The U.S. Air Force's X-37B Space Plane is set to launch on it's third voyage. The details of the mission have been kept a secret.
Secret Missions of the X-37B
Secret X-37B Spaceplane's Mystery Mission!
X-37B Space Plane
X-37B - Orbiter Space Flight Simulator
Secretive Air Force Space Plane X37B Lands | Video
USA-212 Secret Space Plane X-37B found in orbit