NASA's Mars rover Curiosity is driving toward a flat rock with pale veins that may hold clues to a wet history on the Red Planet. If the rock meets rover engineers' approval when Curiosity rolls up to it in coming days, it will become the first to be drilled for a sample during the Mars Science Laboratory mission.

The size of a car, Curiosity is inside Mars' Gale Crater investigating whether the planet ever offered an environment favorable for microbial life. Curiosity landed in the crater five months ago to begin its two-year prime mission.

"Drilling into a rock to collect a sample will be this mission's most challenging activity since the landing. It has never been done on Mars," said Mars Science Laboratory project manager Richard Cook of NASA's Jet Propulsion Laboratory in Pasadena, Calif. "The drill hardware interacts energetically with Martian material we don't control. We won't be surprised if some steps in the process don't go exactly as planned the first time through."

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