(Space) An unmanned military space plane has achieved a major longevity milestone, zipping around Earth for 500 days on a clandestine mission for the United States government.

The U.S. Air Force’s X-37B space plane launched on its third and latest mission on Dec. 11, 2012. As of today (April 24), the vehicle has been aloft for 500 days — far longer than its officially stated maximum orbital lifetime of 270 days.

“Clearly, and understandably, the Air Force initially set the expected parameters [of the X-37B] conservatively, not really knowing what it could do until it was tested — which is what they are doing now,” said Joan Johnson-Freese, professor of national security affairs at the Naval War Collegein Newport, R.I.

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