For NASA's stuck Mars rover, the Spirit may be willing, but the wheels could prove too weak. The space agency on Thursday outlined a rescue plan to try to free the rover Spirit, which has been bogged in a sand trap on the red planet for half a year. The risky operation is expected to last several months.

"If it cannot make the great escape from this sand trap, it's likely that this lonely spot straddling the edge of this crater might be where Spirit ends its adventures on Mars," said Doug McCuistion, who heads the Mars exploration program at NASA headquarters.

The plucky rover was driving backward in April when its wheels broke through the crusty surface and became mired in a patch of talcum-like dirt. It tried to crab its way out, but its wheels sunk deeper.

After rehearsing various escape tries on Earth using prototype rovers, NASA said it was finally ready to provide some roadside assistance

The first driving commands will be sent to Spirit on Monday, but engineers cautioned the wheels likely will have high slippage.

The plans calls for Spirit to drive forward and retrace its steps.

"If we follow our old tracks out, we may be able to make better progress," said rover driver Ashley Stroupe of NASA's Jet Propulsion Laboratory.

Spirit landed on Mars with six working wheels but soon lost movement in its right front wheel. It had been rolling around the red planet dragging the gimpy wheel before it got stuck.

To complicate matters, images snapped by the rover show rocks pushing up against its underbelly, which could make it harder for the wheels to get traction.

Freeing Spirit is the toughest challenge faced by NASA since Spirit and its twin Opportunity parachuted to opposite sides of Mars in 2004. The twin rovers beat expectations by working beyond their three-month warranty.

Efforts to extract Spirit will continue until at least February. If the rover is not free by then, a review panel may decide whether it's worth it to keep on trying, McCuistion said.

It's been a tough year for Spirit. Along with being stuck, it had recurring memory glitches and problems with its antenna.

Despite its predicament, the rover has not been idle. Instead, it has been busy studying the soil to get a better idea of the past environment of Mars.

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On the Net:

Free Spirit page: http://marsrovers.jpl.nasa.gov/newsroom/free-spirit.html