NASA's Mars rover Opportunity is just days away from arriving at the edge of a huge crater after a years-long trek on the Red Planet. Opportunity has been driving for nearly three years toward the crater Endeavour , an immense scar in the Martian surface about 14 miles (22 kilometers) wide. Now the rover is less than 31 feet (50 meters) from the rim and is due to pull up to it later this week, mission scientists said Monday (Aug
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