Mars rovers roll on to five years

It was hoped the robots would work for at least three months; but their longevity in the freezing Martian conditions has surprised everyone.

"These rovers are incredibly resilient considering the extreme environment the hardware experiences every day," said John Callas, project manager for Spirit and Opportunity at Nasa's Jet Propulsion laboratory in Pasadena, California.

Spirit has to drive backwards everywhere it goes because of a jammed wheel; and Opportunity's robotic arm has a glitch in a shoulder joint because of a broken electrical wire.