Humans aren't the only forgetful ones. The Mars Rover Spirit "had a bout of temporary amnesia" on Friday, NASA officials said today.

They have no clue why. And the recalcitrant robot has not offered any clues.

The event is part of a string of problems dating back to an unexpected computer reboot that Spirit initiated a week ago. Another reboot occurred this weekend after the amnesia trouble.

Here's how NASA describes the robot's forgetfulness: Spirit unexpectedly fails to record data into the type of memory, called flash memory, where information is preserved even when power is off. Spirit has had three of these amnesia events in the past 10 days, plus one back on Jan. 25.

Officials don't know if the amnesia is related to the reboots.

The most recent reboot put Spirit back into an autonomous operations mode in which the rover keeps itself healthy, according to today's statement. Spirit experienced no problems in this autonomous mode on Sunday.

The rover team at NASA's Jet Propulsion Laboratory, Pasadena, Calif., revised plans today for regaining Earth control of Spirit's operations and resuming diagnostic and recovery activities by the rover.

"We are proceeding cautiously, but we are encouraged by knowing that Spirit is stable in terms of power and thermal conditions and has been responding to all communication sessions for more than a week now," said JPL's Sharon Laubach, chief of the rover sequencing team, which develops and checks each day's set of commands.

During the past week of diagnostic activities, the rover has successfully moved its high-gain dish antenna and its camera mast, part of checking whether any mechanical issues with those components may be related to the reboots, the amnesia events, or the failure to wake up for three consecutive communication sessions two weeks ago.

Spirit, and its rover twin Opportunity, finished their three-month prime mission on Mars five years ago and have kept operating through multiple mission extensions.

Officials aim to get Spirit back to full operation or, if something is broken, at least get it going in some capacity.

"For example, if we do determine that we can no longer use the flash memory reliably, we could design operations around using the random-access memory," Laubach said.

Spirit has 128 megabytes of random-access memory, or RAM, which can store data as long as the rover is kept awake before its next downlink communications session.