California ? Five years after the NASA rover Spirit landed on Mars, the six-wheel robotic geologist and its twin Opportunity are still on the job.

Expectations were far lower when Spirit made a bouncing landing in a cocoon of air bags on Jan. 3, 2004, followed 21 days later by Opportunity: The goal was to try to operate each solar-powered rover for at least three months.

“That’s an extraordinary return of investment in these challenging budgetary times,” Ed Weiler, associate administrator for NASA’s Science Mission Directorate, said in a December statement.

Combined, the rovers have made more than 13 miles of tracks on Mars’ dusty surface and sent a quarter-million images back to Earth. Their instruments have uncovered evidence that Mars was once a far wetter and warmer place than the frigid, dusty world it is now.