Pix Credit: NASA's twin Mars rovers, Spirit and Opportunity,
are marking the passage of a big milestone—eight years on the surface of Mars.
The golf-cart-size Spirit rover landed on the Red Planet eight years ago today (Jan. 3). Its twin, Opportunity, touched down three weeks later, on Jan. 25, 2004. The two robots were originally supposed to spend 90 days searching for signs of past water activity on Mars.