The U.S. space agency, NASA, says its second Mars rover, Opportunity, is in excellent shape after Sunday's landing, relaying photographs of the terrain around it. NASA computer experts have a new theory about why its twin, Spirit, is malfunctioning, an overloaded memory.

Mission controllers in Pasadena, California say Spirit's computer memory may be clogged with too many files, accumulated during its seven month cruise from Earth and nearly three weeks of ground operations.

Mission manager Jennifer Trosper said engineers will try to nurse the rover back to health by erasing some its the memory. "We're in the process of getting ready to delete a large number of cruise files that were on Spirit, and obviously we recommend to Opportunity that they go ahead and delete those files," she said. "We will be more conscious of this limit that we have. That can easily be managed operationally once we fully understand the problem."

However, Ms. Trosper said there is also a small chance that a hardware problem or rays from a solar flare may have caused Spirit's computer malfunction. She predicts the troubleshooting and repair will take up to three weeks before Spirit can roll again.

The principal scientist for the mission, Steven Squyres of Cornell University, says he is not disappointed by the delay. He points out that the vehicle will probably compensate for the interruption by lasting longer than its planned 90 days. "You have to realize that 90 day number, that's when the warranty expires on this vehicle. That's the design lifetime of these things. That doesn't mean the wheels are going to fall off at 91. We put margin on top of margin on top of margin in the design specifically to allow for the fact that things go wrong when you go to places like Mars," he said.

Both Spirit and the identical Opportunity rover are on a hunt for evidence to determine if the planet once had liquid water that could have supported life. Spirit was sent to what scientists believe is an ancient lake bed. Opportunity's target was a flat plain with extensive deposits of a mineral called hematite, which usually forms in association with water.

Mission scientists are exuberant because Opportunity's initial images show that the spacecraft landed just where they wanted. The Cornell University scientist who built the rover cameras, James Bell, said they reveal a surrounding 20 meter-wide crater rich in hematite on the surface with an underlying exposed treasure, an outcrop of light-colored, layered rock. "We're looking back out across a pretty spectacular landscape. We're seeing outcrops. I mean, this is the Holy Grail for a geologist to be able to see these incredible rocks that are in their native habitat and we're going to go explore them with the rover," he said.

The six-wheeled robot is outfitted with a suite of instruments that researchers will use to analyze the rock, hematite, and soil to determine the water history of the region.

But Opportunity will not drive off its landing platform and begin its search until engineers spend a week or two collecting more photographs and testing its instruments.