Rover engineers check how a test rover moves in material chosen to simulate some difficult Mars driving conditions. The scene is inside the In-Situ Instrument Laboratory at NASA's Jet Propulsion Laboratory, Pasadena, Calif. These tests in early May 2005 were designed to help plan the best way for the rover Opportunity to drive off of a soft-sand dune that the rover dug itself into the previous week. The mixture of sandy and powdery material brought in for these specific tests matched the way the soil underneath Opportunity caked onto wheels, filling the spaces between the cleats on the wheels.