The latest secretive mission of the United States Air Force’s X-37B space plane has cruised beyond 500 days in Earth orbit since its launch last year.

The U.S. military¬†launched the robotic X-37B space plane on May 20, 2015, marking the fourth flight for the Air Force program. A United Launch Alliance Atlas V rocket lofted the spacecraft from Florida’s Cape Canaveral Air Force Station to kick off the OTV-4 mission (short for Orbital Test Vehicle-4).

Exactly what the winged space plane’s duties are while it’s in orbit continues to remain a tight-lipped affair. Similarly, how long the vehicle will remain in orbit has not been detailed.

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