Nasa is celebrating the fifth year of operation for its two Mars rovers. The original plan anticipated that Spirit and Opportunity would be operational for 90 days, but the machines are still in operation nearly five years later.

Over that time they have covered 21 kilometres of the Martian surface and relayed 36GB of data back to Earth.

"The American taxpayer was told that three months for each rover was the prime mission plan," said Ed Weiler, associate administrator for Nasa's Science Mission Directorate at Nasa headquarters in Washington.

"The twins have worked almost 20 times that long. That's an extraordinary return on investment in these challenging budgetary times."

The two solar powered rovers were designed to traverse the Martian landscape for just one season. However, their solar charging capability was underestimated, since Martian storms have cleared dust from the panels allowing them to operate more efficiently.

Nevertheless, there have been problems. One wheel on the Spirit rover jammed in 2007 and its power levels are now very low.

"This last winter was a squeaker for Spirit. We just made it through," said John Callas, project manager for Spirit and Opportunity at Nasa's Jet Propulsion Laboratory.

"We realise 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."