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The Mars Odyssey launch on April 7, 2001. The Odyssey, contracted out to Lockheed Martin, has been orbiting Mars since reaching Mars on October 24, 2001. It's primary mission is to look for evidence of water and volcanic activity. It is the longest surviving spacecraft serving Mars and the longest active spacecraft in orbit around a planet other than Earth. The mission is named as a tribute to Arthur C. Clarke who wrote 2001: A Space Odyssey. (KEYSTONE/SCIENCE SOURCE/NASA)