Nasa scientists say hundreds of computer files that have accumulated on the Mars rover Spirit may be the cause of problems that have crippled it.
Engineers knew the probe was alive
These "cruise files" will now be deleted from the second Mars rover Opportunity before it rolls on to Mars to begin its science mission.
The Spirit rover suffered a major malfunction on Thursday 22 January causing a loss of contact with Earth.
Opportunity landed on Sunday and is apparently in excellent health.
Surface development manager for Spirit, Jennifer Trosper, said the problem involved two types of memory used by Spirit: Ram and flash.
The space required in the rover's Ram memory to manage the data files stored in its flash memory was more than anticipated due to the build-up of files, Ms Trosper told a news conference.
"We have lots and lots of files on the spacecraft," she said. "We've been all the way through cruise [the journey through space], we've been using flash for that whole time. We have some cruise files on the file system.
"We were unaware of [the problem] because of the accumulation that happened during cruise and our 18 sols on the surface."
Project scientists have now re-established contact with the rover and are attempting to send it commands.
Rover health check
Mission controllers have started loading a "script" on the rover designed to locate the root cause of the problem and confirm the scientists' "hunch".
They will try to run a health check on the memory on Tuesday and try to delete some of the problem files on Wednesday.
After an initial reset on Thursday, Spirit became locked in a loop, continually rebooting its computer.
"We don't know yet if Spirit will be perfect again. Our current theory is one in which software will fix the problem, but there are other health checks we need to do," said Ms Trosper.
She added that scientists were still considering that a fault with the rover's high-gain antenna and motor control board that may have occurred during routine checks could have caused Spirit's breakdown.
But this scenario is considered much less likely by scientists working on the rover's recovery.
Halfway around the planet from Spirit, Nasa's Opportunity rover is having better luck.
Nasa released a new colour picture of the rover's landing site at Meridiani Planum, showing smooth, dark soil and a rocky outcrop of bedrock.
Scientists believe Opportunity has landed in a crater about 20 metres (66 feet) across.
They have confirmed that two-way communications between Earth and the rover on Mars are working normally and that the rover had the correct bearings - which will be vital once Opportunity rolls on to the Martian soil and begins driving around.