Space.com X-37B Space Plane: US Air Force’s 4th Mystery Mission in Photos 8 May 2017 fborgosano Leave a comment Share: 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. from Space.com bit.ly/2pdi8Mr Share: