After sending back hundreds of striking photos for nearly three weeks, the Mars rover Spirit has suddenly begun transmitting gibberish, or refusing to communicate with Earth at all.

NASA engineers described the problem as “a very serious anomaly” Thursday and said they were scrambling to identify and correct it. “It’s not clear there is one cause that would explain the observables we’ve seen,” deputy project manager Richard Cook said. “That’s what’s perplexing us at the moment.”

One possible explanation, officials said, would be a strike by cosmic radiation that affected the rover’s hardware and scrambled its computer programs. Mission leaders nevertheless expressed confidence they could find the problem and correct it.

“We are going to do everything we can, knowing this is risky – this is exploration,” said Charles Elachi, director of the National Aeronautics and Space Administration’s Jet Propulsion Laboratory in Pasadena, Calif.

Officials at JPL were encouraged by a series of beeps from the rover Thursday morning. They were sent to Earth on a specific radio channel that Spirit was instructed to use if a malfunction sent it into “safe mode.”

Controllers planned to tune to that same channel this morning to ask the rover to send back diagnostic engineering data that could reveal what went wrong.

“Then we can start taking some corrective action to bring it back up, slowly and methodically, to nominal operations,” Elachi said.

Engineers received their most encouraging news from Spirit on Thursday morning, confirming that it had received queries from Earth. The transmissions were no more than beeps, but they were enough to tell troubleshooters that the rover’s radio, amplifiers, antennas and associated hardware were working.