A spacecraft that blasted off from Cape Canaveral, Fla., under a cone of secrecy Tuesday is likely being used to test top-secret military spy tools, one expert says.

The unmanned X-37B took off Tuesday riding on the nose of an Atlas V rocket. The spacecraft resembles a smaller version of NASA's now-mothballed shuttle program, and follows two earlier successful flights by X-37Bs. One orbited the Earth for several months while another spent more than a year in orbit.

Tuesday's mission sent the third orbital test vehicle (OTV-3) into space. Mission commentary -- which normally provides the public with a glimpse into the inner workings of NASA missions -- ended after 17 minutes when the U.S. Air Force instituted a media blackout.

Randy Attwood, managing editor of Space Quarterly magazine, said the eight-metre-long spacecraft is too small to be carrying weapons, but is likely being used for other military purposes.

"It's not carrying bombs, although it is being put into an orbit, which is exactly where spy satellites go. So it is possible that when it passes over Afghanistan or Iran, it is using cameras and sensing technology to look down and record images from the ground," he told CTV's Canada AM. "But it really isn't big enough to carry any bombs."

Other experts have suggested the spacecraft, which is capable of re-entry, could intercept transmissions of electronic messages, including those sent from terrorist training camps in Afghanistan and other locations.

Attwood said the spacecraft is primarily a "testbed" for future satellites and spy technology to see how the equipment functions in space.

The X-37B has a payload that opens up once it is safely into orbit, extending a large solar panel to power the spacecraft's equipment.

"The last one that was up there was there for over a year, and the challenge is if you put something like a billion-dollar space satellite up and it breaks down, you can't get it back to find out what happened," Attwood said. "So if they can take their technology -- their cameras, their electronics -- into space, expose them to the environment and then bring them back, it's very, very important."

The X-37B is built by Boeing and is reusable, meaning it can re-renter the Earth's atmosphere and land on a runway, much like NASA's shuttles, and still be used for additional missions.

The two previous secret X-37B flights were in 300-kilometre-plus orbits, circling at roughly 40-degree angles to the equator, according to estimations by amateur satellite trackers. That means the craft flew over an area between 40 degrees or so north latitude and 40 degrees or so south latitude.

That would put Russia's far north out of the spaceplane's observing realm, but puts it in sight of the Middle East.

The X-37B program is operated by the Air Force Rapid Capabilities Office and is geared toward space experimentation. The program has been in place since 1999.