After a day of troubleshooting, engineers have not yet been able to restore communications with the Spirit rover, which stopped beaming back science and engineering data Wednesday. Project manager Pete Theisinger at the Jet Propulsion Laboratory in Pasadena, Calif., described the situation as "a very serious anomaly," but said it was too soon to say what might be causing the problem, whether it might be potentially fatal or whether the spacecraft can be restored to normal operation.

"The team has been meeting this morning and through the night working on a set of postulated fault scenarios," Theisinger said. "There is no one single fault that explains all the observables."

He said engineers planned to regroup later this evening to continue troubleshooting and to determine a plan of action.

The loss of contact came at one of the worst possible times for engineers at JPL. An identical rover, named Opportunity, is on target to enter the martian atmosphere around midnight Saturday. But Theisinger said Spirit's troubleshooting would not interfere with Opportunity's descent. The second lander is on target and in good health. Managers are still debating whether to simply stand down on Spirit operations until after Opportunity makes it to the surface.

In the meantime, tension mounted.

"If this problem on Spirit is somehow a software corruption issue, or memory corruption issue that's reflecting itself in software, and there's not a serious power fault, for example, then I think Spirit can go for quite a long time and we can pick up the pieces again," Theisinger said. "But if on the other hand it's had some kind of major power fault ... it may be more difficult to recover from that."

A rainstorm in Australia yesterday interfered with commands being uplinked to Spirit. At that time, the spacecraft sent a short signal indicating it had received the instructions but engineers said the strength of the uplink was much lower than desired and that not all of the commands got through.

The rover's electronic brain is programmed to ignore incomplete commands and engineers thought Spirit would simply stand by, executing previous instructions before calling Earth with daily downlinks of science and engineering data and awaiting further instructions.

Had the rover been operating properly, that would have happened during afternoon and evening overflights by NASA's Mars Global Surveyor and Mars Odyssey orbiters, which are being used as communications satellites for the Mars rover mission.

No data was received during the first overflight. A short signal was received late Wednesday during a Global Surveyor relay pass, but it contained no data. Nothing was heard from Spirit during subsequent passes, either through satellite relay or through programmed direct-to-Earth transmissions using the rover's high-gain antenna.

"There were beeps we heard yesterday afternoon on Mars and then we got the Mars Global Surveyor signal that indicated it heard from the (rover's) UHF link," said Richard Cook, a senior rover manager. "It was not what we expected. We saw a signal, meaning the rover's radio was on, but there wasn't data present. ... The computer wasn't sending information over to it. But we did at least see a signal."

This morning, flight controllers sent a low-data-rate signal to Spirit on the assumption the computer believed it had run into a problem and had loaded fault-protection routines. What may have been a brief response was received. If that data turn out to be valid, it would indicate Spirit is still alive, giving engineers hope they might be able to recover from whatever has gone wrong.

"That would tell us the spacecraft thinks it's on the fault side of the tree somehow, for some reason, that would mean we have positive power, some element of the software is working. ... But, you know, we need to confirm that. So don't take that further than it deserves to go right now."

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