USAF X-37B Space Plane: 1 Year in Orbit

The U.S. Air Force's secretive X-37B space plane program has passed another milestone: Its latest mission marked one year in orbit this week.

The reusable unmanned X-37B space plane, flying what the U.S. military calls the Orbital Test Vehicle 3 (OTV-3) mission, began its second flight on Dec. 11, 2012, launching atop an Atlas 5 rocket from Florida's Cape Canaveral Air Force Station.

 But what it's been doing for the last 12 months remains a mystery. The details of the mission, including its payloads and objectives, have been kept classified.