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.

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

Spirit's wheel stalled during the second step of its thirddrive attempt since the effortto 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 itsensed it was tilting too much and stopped spinning its wheels after just a fewseconds.

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

The stall occurred this time because the wheel's progressfell 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 notactually move that far. The center of the rover moved about 0.2 inch (4millimeters) forward, 0.1 inch (3 millimeters) to the left and about 0.1 inch (3millimeters) down, NASA said. During its second drive attempt, the rover alsomoved forward slightly.

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

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

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