According toSpace.com, both rovers were only supposed to explore the surface of Mars for 90. Twelve years later, Opportunity is still actively exploring. Sister rover Spirit operated for six years, but stopped communicating with Earth in 2010. In 2011, it was declared dead. Powering Opportunity for all these years are solar panels mounted on the golf cart sized rover, harnessing power from the sun. It has even been able to continue working throughout the southern Mars winter, when less sunlight makes it to the planet's surface. John Callas, Mars exploration project manager, says "Opportunity has stayed very active this winter, in part because the solar arrays have been much cleaner than in the past few winters."
The scientists controlling Opportunity have also developed other strategies to keep it going even with low power. They drive the rover to sunnier patches, and try to find windy areas which will clean dust off of the solar panels. Opportunity has traveled further than any other vehicle has on a planet other than Earth, exploring 26.5 miles. It is now again one of two rovers exploring Mars, with the addition of rover Curiosity which landed in 2012.