After nearly three years of dragging through the Martian dust, NASA's Opportunity rover has reached the rim of an expansive and ancient crater.

Since leaving Victoria crater in August 2008, Opportunity has rolled 13 miles to reach the rim of 24-mile-wide Endeavour crater – the biggest of 11 craters the robot has visited. It's the site of an ancient impact that shot out dark rocks onto the crater's rim.

"We're soon going to get the opportunity to sample a rock type the rovers haven't seen yet," said planetary scientist and Mars rover team member Matthew Golombek of NASA's Jet Propulsion Laboratory in a press release. "Clay minerals form in wet conditions so we may learn about a potentially habitable environment that appears to have been very different from those responsible for the rocks [found on] the plains."

Opportunity landed on Mars in 2004 and has far exceeded its 3-month warranty. A faulty front-right wheel forced its Earth-based operators to drive most of the trip to Endeavour backwards.

The robot's twin, named Spirit, stopped phoning home in March. Mission managers considered the robot a goner in May and have refocused their efforts on squeezing as much science as possible out of Opportunity.

*Images: 1) The western rim of Endeavour crater on Mars, as seen by Opportunity looking southward. (NASA) [full-resolution version available] 2) A recent view of the trek Opportunity has made since landing on Jan. 25, 2004. The rover is now near Spirit Point. (NASA/JPL-Caltech/MSSS) ** [full-resolution version available] *

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