"Our current suspicion is that Opportunity rebooted its flight software, possibly while the cameras on the mast were imaging the sun," Mars Exploration Rover Project Manager John Callas of NASA's Jet Propulsion Laboratory, Pasadena, Calif, said in a NASA news update. "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."
Commands have been sent to Opportunity for it to recommence post-conjunction operations, but mission managers will continue work to try to understand why the rover was found in a safe mode state.
PHOTOS: Mars Through Curiosity's Powerful MAHLI Camera
Opportunity, which landed on Mars in 2004 with sister rover Spirit, has roved over 22 miles across Meridiani Planum and is currently working around the rim of Endeavour Crater. Spirit was officially declared lost in 2010 after the rover became stuck in a sand trap at Gusev Crater. Both golf cart-sized rovers have uncovered evidence for water interactions on the Martian surface, providing a huge amount of data on the red planet's wet past.