The U.S. Air Force's unmanned X-37B space plane has now circled Earth for more than 400 days on a hush-hush mission that is creeping closer and closer to the vehicle's orbital longevity record

The X-37B spacecraft launched on Dec. 11, 2012, meaning that it has been aloft for 413 days as of Tuesday, Jan. 28, on the third mission for the program, which is known as OTV-3 (short for Orbital Test Vehicle-3). The endurance record is 469 days, set during OTV-2, which blasted off in 2011.
OTV-2 and OTV-3 have utilized different X-37B vehicle (the Air Force currently has two vehicles). The space plane currently zipping around Earth also flew the program's inaugural OTV-1 mission, which stayed in space for 225 days after launching in 2010.

OTV-1 and OTV-2 both touched down at California's Vandenberg Air Force Base. But the Air Force and Boeing, which builds the X-37B, are eyeing NASA's Kennedy Space Center (KSC) in Florida as a landing site for future missions.

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