For 9 months, the Mars rover Opportunity has inched along the perimeter of an 800-meter-wide crater called Victoria, while NASA scientists debated where—and whether—the robot should attempt a descent into the 70-m-deep hole. Although the downhill journey is risky, the payoff is great: The farther the rover travels, the older the rock it can study in the crater’s walls and the more insights it can gain into the Red Planet’s apparently wet ancient past.
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