Nasa's twin Mars Exploration Rovers, named Spirit and Opportunity, celebrate their fifth year on the surface of the Red Planet this month. Spirit landed on 2004 January 4. Opportunity landed on the opposite side of the planet on January 25. The machines, for their part, pay little heed to our calendar. Their schedules are arranged in sols, a sol being one Martian day.
The most remarkable thing about this milestone is that the rovers were designed to work for only three months. Their willingness to keep going long past expectations has yielded a wealth of new discoveries. Today scientists know much more about geological and weather processes on the planet and the conditions that exist at its surface. This information will be of enormous value in planning future visits to the planet by human beings.
Four weeks ago Spirit and Opportunity emerged from a period of radio silence associated with solar conjuction. During a solar conjuction the sun stands between Earth and Mars, making it impossible for mission controllers to contact the rovers. The dutiful machines stil worked on assignments. Spirit took measurements of atmospheric dust. Opportunity conducted atmospheric studies and collected chemical information about a rock.
Nasa and JPL: Mars Exploration Rover Mission Home Page
Space.com - Amazing Mars Discoveries in 2008
Space.com - Fourth Anniversary of Spirit and Opportunity