Air Force's X-37B Space Plane Mystery Mission Wings by 500 Days in Orbit

The U.S. military launched the robotic X-37B space plane on May 20, 2015, marking the fourth flight for the Air Force program. A United Launch Alliance Atlas V rocket lofted the spacecraft from Florida's Cape Canaveral Air Force Station to kick off the OTV-4 mission (short for Orbital Test Vehicle-4). Exactly what the winged space plane's duties are while it's in orbit continues to remain a tight-lipped affair.