The controllers of NASA's Curiosity rover have waited patiently—and perhaps for too long—to launch a key experiment. Tucked in the rover's belly are nine stainless steel thimbles, each filled with solvent, that are the mission's best shot for detecting signs of ancient martian life. Now, well into its fifth year on Mars, the rover has reached a mountain thought to be a promising hunting ground, and mission scientists are ready to dump freshly drilled dirt into one of their precious wet chemistry cups. But there's a hitch. Since early December 2016, Curiosity hasn't been able to drill.

The problem, likely a stuck brake on the mechanism for extending the drill bit, is serious. "There is apprehension," says Ashwin Vasavada, Curiosity's project scientist at the Jet Propulsion Laboratory in Pasadena, California. But the drill still responds intermittently. "We're not in a situation where it's completely dead."

Still, the clock is ticking for the aging rover, and some outside scientists regret not having used a wet chemistry cup. Rocks have punctured its wheels, and the output of its decaying radioactive power source has dropped by 15%. Jack Mustard, a planetary scientist at Brown University, says he understands the team's hesitance. But he wished the "mission had moved more quickly with the wet chemistry experiments," he says. "I am eager to see what we can learn."

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