NASA's Opportunity Mars rover examines a rock dubbed "Athens" in this image taken on March 25, 2015, by the rover's front hazard avoidance camera.

NASA's Opportunity Mars rover has suffered another bout of amnesia, less than a week after engineers installed a software upgrade intended to fix the robot's memory issues.

The long-lived Opportunity rover began experiencing problems with its flash memory — the kind that can store data even when the power is off — in late 2014. On March 20, mission engineers uploaded new software that aimed to fix the issue by bypassing an apparently faulty "bank." (The rover has seven such flash memory banks.)

But Opportunity experienced another brief amnesia episode on March 25, NASA officials said.

"Although we are a little disappointed at the occurrence of an amnesia event only five days after reformatting, we are not surprised," John Callas, Opportunity project manager at NASA's Jet Propulsion Laboratory in Pasadena, California, said in a statement. There is still no clear understanding of what is causing the problems. Only time will tell if we have been successful in mitigating the most serious flash problems."

Opportunity was experiencing multiple computer resets per day before mission team members began operating the rover in a "no flash memory" mode in December. No such serious issues have resurfaced since the March 20 reformatting, NASA officials said. The brief March 25 event didn't result in the loss of any science data, and Opportunity resumed its work shortly thereafter.

Opportunity and its twin, Spirit, touched down three weeks apart in January 2004 to search for signs of past water activity on Mars. Both rovers found plenty of such evidence and continued to operate long beyond their initial three-month prime missions: Spirit stopped communicating with Earth in 2010, and Opportunity is still rolling along.

Indeed, last week Opportunity completed the first-ever marathon beyond Earth when its odometer ticked over to 26.219 miles (42.195 kilometers). Second place in the off-world driving competition belongs to the Soviet Union's Lunokhod 2 rover, which traveled 24.2 miles (39 km) on the moon in 1973.

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