From Dave Mosher of Space.com:
Two robots the size of golf carts were given 90 days to squeeze as much science as possible from the barren, dust-swept terrain of Mars. After that, scientists expected nothing more from them than death.
Nearly four years after their warranties expired, however, the Mars Explorations Rovers (MERs) "Spirit" and "Opportunity" continue to play productively in the red dirt.
Spirit celebrates its fourth anniversary of Martian work on Jan. 4, the day it landed in 2004, followed by Opportunity on Jan. 25. Those four Earth years since landing convert to 2.25 Martian years, or 1,422 Martian days called "sols."
"We never thought we'd still be driving these robots all over Mars," said Mark Lemmon, a planetary scientist at Texas A&M University and member of the rover science team. "We joked about driving Opportunity into Victoria Crater, but now we're there, and we're looking at doing even more science. Each day they still work is an amazing one."
Our conception of Mars will never be the same.