via Livescience, this month is the 4th anniversary of the landing of both the Mars rovers. At the time, NASA researchers expected the golf cart sized robotic explorers to last just 90 days.
"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."
The rovers have traveled nearly 12 miles on the Martian surface and collected over two hundred thousand images. More than 100 scientific studies on the planets past have been compiled due to the data sent by the rovers. The most important discovery of the rovers has been the findings of evidence of water on Mars, at least at some point in the past, and probably still found deep underground.
Martian weather has proven challenging to the explorers, with a massive dust storm obscuring the Martian sky and covering their solar arrays with a particularly sticking dust that limited the light reaching the arrays by 96%. The rover Spirit has an additional problem; with one of its front wheels jammed, it has been forced to drive backwards into a winter "resting" position facing northward to gather energy from its solar panels on a rock outcrop named Home Plate, but is still making observations. Sister rover Opportunity, on the other hand, has much cleaner panels and is currently activley exploring Victoria Crater on the red planet.