Washington - The US space agency's Mars rovers Spirit and Opportunity mark their fifth anniversary this month on the red planet where they have endured harsh conditions and revealed a deluge of information.
The twin robots, which landed on Mars in January 2004, were initially expected to have just 90-day missions.
The machines have sent back to Earth 250 000 images and 36 gigabytes of data, and have greatly advanced Nasa's understanding of Mars's geology, including peeks into the planet's wet and habitable past.
Analysts say the wealth of information data will keep scientists busy for years.
The machines have covered 21km of Mars's characteristic red rock desert, toured mountains and craters and survived violent dust storms.
Driving almost centimetre by centimetre to avoid chasms and rocky obstacles, the robots have picked up samples and snapped 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, the project manager of the Spirit and Opportunity rovers.
While the machines have had relatively balmy 20C summers, they have had to endure frigid extremes, where temperatures of -100C in winter are common.
Harsh Martian winds, however, have provided an occasional cleaning job to the rovers' vital solar panels.
But the Spirit machine's panels are hardly clear enough to survive its third southern hemisphere winter, which ended in December.
Although the $820-million (R7,6-billion) project's mission began as scientific, 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 data provided by the two 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 (R21-billion) Mars science laboratory is now expected to be launched in 2011.
Launch opportunities for Mars come only every 26 months, when the planet is in the correct alignment with Earth. - Sapa-AFP