The older, smaller cousin of NASA's huge Mars rover Curiosity is quietly celebrating a big milestone today (Jan. 24) nine years on the surface of the Red Planet.

NASA's Opportunity rover landed on Mars the night of Jan. 24, 2004 PST (just after midnight EST on Jan. 25), three weeks after its twin, Spirit, touched down. Spirit stopped operating in 2010, but Opportunity is still going strong, helping scientists better understand the Red Planet's wetter, warmer past.

"No one could've imagined how good the exploration and scientific discovery would be for this vehicle, looking from the perspective of nine years ago," said John Callas, Opportunity's project manager at NASA's Jet Propulsion Laboratory in Pasadena, Calif. "It's been a phenomenal accomplishment."

The headline-stealing Curiosity rover, for its part, touched down on Aug. 5, 2012, marking the next step in Mars exploration. The car-size Curiosity weighs about 1 ton five times more than either Spirit or Opportunity.

Long-lived rovers

Spirit and Opportunity were originally supposed to spend three months searching for evidence of past water activity on the Red Planet. The golf-cart-size robots found plenty of such signs at their separate landing sites, showing that Mars was not always the cold and arid planet we know today. [Most Amazing Discoveries by Spirit and Opportunity]

For example, in 2007 Spirit uncovered an ancient hydrothermal system in Gusev Crater, suggesting that two key ingredients for life as we know it liquid water and an energy source were both present in some parts of Mars long ago.

And Opportunity is currently inspecting clay deposits along the rim of Mars' huge Endeavour Crater. Clays form in relatively neutral (as opposed to acidic or basic) water, so the area may once have been capable of supporting primitive microbial life, researchers say.

"This is our first glimpse ever at conditions on ancient Marsthat clearly show us a chemistry that would've been suitable for life at the Opportunity site," Opportunity principal investigator Steve Squyres, of Cornell University, said of the discovery at a conference last month.

The rovers rolled far beyond their 90-day warranties. Spirit finally stopped communicating with Earth in March 2010, after getting mired in soft sand and failing to maneuver into a position that would allow it to slant its solar panels toward the sun over the 2009-2010 Martian winter. NASA declared the rover dead in 2011.

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NASA's Opportunity Rover Begins Year 10 on Mars