This blink comparison aids evaluation of a test of the right-front wheel of NASA's Mars Exploration Rover Spirit during the rover's 2,113th Martian day, or sol (Dec. 12, 2009). The test of electrical resistance in the wheel's drive motor, planned for comparison with results of tests on the right-rear wheel, surprisingly indicated normal resistance and produced a slight wheel movement of about one-fourth of one degree. Slight wheel movement is expected during a resistance test for an operating wheel actuator, but the right-front actuator (the combination of motor and gearbox) was expected to be non-operational because it had stopped working in April 2006.

The results of the resistance test do not enable a conclusion about whether the right-front wheel is usable for driving. Further tests are planned for both the right-front wheel and for the right-rear wheel, which has not moved since it stalled on Sol 2099 (Nov. 28, 2009).

The two wide-angle views shown one after the other in this comparison come from Spirit's front hazard-avoidance camera, one taken before the resistance test and the other after the test. The most obvious change is in the position of shadows, a change unrelated to the wheel's movement during the drive. The shaded area gets smaller in the "after" image. The very small amount of wheel movement discernible -- and also indicated by telemetry from the rover -- is equivalent to about one full rotation of the drive motor at the wheel's gear ratio of about 1,500 motor rotations for each full wheel rotation.

The rover team began commanding extrication drives in November after months of Earthbound testing and analysis to develop a strategy for attempting to drive Spirit out of this soft-soil site, called "Troy." The extrication drives are expected to make slow, if any, progress in coming weeks, and the probability of success in escaping from Troy is uncertain.