The U.S. Air Force's X-37B space plane has now been circling Earth for more than 600 days on its latest mystery mission.

The reusable robotic vehicle, which looks like a miniature version of NASA's space shuttle orbiters, launched atop a SpaceX Falcon 9 rocket from NASA's Kennedy Space Center in Florida on Sept. 7, 2017.

As of today (April 30), the space plane has been aloft for 601 days, on a mission known as Orbital Test Vehicle 5 (OTV-5) because it's the fifth flight of the X-37B program.

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https://www.space.com/x-37b-military-space-plane-otv5-600-days.html