In recent days, controllers directed NASA's Mars Exploration Rover Spirit to back up and turn to try to dislodge a potato-size rock from Spirit's right rear wheel. The rock did not present a threat -- it was sort of like having a pebble stuck in your shoe -- but the rover team was taking no chances that the rock might work its way deeper inside the rover's wheel well. The rock can be seen in the lower left side of this image, which Spirit took with its right rear hazard avoidance camera on martian day, or sol, 345 (Dec. 21, 2004). By the following day, the rock had rolled out onto the martian sand.