Five years into what was supposed to be a 90-day mission on Mars, the two rovers, Spirit and Opportunity, are beginning to face a series of problems associated with old age. "On Sunday, during the 1,800th Martian day, or sol, of what was initially planned as a 90-sol mission on Mars, information radioed from Spirit indicated the rover had received its driving commands for the day but had not moved," NASA said.

However, scientists said other behavior was even more unusual: Spirit apparently did not record the day's main activities into non-volatile memory (that part of its memory that persists even when power is off).

The National Aeronautics and Space Administration scientists said they plan to run diagnostic tests to determine why Spirit didn't report as scheduled, including failing to determine its orientation after an incomplete drive.

Troubleshooters at JPL wondered whether the rover failed to drive because it wasn't properly oriented to the sun. So on Monday engineers ordered Spirit to locate the sun with its camera. Spirit reported back that it could not find the sun.

Moreover the rover did not record actions, as it otherwise always does, to the part of its computer memory that retains information even when power is turned off, the so-called nonvolatile memory. “It’s almost as if the rover had a bout of amnesia,” said John Callas, the project manager for the rovers.

However Spirit's fellow Martian rover Opportunity still appears to be in fine condition. Both bots have been roving the midsection of Mars for more than 20 times the planned 90-day length of their missions. The NASA-JPL team will run diagnostic tests on the rover this week. The problems could be short-lived. It's possible that cosmic rays hit some vital electronics, causing them to temporarily malfunction.