Possible rest stops for the rover are nearby on the northern edge of Home Plate, and scientists have a couple places in mind, he said. (See a Mars map.)
Once in position, the rover will idle as sunlight declines until the middle of the Martian winter next June. Meanwhile, more dust is likely to accumulate on its panels.
"We won't be doing much, and more dust [than we expect] could make that dicey," Callas said.
Still, the scientists have a few tricks up their sleeve. Shutting down a heater to one of the scientific instruments would cut the amount of power needed for bare-minimum survival by about a third.
A riskier action would be turning down the heat on the rover's electronics.
Spirit's processors are able to work at temperatures as low as -40 degrees Fahrenheit (-40 degrees Celsius), but they might be able to go lower.
"We've tested them to -67 degrees Fahrenheit [-55 degrees Celsius] in ground testing before launch," Callas said.
"But that was a brand-new rover, and this one's four years old. We'd be taking a risk."
Plenty of Power for Opportunity
The other rover, Opportunity, faces no such crisis.
That's because it is closer to the Martian equator, where sunlight is more consistent across seasons, said Steve Squyres of Cornell University, who heads Opportunity's science team. (See a photo of the crater.)
Also, Opportunity's solar panels are much more frequently cleaned by wind gusts, he added. At Spirit's position near Home Plate, panel-scouring winds are rare.
"Opportunity is going to die someday, but it's not going to be from dust on the panels," Squyres said.
If Spirit's solar panels hold out through the winter, it too might bring another year's worth of discoveries.
Other than the dust, Callas of JPL said, "both rovers are in very good health. We have had no major [equipment] failures since we lost the right front wheel on Spirit almost two years ago."
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