Testing at NASA's Jet Propulsion Laboratory in August 2009 is assessing possible maneuvers that the Mars rover Spirit might use for escaping from a patch of soft soil where it is embedded at a Martian site called "Troy".
A second, lighter-weight test rover has entered the testing setup at JPL where rover team members are assessing strategy for getting Spirit out of soft soil where it is embedded on Mars.
The rover team has begun using a test rover that does not carry a science payload or robotic arm, as do Spirit and Opportunity on Mars, and the primary engineering test rover at JPL. While the primary test rover's weight on Earth is greater than Spirit's weight on Mars, the second rover is even lighter on Earth and closer to the weight of Spirit on Mars.
Making comparisons between motions of the two test rovers in duplicated drives will aid the rover team in interpreting effects of differing gravity on rover mobility. The testing team plans to run such comparisons both in the soft, fluffy material being used to simulate the soil at Spirit's current location and also on coarser, crushed rock that offers better traction.
"There is no perfect Earth analog for Spirit's current situation," said JPL's John Callas, project manager for the twin Mars Exploration Rovers. "There's less gravity on Mars, little atmosphere, and no moisture in the soil where Spirit is. It is not anything like being stuck in sand or snow or mud on Earth. Plus, since the rover moves only about as fast as a tortoise, you cannot use momentum to help. No rocking back and forth as you might do on Earth."
The comparison experiments with the two test-rover siblings to Spirit and Opportunity precede a planned "dress rehearsal" long-duration test of driving as far in the test setup as the distance that Spirit would need to achieve on Mars to escape its predicament at the site called "Troy."
The team has also made further assessments of the position of a rock underneath Spirit relative to the rover's center of gravity. Part of the strategy for getting Spirit free will be to avoid getting in a position with the center of gravity directly over a rock touching the rover.