NASA's Mars Exploration Rover Opportunity captured this view of its afternoon shadow stretching into Endeavour Crater during the 3,051st Martian day, or sol, of Opportunity's work on Mars on Aug. 23, 2012.

As the world rang in the Near Year this week, NASA was looking forward to a big milestone of its own — nine years and counting on the surface of Mars for an overachieving Red Planet rover mission.

The golf-cart-size Spirit rover landed on Mars on Jan. 3, 2004, PST. Its twin, Opportunity, touched down at another Martian locale three weeks later, joining Spirit on a 90-day quest to search for signs of past water activity on the Red Planet. Together, the two robots make up the Mars Exploration Rover mission, the precursor to the huge Mars rover Curiosity, which arrived at the planet last August.

The two NASA robots found plenty of such evidence, helping scientists confirm that Mars — now a frigid and seemingly bone-dry place — was warmer and wetter billions of years ago. Spirit even stumbled onto an ancient hydrothermal system, where heat energy and liquid water may have created conditions capable of supporting life as we know it.

Both rovers kept on chugging long after their warranties expired. Spirit finally stopped communicating with Earth in March 2010 and was declared dead a year later. Opportunity is still going strong, exploring clay deposits on the rim of Mars' Endeavour Crater. [Latest Mars Photos From Rovers Spirit & Opportunity]

Together, the two robots have covered 26.82 miles (43.16 kilometers) to date, with Opportunity racking up the lion's share (22.02 miles, or 35.44 km). While Opportunity is showing some signs of its advanced age, such as an arthritic arm, the rover remains in good health and continues to return interesting data to its handlers back on Earth.

"Every day is a gift at this point," rover mission principal investigator Steve Squyres, of Cornell University, said last month at the annual fall meeting of the American Geophysical Union. "We're just going to push the rover, and push ourselves, as hard as we can."

The nine-year anniversary may bring some attention back to Opportunity, which ceded the Mars rover spotlight to its car-size cousin Curiosity last summer.

The $2.5 billion Curiosity rover landed inside Mars' huge Gale Crater on Aug. 5, 2012, on a mission to determine if the area has ever been capable of supporting microbial life. The 1-ton robot — which weighs five times as much as Spirit or Opportunity — has already discovered an ancient streambed where water likely flowed continuously for thousands of years long ago.

Curiosity's surface mission was originally slated to last about two Earth years, but NASA officials recently announced that they would let the robot roam as long as it was scientifically viable. If the performance of Spirit and Opportunity are any guide, Curiosity could be roving for many years to come.

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