This picture shows a model of the Mars Exploration Rover Spirit being tested for performance on five wheels at NASA's Jet Propulsion Laboratory. Spirit's right front wheel, now operating at six times its design life, has been showing signs of age, so rover planners devised a creative approach to keep the rover moving. They will drive Spirit backwards on five wheels, engaging the sixth wheel only sparingly to ensure its availability for tougher terrain. Tests performed at JPL allowed the rover planners to come up with this roundabout solution, and to develop commands that will help the five-wheeled rover steer.