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​The Air Force's X-37B Orbital Test Vehicle mission 4 lands at NASA 's Kennedy Space Center, Fla., May 7, 2017. Air Force photo.

​The Air Force’s X-37B Orbital Test Vehicle landed at NASA’s Kennedy Space Center, Fla., on Sunday after nearly two years in low-earth orbit. The mission, which launched on May 21, 2015, was the fourth completed flight for the OTV program. Each successive mission has been longer in duration than the last—the first was 224 days, the second was 468, and the third was 675. OTV-4 remained in orbit for 718 days, during which time it flew at altitudes ranging between 190 and 225 miles, reported Space Flight Now. The length of its mission was in part related to the assessment of an electric propulsion system—called a Hall Effect thruster. The thruster is slow to start up and the Air Force wanted to test it for long-term exposure to the space environment. It is designed to get more propulsive effect out of the same mass of fuel, which would allow future spacecraft to be lighter and cheaper to launch, and not run out of fuel and maneuvering capability as quickly. The X-37B is managed by the Air Force’s Rapid Capabilities Office and conducts experiments, performs risk reduction activities, and does concept of operations development for reusable space vehicle technologies. Sunday’s landing was the X-37B’s first in Florida. The Air Force plans to launch the fifth OTV mission from Cape Canaveral before the end of 2017, according to a press release.