NASA (search) gave its twin Mars (search) rovers the green light to continue searching the Red Planet for evidence it once was a wetter place hospitable to life.

The five-month, $15 million extension to the already $820 million mission means Opportunity and Spirit could keep exploring Mars through September — nearly three times longer than originally budgeted.

Dust, cold and mechanical wear-and-tear could curtail the lifetime of either or both rovers, however, said Firouz Naderi, manager of the Mars exploration program at NASA's Jet Propulsion Laboratory.

For Opportunity, the move gives it time to build on the evidence it already has found that water once bathed its landing site on Mars, allowing it to visit at least four outcrops of rock, said Washington University's Ray Arvidson, the mission's deputy main scientist.

The first outcrop Opportunity analyzed, within the small crater it landed in Jan. 24, revealed that standing water, perhaps a salty sea or swamp, once covered Meridiani Planum.

For Spirit, working since Jan. 3 in Gusev Crater on the other side of planet, similar evidence of abundant past water activity has been elusive.

NASA will lose touch with the rovers Sept. 13, when Mars passes behind the sun. The blackout should leave Earth out of contact with the two rovers for a week to 10 days, Naderi said.

If NASA can re-establish contact with the rovers once Mars pops back into view, the agency could further extend operations, Naderi added.