By Leonard David, Space.com’s Space Insider Columnist

The latest mystery mission of the U.S. Air Force’s robotic X-37B space plane has now passed the 400-day mark .

This mission — known as Orbital Test Vehicle (OTV-5) — was rocketed into Earth orbit on Sept. 7, 2017, atop a SpaceX Falcon 9 rocket from Launch Complex 39A at NASA’s Kennedy Space Center in Florida.

The uncrewed space plane is carrying out secretive duties during the X-37B program’s fifth flight. [The X-37B Space Plane: 6 Surprising Facts]

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