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Mars Rover Spirit Down a Wheel
riflemann writes "NASA is reporting that two years into its 90-day mission, Spirit has lost one wheel and is now running on five wheels, dragging the broken wheel. With this reduced mobiity, the rover still needs to make its way to a slope where it can catch enough sun over the Martian winter to keep it operating. 'Even though the rovers are well past their original design life, they still have plenty of capability to conduct outstanding science on Mars.', says project leader Dr. John Callas."
Doesn't really LOOK like it's free spinning:
1 /2F195698779EFFAQ15P1210L0M1.JPG
http://marsrovers.jpl.nasa.gov/gallery/all/2/f/78
It's not that simple. The rovers are full of fairly sophisticated sensor packages, most of which can't handle the extremely low temperatures on the Martian surface. They need the batteries to basically, well, run the heater.
The principle investigator for the missions has written a book, "Roving Mars", that really is worth the read.
They stopped using the wheel about a year ago for a while because it was having problems as if the lubricant was wearing off, and it indeed did *not* free-spin when power was not sent to it. They simply dragged it around by running the rover backward. They found it easier to control the rover by dragging the bum wheel rather than by pushing it. They only used the wheel for close-up control when rocks were being targeted. Eventually it started working properly for a while, and now won't turn at all. It does not appear they have a "free spin" mode. Dragging is it.
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I beleive they have a video about the last time the wheel was left dragging. They did some test-bed simulations of an Earthly rover copy. Page down to the "Driving Uphill Backwards" portion, about half-way down the page:
http://marsrovers.jpl.nasa.gov/gallery/video/spir
Table-ized A.I.