NASA's Mars exploration rover Opportunity used its front hazard-avoidance camera to take this picture showing the rover's arm extended towards a light-toned rock, "Tisdale 2," during the 2,695th martian day, or sol, of the rover's work on Mars (August 23, 2011). Tisdale 2 is about 12 inches (30 centimeters) tall. Credit: NASA/JPL-Caltech