It is the second flight of the second Boeing X-37B vehicle and the fourth flight of the program. United States Air Force Orbital Test Vehicle is an unmanned 5000 kg, 8.8 m-long reusable mini-spaceplane capable of autonomous re-entry and landing. The OTV-4 mission was to test Aerojet Rocketdyne's XR-5A Hall-effect thruster and conduct a NASA investigation for testing various materials in space. The mission concluded after 717 days in orbit, marking the first time X-37B landed on the Shuttle Landing Facility at the Kennedy Space Center.