Images: Mars rovers' greatest hits

NASA is celebrating the third Earth-year anniversary of the landing of the rovers Spirit and Opportunity on Mars with a contest to pick the best photo taken by the twin robots. The voting began on January 3, three years after Spirit arrived on the Red Planet. It will end on January 24, the third anniversary of Opportunity's landing.

The original mission was scheduled to last 90 Martian days, but that estimate has proved to be far short of the rovers' capabilities. A Martian day lasts 24 hours and 37 minutes of Earth time while a Martian year lasts 686.98 Earth days.

Opportunity traveled almost five miles over nearly three Earth years before it arrived at Victoria Crater and took this photo (left) of a rock outcropping named Cape Verde.