The U.S. Air Force’s secretive robotic X-37B space plane mission continues to chalk up time in Earth orbit, nearing 430 days of a spaceflight that — while classified — appears to be an unqualified success.

The space plane now circuiting Earth is the second spacecraft of its kind built for the Air Force by Boeing’s Phantom Works. Known as the Orbital Test Vehicle 2, or OTV-2, the space plane’s classified mission is being carried out by the Air Force Rapid Capabilities Office.

The robotic X-37B space plane is a reusable spacecraft that resembles a miniature space shuttle. The Air Force launched the OTV-2 mission on March 5, 2011, with an unmanned Atlas 5 rocket lofting the space plane into orbit from the Cape Canaveral Air Force Station in Florida.

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