NASA's Spirit rover has stopped transmitting data from Mars in an ominous turn that baffled engineers and sent them scrambling Thursday to figure out what brought the mission to a potentially calamitous halt.

This image mosaic taken by the panoramic camera on board the Mars Exploration Rover Spirit shows the rover's landing site, the Columbia Memorial Station, at Gusev Crater, Mars. To the right are the east hills, about 3 kilometers away from the lander. The picture was taken on the 16th martian day, or sol, of the mission (Jan. 18/19, 2004). A portion of Spirit's solar panels appear in the foreground. Data from the panoramic camera's green, blue and infrared filters were combined to create this approximate true color image. [AP]

NASA received its last significant data from the unmanned Spirit early Wednesday, its 19th day on the surface of Mars. Since then, the six-wheeled vehicle has sent either random, meaningless radio noise or simple beeps acknowledging it has received commands from Earth.

"We now know we have had a very serious anomaly on the vehicle," project manager Pete Theisinger said at NASA's Jet Propulsion Laboratory.

Engineers struggled to diagnose what was wrong with the rover. Among the possible causes: a corruption of its software or computer memory.

If the software is awry, NASA can fix it from Earth by beaming patches across more than 100 million miles of space or by rebooting the rover's computer. But if the problem lies with the rover's hardware, the situation would be far more grave — perhaps beyond repair.

"Yes, something could break, something certainly could fail. That's a concern we have — that's quite a serious event," Theisinger said.

Spirit is one-half of an $820 million mission. Its twin, Opportunity, is expected to land on Mars late Saturday. The twin rovers are supposed to examine the Red Planet's dry rocks and soil for evidence that it was once wetter and more hospitable to life.

Until Wednesday, Spirit had functioned almost flawlessly and NASA scientists and engineers had been jubilant.

Cushioned by its air bags, the rover made a bull's-eye landing on Mars, surviving what was by far the most dangerous part of the mission — the descent through the atmosphere at 12,000 mph. Then on Jan. 15, in another nail-biting moment for NASA, the rover safely rolled down a ramp onto Mars' ruddy soil without becoming snagged.

It has snapped thousands of pictures, including breathtaking panoramic views and microscopic images of the martian soil. It also carried out preliminary work analyzing the minerals and elements that make up its surroundings.

Steven Squyres, of Cornell University, the mission's main scientist, cautioned that communications problems are common on spacecraft. "While it is cause for concern, it is not cause for alarm," he said.

NASA last heard from Spirit as it prepared to continue its work examining its first rock, just a few yards from where it landed.

Early Thursday, NASA initially heard nothing from Spirit that would indicate it was in "fault mode," a state that the rover enters by itself when it has experienced a problem. Later, NASA send a command to Spirit as if it were in fault mode, anyway. Spirit acknowledged with a beep that it received the command, indicating an onboard problem. That puzzled engineers.

"It is precisely like trying to diagnose a patient with different symptoms that don't corroborate," said Firouz Naderi, manager of JPL's Mars exploration program.

The rover missed several scheduled opportunities to communicate, both directly with Earth and by way of two NASA satellites in orbit around Mars. As of midday Thursday, it had sent no engineering or science data for more than 24 hours. NASA scheduled two other attempts Thursday night to communicate with Spirit.

Preliminary indications suggested the rover's radio was working, and it continued to generate power from the sun with its solar panels. Spirit's internal clock also was running and had roused the rover several times on cue to communicate with NASA's Mars Global Surveyor and 2001 Mars Odyssey as they zipped overhead.

Engineers hoped to receive data on how the spacecraft is functioning by early Friday, when a window of communication with the rover opens, JPL director Charles Elachi said in a television interview broadcast by NASA.

"We can do a diagnostic and understand what happened, what are the corrective actions that need to be done and how do we bring it carefully and thoughtfully to its normal operation mode," Elachi said.

"There is nothing rushing us to do the fix immediately, other than people being anxious," he added.

Initially, engineers believed bad weather on Earth — a thunderstorm near a Deep Space Network antenna in Australia — had caused the communications glitch. But the weather was later discounted as the source,

The rover had been scheduled Thursday to grind away a tiny area of the weathered face of a sharply angled rock dubbed Adirondack. Examination of the rock beneath could offer clues to Mars' geologic past.

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