Complicating the work to track down the problem, "the indications we have on two occasions is that the thing that causes the reset is not always perceived to be the same," Theisinger said. "We are confused by that, but that's the facts as we presume them to be right now."

The reset sequence, similar to repeatedly unplugging one's personal computer and forcing it to restart, began Wednesday morning on Mars when a calibration of the spectrometer motors ended prematurely. An anomaly team has been formed to study the telemetry and to decide what readings to request from Spirit to help narrow down the range of possible failures.

"I think we should expect that we will not be restoring functionality to Spirit for a significant period of time," Theisinger said, "I think many days, perhaps a couple of weeks, even in the best of circumstances, from what we see today."

In the meantime, he said, Spirit remains in "critical" condition.

"We do not know to what extent we can restore functionality to the system because we don't know what's broke," Theisinger said. "We don't know what started this chain of events and I think, personally, that it's a sequence of things, and we don't know, therefore, the consequences of that. I think its difficult at this very preliminary stage to assume we did not have some type of hardware event that caused this to start and therefore, we don't know to what extent we can work around that hardware event and to what extent we can get the software to ignore that hardware event if that's what we eventually have to do.