The twin robots, which landed on Mars three weeks apart in January 2004, were initially expected to have just 90-day missions, but have since sent back to Earth a quarter-million images, toured mountains and craters and survived violent dust storms.

"The American taxpayer was told three months for each rover was the prime mission plan. The twins have worked almost 20 times that long," said NASA assistant administrator Ed Weiler in a statement.

"That's an extraordinary return of investment in these challenging budgetary times."

The rovers, which along with 250,000 images have sent back to Earth some 36 gigabytes of data, have greatly advanced NASA's understanding of Mars' geology, including peeks into the planet's wet and habitable past.

Analysts say the wealth of information data will keep scientists busy for years as they further unravel the vast banks of data.

Since 2004 the machines have covered 21 kilometers (13 miles) of Mars' characteristic red rock desert, driving inch by inch to avoid chasms and rocky obstacles, picking up samples and snapping images to beam back to mission control on Earth.

"These rovers are incredibly resilient considering the extreme environment the hardware experiences every day," said John Callas, project manager for Spirit and Opportunity.

"We realize that a major rover component on either vehicle could fail at any time and end a mission with no advance notice, but on the other hand, we could accomplish the equivalent duration of four more prime missions on each rover in the year ahead."

While the machines have had relatively balmy 20 degree Celsius (68 degree Fahrenheit) summers, they have had to endure frigid extremes, where temperatures of minus-100 degrees Celsius (minus-148 degrees Fahrenheit) in winter are common.

Harsh Martian winds, however, have provided an occasional cleaning job to the rovers' solar panels -- critical instruments to power the machines.

This unconventional aid, however, has not be reliable, with the Spirit machine's panels hardly clear enough to survive its third southern hemisphere winter, which ended in December.

Although the 820-million-dollar project's mission began as scientific, the it has become something much larger, according to Steve Squyres of Cornell University, the rover mission's principal investigator.

The journeys "have led to something else important," he said. "This has turned into humanity's first overland expedition on another planet.

"When people look back on this period of Mars exploration decades from now, Spirit and Opportunity may be considered most significant not for the science they accomplished, but for the first time we truly went exploring across the surface of Mars."

The continuing wealth of data provided by the rovers is a welcome hold-over for the US space agency, which was forced to delay a landmark mission to Mars by 26 months last month.

The 2.3- billion-dollar Mars Science Laboratory (MSL) is now expected to be launched in 2011.

Launch opportunities for Mars come only every 26 months, when the planets are in right alignment.

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