Washington - When the Spirit rover landed on Mars five years ago, no one expected it or its sister rover Opportunity to make it to their first birthdays, let alone their fifth.

The rovers had predicted life spans of just 90 days, but instead US space agency NASA this month marks the fifth anniversary of the mission that is still going strong.

Spirit landed on January 3, 2004 and was followed by Opportunity three weeks later. Since then, the golf cart-sized crafts have provided scientists with valuable information about the Red Planet's wet history, while sending back 250,000 images and driving more than 13 miles, in what scientist Steve Squyres characterized as "humanity's first overland expedition on another planet."

Perhaps the most important discovery was silica in Mars' soil that was uncovered by a dragging wheel on the Spirit rover. The mineral was seen as a likely product of a damp environment produced by hot springs or steam vents.

"The American taxpayer was told three months for each rover was the prime mission plan, Ed Weiler, associate administrator for NASA's Science Mission Directorate said. "The twins have worked almost 20 times that long. That's an extraordinary return of investment in these challenging budgetary times."

But that long life span has not come easy, in 2007 NASA almost lost contact with Opportunity as it battled to survive a Martian dust storm that cut off its solar power source. Spirit has also faced some difficulties from dust, and it barely survived its third Martian winter that ended in December as its solar panels became coated with dirt.

The crafts' survival is even more remarkable because nearly two- thirds of Mars missions in the past have failed, with a European craft Beagle 2 launched just before Spirit and Opportunity failing to make any contact with scientists back home.

But scientists still have big plans for the small rovers. Opportunity is headed to the so-called Endeavour Crater, which is 23 kilometres in diameter or about 20 times larger than Victoria Crater, which the craft has explored extensively over the past two years.

Spirit will travel 200 yards south to study a mound and a pit called Goddard that scientists suspect might be a volcanic explosion crater.