China's state run media has run reports speculating that the X-37B is an orbiting space weapon, capable of dropping bombs or disabling satellites.
It's the second flight for this original X-37B space plane. It circled the planet for seven months in 2010. A second X-37B spacecraft spent more than a year in orbit.
These high-tech mystery machines -- 29 feet long -- are about one-quarter the size of NASA's old space shuttles and can land automatically on a runway. The two previous touchdowns occurred in Southern California; this one might end on NASA's three-mile-long runway once reserved for the space agency's shuttles.
The military isn't saying much if anything about this new secret mission. In fact, launch commentary ended 17 minutes into the flight.
But one scientific observer, Harvard University's Jonathan McDowell of the Harvard-Smithsonian Center for Astrophysics, speculates the space plane is carrying sensors designed for spying and likely is serving as a testbed for future satellites.
The Air Force said when this mission is complete, they may land the X-37B back in Cape Canaveral, as opposed to California, where it has landed the last two times.