LOS ANGELES – The Mars rover Spirit is racing against time to reach a resting spot for the winter after a giant dust storm drained much of its energy, scientists said Monday. Spirit has until Christmas to drive to the sunny slope of a low plateau where it will park itself with its solar panels pointed at the sun and hunker down for the winter.

The trek is difficult because the rover needs one day to recharge after every day it moves. That means ground controllers on Earth have seven opportunities to direct the robot into position for it to have a chance to survive.

"It's scramble right now because we're losing sunlight,'' said rover project manager John Callas of NASA's Jet Propulsion Laboratory, which manages the mission.

Spirit's situation was detailed during an American Geophysical Union meeting in San Francisco.

Spirit and its twin rover Opportunity faced their biggest challenge yet this summer when a series of dust devils blanketed their solar panels and limited their movement. Winds managed to clean off Opportunity, but Spirit is still covered in gunk and working at 42 percent capacity.

Callas predicted further dust accumulation could cause Spirit's solar array performance to drop to 30 percent by the winter.

By contrast, the rover was operating at 70 percent capacity during its first Martian winter and 50 percent last year. That likely means Spirit will be stationary for more than seven months to conserve energy.

In October, NASA extended the rovers' mission for the fifth time since the machines landed on opposite sides of the planet in 2004.

Spirit has been exploring a plateau called Home Plate where it discovered silica-rich soil earlier this year. Meanwhile, Opportunity is analyzing exposed rock layers inside a giant crater near the equator.

The silica likely came from an ancient hot spring or fumarole and represents the best evidence yet of a past habitable environment, said principal investigator Steve Squyres of Cornell University.