NASA said Monday that the rover had switched itself into safe mode two days earlier when a command file running on its backup, or "B-side," computer failed a size-check by protective software. The agency determined that a software bug had attached an unrelated file to the one being checked, causing a file size mismatch.
The SUV-like rover is being operated by its B-side computer because its primary, A-side computer suffered a memory glitch on Feb. 27, causing the Curiosity project team at NASA's Jet Propulsion Laboratory to switch to the backup system and put the vehicle into safe mode for two days.
NASA is still working to determine the cause of the A-side computer problem. "We still don't know exactly what that is yet, whether it's a hardware issue or was a radiation event," said John Grotzinger, Mars Science Lab project scientist, speaking from the Lunar and Planetary Science Conference in The Woodlands, Texas.
[ Rock testing from the Curiosity rover shows elements on Mars consistent with life. Read more at NASA Curiosity Rover Finds Life-Enabling Basics On Mars. ]
For the time being, the A-side computer is operating as a backup to the B-side computer. Grotzinger said the Curiosity project team has uploaded software patches to the B-side computer to protect it from the problems that disabled the A-side computer.
NASA officials downplayed the seriousness of Curiosity's latest software glitch, saying that the rover is stable, healthy and communicating with JPL engineers. "This is a very straightforward matter to deal with," said project manager Richard Cook. "We know how to keep this from occurring in the future."
However, the snafus have put Curiosity's scientific observations on hold for nearly three weeks and counting. Those observations have been suspended since the February 27 shut down, and NASA said it will take a "couple of days" more to bring the rover out of safe mode after the latest setback.
In the near term, NASA researchers have a narrowing window of opportunity for experimentation. From Earth's perspective, Mars is about to pass behind the sun, raising the potential for interference in communications between Curiosity and mission control. As a precaution, NASA won't send commands to the rover for four weeks beginning on April 4.
The technical complications come on the heels of NASA's recent announcement that Curiosity's first rock samples revealed evidence that life-enabling conditions -- including the presence of carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur -- once existed on the Red Planet.
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