NASA engineers are scratching their heads over some unexpected behavior from the long-lived Spirit rover, which began its sixth year exploring Mars this month.

Spirit failed to report in to engineers at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, Calif., last weekend, prompting a series of diagnostic tests this week to hunt the glitch's source.

The aging Mars rover did not beam home a record of its weekend activities and, more puzzlingly, apparently failed to even record any of its actions on Sunday, mission managers said.

"We don't have a good explanation yet for the way Spirit has been acting for the past few days," said NASA's Sharon Laubach, who leads the JPL team that that writes and checks commands for the rover and its robotic twin Opportunity. "Our next steps will be diagnostic activities."

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Sunday marked Spirit's 1,800th Martian day, or sol, exploring a region known as "Home Plate" in the planet's expansive Gusev Crater.

Spirit and its twin Opportunity were initially expected to spend just 90 days exploring the Martian surface when they landed in succession more than five years ago this month. Opportunity is currently headed for the monster crater Endeavour on the other side of Mars.

On Sunday, Spirit apparently received commands to drive to its next waypoint, but failed to move an inch, mission managers said.

While that glitch can have any number of causes, such as Spirit not properly perceiving it was ready to drive, the rover's failure to record its daily work in its non-volatile computer memory is perplexing, they added.

By Monday, Spirit's mission controllers decided to tell the rover to find the sun with its camera on Tuesday to determine its location on Mars.

Early Tuesday, the rover beamed back that it had tried to follow the instructions of its human handlers, but couldn't find the sun.

NASA engineers believe Spirit's woes may be due to a transitory cause, such as a high-energy cosmic ray hitting the rover's electronics. On Tuesday, the rover's non-volatile memory appeared to be working fine, mission managers said.

The rovers Spirit and Opportunity have lasted more than 20 times their initial three-month mission plan, with each suffering from aches and pains associated with their longevity.

Spirit initially bounced back from a worrying computer glitch early in its mission and has since survived frigid winters on Mars and scaled a nearby hill. Both rovers have expanded scientists' knowledge of the history of liquid water on Mars during their respective missions.

While puzzling, Spirit's new glitches don't appear to be a serious concern at present, according to NASA's rover mission chief John Callas.

"Right now, Spirit is under normal sequence control, reporting good health and responsive to commands from the ground," he added.

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