Pasadena, Calif. ? NASA’s Spirit rover stopped transmitting data from the surface of Mars, baffling engineers Thursday and bringing the vehicle’s mission to a potentially calamitous halt.

NASA received its last significant data from the unmanned Spirit early Wednesday, its 19th day on Mars. Since then, the six-wheeled rover 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. 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.

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.