Mars Exploration Rover team members on July 21, 2009, tested how altering the order in which individual wheels turn for steering affects how those turns dig the wheels deeper into soft soil. From left: Alfonso Herrera, Vandana Verma, Bruce Banerdt.

This work was part of a series of tests at NASA's Jet Propulsion Laboratory, Pasadena, Calif., designed to determine the best way to get Spirit out of a Martian patch of soft soil called "Troy," where Spirit's wheels have dug in. The test setup simulates the situation at Troy.

To see updates on the efforts to free the Spirit rover, visit the JPL Free Spirit website.