In the struggle to free the Mars rover Spirit from a sand trap, NASA engineers are bringing out the reserve troops. A second, lighter duplicate rover slid into a sandbox for testing this week, delaying any attempt to free Spirit by as much as three weeks, to mid-September.

Spirit has been stuck in a sandpit for nearly four months. Since late June, engineers have been trying to determine the best moves to extricate it by driving a test rover around a sandbox at NASA's Jet Propulsion Laboratory (JPL) in California.

Rover engineers had earlier announced that they were almost done with the testing and would be ready to move Spirit around 10 August, but they backtracked in a meeting on 6 August.

"We've come up with additional tests that we want to do, and additional computer modelling that we want to do as well," says rover project manager John Callas at JPL. "Now we're looking at the middle of September."