CAPE CANAVERAL, FLA. - The military's top-secret version of the space shuttle was launched into orbit on Tuesday for a repeat mystery mission, two years after making the first flight of its kind.

The Air Force launched the unmanned spacecraft atop an Atlas V rocket. It is the second flight for this original X-37B spaceplane. The craft circled the Earth for seven months in 2010. A second X-37B spent more than a year in orbit.

These high-tech machines 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 a NASA runway once reserved for the space agency's shuttles.

The military isn't saying much about this new mission known as OTV-3, or Orbital Test Vehicle, flight No.3. In fact, launch commentary ended 17 minutes into the flight and a news blackout followed.

But one scientific observer, Jonathan McDowell of the Harvard-Smithsonian Center for Astrophysics, speculated that the spaceplane is carrying sensors designed for spying and likely is serving as a testbed for future satellites. He dismissed rumors of "exotic ideas" for the X-37B as weaponry or shadowing a Chinese satellite.

The beauty of a reusable spaceplane is that it can be launched on short notice, McDowell said. What's important about this flight is that it is the first reflight.

"That is pretty cool," McDowell said, "reusing your spacecraft after a runway landing. That's something that has only really been done with the shuttle."

The X-37B program, which dates back to 1999, is geared toward space experimentation.