Opportunity's mechanical arm is jammed at the "shoulder joint" and engineers are working to diagnose the problem, said John Callas of NASA's Jet Propulsion Laboratory on Dec. 5 at a meeting of the American Geophysical Union.
Tests run on the arm's motor Friday confirmed a possible cause for the sticking that, once addressed, will restore some movement, according to a JPL status report.
Project managers hope to restore four degrees of movement in the arm, which holds four rock inspection tools, so that the rover will be able to conduct its work, Callas said.
While engineers are working on the problem, Opportunity is continuing to photograph its surroundings in the Erebus Crater.
Spirit is faring better with only a worn out grinding bit. The tool was designed to take samples from rocks three times but lasted for 15 grindings, Callas said.
Located on the other side of Mars from Opportunity, Spirit recently reached a peak in the Columbia Hills, where the rover has encountered nine different rock types, indicating the planet's geologic complexity, said Dr. Steve Squyres, a Cornell University professor and principal investigator for the Mars Exploration Rovers.
Both rovers, which were expected to last only three months, are still functioning after entering their second Martian year -- equivalent 687 Earth days -- on Monday for Opportunity and three weeks ago for Spirit.
"We drive each day like there's no tomorrow," said Squyres.
Soon after the rovers landed on Mars in January 2004, they found evidence in the composition of rocks and soil that water -- considered a building block of life -- likely once covered large areas of the planet millions of years ago. They also have relayed back to Earth about 130,000 images of Mars' terrain.
One of the most remarkable aspects of the rovers' mission is it "has made Mars a real place to people," Squyres said.