The Mars rover Spirit is ambling along just fine, after a recently reported glitch that turned out to be a minor "benign event," according to Steven Squyres, the Goldwin Smith Professor of Planetary Science and science team leader for the Mars rover mission.

That means that both rovers, designed to explore Mars for a mere 90 days, are still up and running some five years after landing.

Exactly what happened to Spirit recently may never be resolved, Squyres said, "Right now we're proceeding as if everything's fine."

On sol 1800 (the 1,800th Martian day after Spirit landed), the rover's computer booted up in a mode that prevented it from writing to its memory, so there was no record of what it had been doing for a period of time.

"This makes it hard to troubleshoot," Squyres said.

The most likely explanation, he said, was that a cosmic ray passing through the chip briefly disrupted processing.

Spirit is currently at the northern edge of a plateau informally called Home Plate, where it spent the recent Martian winter essentially hibernating, because there was not sufficient sunlight to allow the solar-powered rover to drive. With the advent of Martian spring, Spirit is now driving again, heading toward unexplored terrain south of Home Plate. A recent wind gust removed some of the Martian dust that has coated Spirit's solar arrays, increasing the power output by about 15 percent.

On the other side of the planet, rover Opportunity is in good health, according to Jim Bell, Cornell professor of astronomy and leader of the Pancam (panoramic camera) team for the rover mission. There has been some fraying of the cables on the robot arm, a result of overuse of a machine originally expected to last only three months.

"These are just inconveniences, and the team has figured out ways to work around them," Bell said. "I don't think there's anything mission-threatening."

Opportunity continues to drive southeast toward the huge Endeavour Crater, with another 10 kilometers or so to go. The drive will take at least two years, Squyres said.

Even if it doesn't get there, the journey is worth a try, Squyres said. "[Endeavour is] the deepest window into the subsurface of Mars that we could possibly ever see," he said.