An attempt to clear the Spirit rover's (search) air bags from its path to the Mars surface has failed, and engineers are working on a last-ditch shot at removing the obstacle before choosing a more difficult route, NASA said Thursday.

Two sections of the air bags partially block the ramp that the six-wheeled robot would follow if it rolled straight off its lander. Engineers at NASA's Jet Propulsion Laboratory (search) consider that the safest route for the golf cart-sized rover to follow.

They fear the puffed-up sections of the air bags, woven of a polymer fiber stronger than steel, could snag on the rover's solar panels or robotic arm during the 10-foot roll to the ground. The air bags were used to cushion the rover's landing last week.

NASA (search) had hoped that lifting the ramp and then further retracting the air bags would gather them inward sufficiently to clear the path. That "lift and tuck" operation failed Wednesday.

"We were not successful in doing that yesterday," said Matt Wallace, mission manager for Spirit's sixth day of operations.

The newest pictures taken by the rover's cameras show a tilt to the distant martian horizon because of the attempts to clear Spirit's air bags.

If the final effort succeeds, Spirit could roll onto the ruddy dirt of Mars on Wednesday, its 12th day on the planet. Should engineers decide to take an alternate ramp, Spirit might not leave its lander until one to three days after that.

The alternatives would require Spirit to either pirouette 120 degrees to the right, allowing it to roll straight off, or turn 60 degrees to the left and roll off backward, Wallace said. Either choice could require further air bag retractions as well, he added.

NASA prefers that Spirit follow the safest path to the ground and simply drive straight off the lander. "There is risk inherent in getting off the vehicle," Wallace said.

While parked, Spirit has completed a 360-degree, color panorama of its surroundings and transmitted 40 percent of it to Earth.

The rest of the panorama should arrive over the next two to four days, said Jim Bell, of Cornell University, a camera scientist.

The Mars Exploration Rover project includes a second rover named Opportunity, which should land on Jan. 24. They were sent to prospect for evidence that Mars may have been a wet world conducive to life in its ancient past.