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LOS ANGELES — Scientists plan to put one of the twin Mars rovers to sleep and limit the activities of the other robot to fulfill a NASA order to cut $4 million from the program's budget, mission team members said Monday.

The news comes amid belt-tightening at NASA headquarters, which is under pressure to juggle Mars exploration and projects to study the rest of the solar system.

The solar-powered rovers Spirit and Opportunity have dazzled scientists and the public with findings of geologic evidence that water once flowed at or near the surface of Mars long ago.

Both rovers were originally planned for three-month missions at a cost of $820 million, but are now in their fourth year of exploration. It costs NASA about $20 million annually to keep the rovers running.

The Mars rovers program received its latest extension, $22 million, in fiscal year 2008 and has used up half money. The latest directive from NASA to cut $4 million of the remaining funds means Spirit will be forced into hibernation in the coming weeks, said principal investigator Steve Squyres of Cornell University.

"It's very demoralizing for the team," Squyres said.

Spirit has parked itself on a sunny slope for the Martian winter and had planned to gather atmospheric measurements before the budget cut. Instead, it will now stay in sleep mode for most of the winter and stop all science gathering.

The funding cut was announced in a letter delivered Wednesday to NASA's Jet Propulsion Laboratory in Pasadena.

The cut comes at a time when the robots are in the midst of "extensive exploration campaign," said deputy principal investigator Ray Arvidson of Washington University in St. Louis.

"We're not done. There is still a lot to explore," Arvidson said.

JPL, which manages the rovers, plans to appeal the cut, but it's unclear whether it would work.

Besides resting Spirit, scientists will also likely 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.

"Any cut at any time when these rovers are healthy would be bad timing," Callas said. "These rovers are still viable capable vehicles in very good health."