Since landing on opposite sides of Mars in January 2004, the US space agency's two robotic explorers have plowed over five kilometers (three miles) of the planet's surface and sent back more than 130,000 pictures, many of them stunning depictions of a desolate, arid but highly varied landscape.

The two also continue to reveal Mars' geological secrets, digging into soil and overturning rocks to provide evidence that water once featured on the planet's surface, creating possibly habitable conditions.

But what has equally pleased the National Aeronautics and Space Administration is that the two rovers have far exceeded their 90-day missions, lasting through one full Mars year -- 669 Mars days, the equivalent of 687 Earth days.

"The rovers went through all of the Martian seasons and are back to late summer. We're preparing for the challenge of surviving another Martian winter," John Callas, NASA's deputy rover project manager said on NASA's website.

But recently the two explorers have shown signs of fatigue, and the Mars winter could test their solar batteries' ability to keep functioning.

The two robots continue to discover new variations in the sedimentary rocks on the Mars surface and more evidence of the former presence of water.

Their discoveries were confirmed in November by data from the European Space Agency's Mars Express orbiter which detected, for the first time, water ice under the soil of Mars' northern pole.

On December 5, ESA reported that data from Mars Express indicates that some 3.2 billion years ago the planet underwent a cataclysmic change that turned it from humid, with significant amounts of surface water, to dry and cold as it is today.

NASA's Spirit rover, which landed in the Gusev crater on January 3, 2004, was recently moved down from the 46 meter (151 foot) apex of 'Husband Hill', from where it had surveyed the landscape for one month.

The robot's controllers on Earth were preparing to move it south to another hill where they hope to maximise its solar-cell output during the winter.

Opportunity, which alit on the Meridiani Planum around Mars' other side from Spirit on January 24, 2004, has been examining bedrock exposures along a route between the craters Endurance and Victoria.

The data collected by Opportunity suggests cyclical changes at one time from drier to wetter conditions, NASA said.

But the two rovers' performance has begun to sag, long after the maximum lifespan of 270 days that NASA had projected for them in tests.

In late November a mechanical arm on Opportunity stopped working, as one of its key driving motors stalled. It took NASA engineers two weeks to get the arm operational, but they are still concerned that it might fail again.

"If it has failed, it will be a significant hit. It is the contact arm of the mission," Callas said.

He also said that part of the robot's directioning system was broken, but that it would not prevent NASA from directing the machine.

Spirit has worn better, with only an instrument used to scrape rocks showing wear and tear, Callas said. It was expected to undertake only three tests, but has done 15 so far.

But with the two Mars explorers having surpassed their expected lifespans by more than double, NASA scientists are fatalistic about their prospects.

"We drive it every day as if there were no tomorrow," said Stephen Squyres of Cornell University, the principal investigator for Spirit's scientific instruments.

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