The U.S. military's small, top-secret version of the space shuttle rocketed into orbit Tuesday for another mystery mission, two years after making the first flight of its kind.

The air force launched the unmanned spacecraft Tuesday hidden on top of an Atlas V rocket.

It's the second flight for this X-37B spaceplane, which 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 — which are eight metres long — are about one-quarter the size of the U.S. space agency's old space shuttles and can land automatically on a runway.

The military isn't saying much, if anything, about this new mission. Launch commentary ended 17 minutes into the flight.

The X-37B unmanned spacecraft, shown in this undated image, was launched at Cape Canaveral on Tuesday hidden on top of an Atlas V rocket.The X-37B unmanned spacecraft, shown in this undated image, was launched at Cape Canaveral on Tuesday hidden on top of an Atlas V rocket. (U.S. Air Force, File/Associated Press)

But one scientific observer, Harvard University's Jonathan McDowell of the Harvard-Smithsonian Center for Astrophysics, speculates the spaceplane is carrying sensors designed for spying and likely is serving as a testbed for future satellites.

While acknowledging he does not know what the spaceplane is carrying, McDowell said on-board sensors could be capable of imaging or intercepting transmissions of electronic emissions from terrorist training sites in Afghanistan or other hot spots.

The beauty of a reusable spaceplane is that it can be launched on short notice based on need, McDowell said.

The two previous secret flights were in orbits roughly 321 kilometres high, circling at roughly 40-degree angles to the equator. That means the craft flew over the swath between 40 degrees or so north latitude and 40 degrees or so south latitude.

McDowell speculates that this newest flight will follow suit.

"It might be studying Middle Eastern latitudes or it might just be being used for sensor tests over the United States," McDowell said.