Both devices represent a remarkable engineering feat as they continue to function nearly 1 1/2 years past their predicted expiration date, the Christian Science Monitor reported Monday.

Now the mission's lead scientist, Steve Squyres, believes the biggest discoveries lie ahead for the rovers. But, at the same time, Squyres is aware both machines continue to operate in an environment in which temperatures rise and fall daily by more than 200 degrees F.

Since the rovers have continued to function long after their projected expiration, scientists have stopped predicting when they will stop working. So far, Opportunity has driven more than 3 1/2 miles across Mars; while Spirit has driven 3 miles, the Monitor said.

Squyres says he believes they've lasted this long only because Martian windstorms have unexpectedly wiped away dust accumulating on their solar panels.

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