Astronaut Steve Bowen works outside the International Space Station's Quest airlock during Monday's spacewalk. Bowen's spacewalk was unexpectedly extended by a computer malfunction. NASA TV hide caption

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Astronauts aboard the space shuttle Discovery, seen here lifting off from Kennedy Space Center in Florida, are preparing to dock with the International Space Station Saturday. Stan Honda/AFP/Getty Images hide caption

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Stan Honda/AFP/Getty Images

NASA Ceremony Marks 25th Challenger Anniversary

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May 28, 2008, file photo: Space shuttle Discovery commander Mark Kelly, right, gestures as he walks with his twin brother astronaut Scott Kelly, left, and mission specialist Ron Garan, after arrival at Kennedy Space Center in Cape Canaveral, Fla. Terry Renna/AP hide caption

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Terry Renna/AP

The space shuttle Atlantis docked to the International Space Station in May 2010. NASA has announced that Atlantis will fly one last mission in June 2011 before retirement. NASA/Getty Images hide caption

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NASA/Getty Images