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. That can happen for many reasons, including the rover properly sensing that it is not ready to drive. However, other behavior on Sol 1800 was even more unusual: Spirit apparently did not record the day's main activities into the non-volatile memory, the part of its memory that persists even when power is off.
Now that's rocket science -- getting a probe designed for 90 days to go 1800.
Spirit isn't written off yet, but this is probably a sign that the end of the probe's usefulness is coming to an end.
