Why Is The US Air Force's Secret Space Plane Orbiting The Earth In 500 Days?

he mysterious U.S. Air Force space plane known as X-37B is on its fourth secretive mission. It has spent 500 days in space orbiting the planet Earth for unknown reasons. Advertisement Space reports that the robotic X-37B space plane was launched on May 20, 2015, wherein the United Launch Alliance Atlas V rocket-propelled the spacecraft from Florida's Cape Canaveral Air Force Station. It is on the secretive mission known as OTV-4 mission or Orbital Test Vehicle-4. In its third mission, it orbited the Earth in 674 days and landed on October 17, 2014. It was constructed with the aid of Boeing. So, what is this strange and enigmatic space plane doing up there? More

(Source: Science World Report - Oct 16)