'Spirit' mission marks 9 years on planet Mars

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Interest in Mars exploration was revived when NASA successfully landed the Curiosity Rover in the Gale crater last year, with millions of people watching the "seven minutes of terror" descent of the rover to the surface.

But Curiosity does not roam the Red Planet entirely alone: the chatty robotic lab joined Spirit and Opportunity, twin rovers who this week celebrate their 9th anniversary exploring the Martian landscape.

Spirit and Opportunity landed on Mars in January 2004, were initially slated for a 90-sol (90 Mars days) mission but ended up lasting more than 20 times that--about six years--until it got stuck in soft soil and became unrecoverable. Opportunity continues to move and gather data.

The two rovers were instrumental in confirming the past presence of water on a planet that is now too cold and dry to sustain life.