Attempt to Free Mars Rover Snagged by Wheel Stall
This image shows the progress of Spirit after its second drive attempt. (Click on the image for a "before" and "after" animation.) In the "after" image, Spirit's left-front wheel has become slightly less buried in the soft soil in which the rover had become embedded about six months ago. The right-front wheel, which has not been usable for driving since 2006, has been pushed perceptibly forward by the drive. The amount of forward motion is less than one percent of the distance that would have been covered on firm ground by the amount of wheel rotation commanded in the drive.
Credit: NASA/JPL-Caltech

NASA's bogged-down Mars rover Spirit hit another snag in its attempt to free itself from a sand trap when one of its wheels stalled as it tried to move forward, NASA reports.

Spirit's wheel stalled during the second step of its third drive attempt since the effort to free it from a sandy feature called Troy began in earnest on Nov. 17. The rover also hit a snag during its first drive attempt last week when it sensed it was tilting too much and stopped spinning its wheels after just a few seconds.

The wheel that stalled during the third attempt was the rover's right-rear wheel; Spirit's left-middle wheel also stalled back in May, but has since worked normally.

The stall occurred this time because the wheel's progress fell behind the expected rotation rate. The rover had completed about 13 feet (4 meters) of commanded wheel spin before the stall stopped the drive.

Though the wheel spun for that distance, the rover did not actually move that far. The center of the rover moved about 0.2 inch (4 millimeters) forward, 0.1 inch (3 millimeters) to the left and about 0.1 inch (3 millimeters) down, NASA said. During its second drive attempt, the rover also moved forward slightly.

On Monday, the rover team planned to run a set of diagnostics to explore the right rear wheel stall. The diagnostics will include a rotor resistance test, a possible steering test, a small backward rotation of just the right-rear wheel and a short (about 3 feet, or 1 meter) forward commanded motion of the rover.

Spirit won't attempt to drive again until Wednesday at the earliest, according to a NASA statement.

Spirit and its rover twin, Opportunity, have been on Mars for nearly six years now.