National Aeronautics and Space Administration scientists said Spirit's solar array produced only 89 watt hours of energy during the rover's 1,725th Martian day, which ended Sunday. That is the lowest output by either Spirit or its twin, Opportunity, during their nearly five years on Mars, and much less energy than Spirit requires daily.

NASA said the charge level of Spirit's batteries is dropping so low, it risks triggering an automated protective shutdown.

"The best chance for survival for Spirit is for us to maintain sequence control of the rover, as opposed to it going into automated fault protection," said John Callas of NASA's Jet Propulsion Laboratory in Pasadena, Calif., project manager for Spirit and Opportunity.

"Mission controllers are commanding Spirit to turn off some heaters, including one that protects a science instrument -- the miniature thermal emission spectrometer -- and take other measures to reduce energy consumption," the space agency said in a statement. "The commands will tell Spirit not to try communicating again until Thursday.

"While pursuing that strategy the team also plans to listen to Spirit frequently during the next few days to detect signals the rover might send if it does go into a low-energy fault protection mode."