An illustration of NASA's Mars Exploration Rover Spirit. Credit: NASA JPL

NASA's Mars Exploration Rovers, Spirit and Opportunity, have been trekking across the Martian surface for the past half decade, surviving dust storms, sand traps, and three freezing winters with only minor setbacks.

Now Spirit, having just received much fanfare in celebration of its five-year anniversary on the planet, appears to be running awry, and its operating team is concerned. It plans to conduct diagnostic tests on the rover later this week.

Engineers first noticed Spirit's peculiar behavior on Sunday. The rover had radioed to say that it had received its driving commands for the day, but strangely, it had not moved. While NASA says that this can happen for a number of reasons, the rover also failed to record its day's activities to its nonvolatile memory--storage that is retained even when the rover is powered off. The next day, the team asked the rover to determine its orientation by locating the sun. Spirit found the sun, but it inaccurately reported its location.

The Spirit team does not yet have an explanation for why the rover may be a little out of whack, but one hypothesis is that it could be suffering the fleeting effects of cosmic rays hitting its electronics. Diagnostic tests should provide a more definitive answer soon.

Spirit, like Opportunity, is a warrior of the Red Planet. Both rovers, launched in January 2004, were scheduled to last a minimum of three months and a maximum of six. Now, after five years, the rovers have turned Mars into what seems like a next-door neighbor--not the alien planet that it once was.

Since landing, the rovers have made important scientific discoveries. Spirit discovered deposits of salts and minerals such as sulfur and silica, which only form with water. This happened when it inadvertently dug a trench behind itself while dragging a broken right front wheel. This video highlights Spirit's adventures:

Opportunity, the crater-exploring rover, was fortunate to land on exposed bedrock that was determined to be laid down in water some 3.5 to 4 billion years ago. This was the first evidence of ancient surface water. It also discovered tiny balls of material that appear to have formed in the presence of water. This video highlights Opportunity's activities:

Scott Maxwell, a rover driver at NASA's Jet Propulsion Laboratory, says that "the mission just keeps getting better and better the longer it goes."

"Mars is such a complex place, and these are such capable vehicles that there will never come a time when we're done; five years from now there is going to be some wonderful, tantalizing thing just beyond our reach that we didn't quite get to," adds Steve Squyres, principal investigator of the rovers at Cornell University.

Videos by NASA