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The United States has participated in the International Space Station beginning in the 1990s, the Space Shuttle program has continued, although the Space Shuttle Columbia disaster has led to the planned retirement of the Space Shuttle in mid-2011. There is a current debate on the post-Space Shuttle future of the civilian space program: the Constellation program of the George W. Bush administration directed NASA to create a set of new spacecraft with the goal of sending astronauts to the Moon and Mars, but the Obama administration cancelled the Constellation program, opting instead to emphasize development of commercial rocket systems.