The U.S. Air Force's fourth mystery mission of its X-37B space plane launches on May 20, 2015. How long the mission will last, and its exact purpose, is a mystery. See photos from the mission here.
The U.S. Air Force's fourth mystery mission of its X-37B space plane launches on May 20, 2015. How long the mission will last, and its exact purpose, is a mystery. See photos from the mission here.
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