Efforts to free the stuck Spirit rover on Mars have been dragging on since May and a NASA official said Sept. 14 the robot may never get free.

“We are proceeding very cautiously and exploring all reasonable options,” said John Callas, NASA project manager for Spirit and its twin, Opportunity. “There is a very real possibility that Spirit may not be able to get out, and we want to give Spirit the very best chance.”

Callas and colleagues at the Jet Propulsion Laboratory have been testing ideas on a model of Spirit at the Pasadena, Calif., facility, in a pit designed to simulate the surface of Mars. It’s tricky though, because of the difference in gravity of the two planets. The rover team is also refining a detailed computer model of rover mobility, calibrated with results from testing and measurements from Mars.

“The computer modeling will allow us to connect the results from tests performed in Earth gravity with what to expect from the rover in Mars gravity,” Callas said in a statement.

Spirit has already far outlived expectations. It became embedded in soft soil at a site called Troy in early May, more than five years into a mission on Mars that was originally scheduled to last for three months. The rover team suspended further driving attempts with Spirit while evaluating how to free it.

The engineers are trying to figure out how to move Spirit while avoiding putting the rover’s center of gravity directly over a rock that is touching or nearly touching the machine’s underbelly. In the laboratory, meanwhile, engineers are using a lighter-weight test rover than the one used for most of the testing this summer. A complete “dress rehearsal” test of the extrication strategy judged to hold the best chance of success is planned before the team commands Spirit to begin driving. That test and subsequent review of its results are expected to take several weeks.

Moves by Spirit will not begin before October, according to current plans.

A dust storm that had reduced the electrical output from Spirit’s solar panels by nearly half during late August is still having some lingering effects, NASA said.