-II2 (at least I think that's its designation, JAXA is as bad as Apple is becoming with naming its products), actually has 4 different ways of moving jammed into its little body. Those include a 'leaf-spring buckling mechanism', which sounds exciting even if a bit Victorian.
The engineering talent that has gone into Hayabusa 2 is as fascinating as the missions scientific goals. I also find it an intriguingly different philosophical approach than many US or European missions - full of ideas, intricate devices, and a kind of seat-of-your-pants way of doing things, with backup tools and activities, and complexity. Given the largely uncharted terrain of asteroid environments this feels like a pretty good way to pursue the adventure of exploration.