During a moratorium on commanding this month while Mars passed nearly behind the sun - a phase called solar conjunction - NASA's Mars Exploration Rover Opportunity entered a type of standby mode.

Mission controllers learned of the changed status on April 27 when they first heard from Opportunity after the period of minimized communication during the solar conjunction. They prepared fresh commands April 29 for sending to the rover to resume operations.

Initial indications suggest the rover sensed something amiss while doing a routine camera check of the clarity of the atmosphere on April 22.

"Our current suspicion is that Opportunity rebooted its flight software, possibly while the cameras on the mast were imaging the sun," said Mars Exploration Rover Project Manager John Callas of NASA's Jet Propulsion Laboratory, Pasadena, Calif.

"We found the rover in a standby state called automode, in which it maintains power balance and communication schedules, but waits for instructions from the ground. We crafted our solar conjunction plan to be resilient to this kind of rover reset, if it were to occur."

Opportunity has been working on Mars for more than nine years. NASA's other Mars rover, Curiosity, which landed last year, is also nearing the end of its solar conjunction moratorium on commanding.

Curiosity has reported coming through the conjunction in full health. Controllers plan to send Curiosity's first set of post-conjunction commands on May 1.