The U.S. Air Force's robotic X-37B space plane has broken its own all-time endurance record in orbit after more than 470 days of circling the Earth on a mystery mission for the American military.

The X-37B space plane currently in orbit in flying the Orbital Test Vehicle 3 (OTV-3) mission, the third long-duration flight of the unmanned Air Force spaceflight program. The miniature space shuttle launched on Dec. 11, 2012 and is surpassed the record for longest X-37B spaceflight on Wednesday (March 26).

Until now, the record for the longest X-37B mission is 469 days, set by the program's OTV-2 mission that was launched in 2011. [See photos from the X-37B's OTV-3 mystery flight]

The OTV-3 mission in orbit today now uses the first of the Air Force's two X-37B space plane vehicles. The same spacecraft was used to fly the first-ever X-37B mission (the 225-day OTV-1 flight in 2010), while a second vehicle flew the longer OTV-2 mission a year later.

The U.S. military's X-37B space planes launch into orbit atop an unmanned Atlas 5 rocket from a pad at the Cape Canaveral Air Force Station in Florida. During launch, the space planes are encapsulated inside a protective shroud.

At the end of their respective flights, both the OTV-1 and OTV-2 craft missions flew themselves back to Earth on autopilot, each time touching down on a tarmac at California's Vandenberg Air Force Base.

X-37B space plane's Florida landing

Earlier this year, the X-37B supplier Boeing Space & Intelligence Systems announced plans to study the possibility of using NASA's Kennedy Space Center (KSC) in Florida as a landing site for future space plane missions.

Boeing officials said that investments will be made to convert a former space shuttle hangar, called the Orbiter Processing Facility (OPF-1), into a structure that would enable the U.S. Air Force "to efficiently land, recover, refurbish, and re-launch" the X-37B Orbital Test Vehicle.

Construction work at KSC for consolidating the prepping, takeoff and landings of X-37B spacecraft would be completed by the second quarter of 2015, said Boeing spokeswoman Diana Ball.

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US Air Force's Secretive X-37B Space Plane Shatters Orbital Endurance Record