With more than 10 years in orbit and counting, the 2001 Mars Odyssey spacecraft has spent more time in orbit around the Red Planet, collecting data on Mars' climate and geology, than any other spacecraft in history. It also serves as a key communications relay for NASA's Mars Exploration Rover Opportunity.
In 2002, the orbiter's discovery of hydrogen just below the Martian surface prompted the creation of NASA's Phoenix Mars Lander mission, which in 2008 confirmed theories that the element was contained in frozen water in Mars' high-latitude regions.
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