Tests of possible maneuvers for use by NASA's rover Spirit on Mars include use of this lightweight test rover at NASA's Jet Propulsion Laboratory, Pasadena, Calif. In this scene from Sept. 8, 2009, rover team member Walter Hoffman is checking for a change in the vehicle's tilt after an arc-backwards maneuver.

This test rover, called the Surface System Testbed Lite, weighs about the same on Earth as Spirit does on Mars. Unlike the primary test rover in use at JPL, called the Surface System Testbed, the lighter model does not carry science instruments or a robotic arm. An object that weighs 10 pounds on Earth weighs just 3.8 pounds on Mars, due to the smaller mass of Mars compared to Earth.

Computer modeling using results from both test rovers and data from Mars is helping the rover team plot a strategy to try getting Spirit out of a patch of soft Martian soil where Spirit has been embedded for more than four months.

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