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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