Because of the mechanical failure, controllers at NASA's Jet Propulsion Laboratory said they are having a difficult time getting the rover to move toward a north-facing slope to survive the cold and dark Martian winter, which is still more than a month away.

Spirit must position its solar panels northward in winter to collect enough solar energy to operate while the Sun lays low along the red planet's horizon.

"Essentially, it's a race against time," controllers said in a statement. Even though the period of minimum sunshine in the Martian winter is more than 100 days away, Spirit already receives only enough power to drive for about an hour a Martian day, or sol, on flat ground.

"The climate is changing rapidly, and we have to put our primary emphasis on keeping the rover safe," said Steve Squyres, the principal investigator for both rovers. "Assuring survival has to take priority over science until we've got the vehicle on safer ground."

The problem is the route Spirit must take to reach that safer ground seems more and more difficult, because of the softness of the soil. That dragging front wheel has made progress painfully slow, and could paralyze the rover, perhaps permanently.

So the team has devised what might turn out to be a last-ditch strategy: They have reversed Spirit's course, driving it back downhill for a while before moving laterally to a different slope that should be easier to climb and will provide the maximum available sunlight for the rover's solar panels.

So far, the results are mixed. Spirit drove about 10 meters (33 feet) in one day after the wheel stopped working, but then lost traction on following days because of wheel slippage in the loose soil. A further problem has been caused by the rover's suspension system, which keeps all six wheels in contact with the ground.

"Engineers designed the rovers so well that Spirit's weight remains evenly distributed on all six wheels," said Al Herrera, a computer sequencing specialist for Columbus Technologies and Services Inc. who oversees uplink commands to the rover. "That keeps the rovers balanced so they don't tip or roll backward."

It also prevents relief against the dragging right-front wheel disrupting Spirit's progress, however, because it continues to maintain equal weight on it.

Spirit's wheel difficulty emerged March 13, on sol 779, when the motor suddenly began drawing more electrical current than normal, then stopped drawing current altogether while the rover was turning to adjust the orientation of its communications antenna.

Engineers determined that the fault occurred in a drive actuator, one of 10 motors that operate the six wheels. Following tests in the duplicate rover facility at JPL, engineers determined the motor could no longer be used.

Spirit's problem is long overdue. The rover and its twin, Opportunity, have lasted more than two years, or nearly nine times longer than their mission design. Spirit's wheel motors have rotated more than 13 million times.

Still, controllers are trying to maintain their optimism. "It is too early to tell how serious this is," said John Callas, the rover's project manager. "The nature of the terrain is a dominant factor."

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