Budget cuts at NASA means that one of the two Mars rovers will be shut down, even though it's still doing useful science.
Besides resting Spirit, scientists also likely will have to reduce exploration by Opportunity, which is probing a large crater near the equator. Instead of sending up commands to Opportunity every day to drive or explore a rock, its activities may be limited to every other day, said John Callas, the Mars Exploration Rover project manager at JPL.
The rovers were originally deployed for three-month missions but have operated for more than four years.
Update: NASA decided not to go through with Mars rover budget cuts. (thx, jeff)