Exactly what the X-37B did during those four missions, or what it will do during the newly launched OTV-5, is a mystery; most X-37B payloads and activities are classified. This secrecy has spawned some speculation that the vehicle may be a space weapon of some sort. But the Air Force has vigorously denied this idea, stressing that the space plane is just testing technologies for future spacecraft and hauling experiments to orbit.
The U.S. Air Force's X-37B space plane fleet has begun its fifth secret flight with the launch of the OTV-5 mission on Sept. 7, 2017. Credit: U.S. Air Force
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