People could land on Mars in the next 20 to 30 years provided scientists can find water on the red planet, the head of NASA’s surface exploration mission said on Wednesday.

Two partially solar-powered “robot geologists” — Mars Exploration Rovers, or MERs — have been trundling across 3 miles of the planet and into craters since January, beaming back data about the makeup of what scientists believe is Earth’s sister planet.

Asked how long it could be before astronauts land on Mars, Arthur Thompson, mission manager for MER surface operations, told Reuters in an interview in Lima, “My best guess is 20 to 30 years, if that becomes our primary priority.”

The two MER robots, dubbed Spirit and Opportunity, have found ancient evidence that water was once plentiful — important for scientists hoping to know if there was once — or could still be — life on Mars.

Without water, the dream of sending astronauts to the often dusty planet, which has rust-colored rocks and where the sky is red and sunsets are blue, could unravel.

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