New York (CNN Business) An Air Force X-37B spaceplane just completed its 718th day in orbit, making it the longest mission yet for a secretive military test program.

The US military has launched five uncrewed X-37B spaceplanes into orbit over the past decade, and each flight has been longer than its predecessor.

The current mission has no specified end date, according to Air Force spokesman Major William Russell. The spacecraft will return to Earth only after its completed all its objectives, he said.

The details of those objectives remain a closely guarded secret.

The X-37B program tends to attract public interest because of that secrecy: The Air Force does not share the locations of the X-37B planes while they're in orbit, although amateur astronomers have made a sport out of spotting the spacecraft with telescopes.