The X-37B is about to complete its 3rd mission orbiting around the Earth
The X-37B is about to complete its 3rd mission orbiting around the Earth, but what exactly is it doing up there?Reuters

The unmanned X37-B space plane owned by the US Air Force has been orbiting the earth for over 400 days but even though the spacecraft is now on its third mission, we still have no idea what it's doing up there.

The US Air Force owns two of these solar-powered robot drones, which were built by aircraft manufacturer Boeing, are about the size of a bus and looks like a cross between an aircraft and a space shuttle.

One is in storage while the second has been in orbit since December 2012, its third time since the X-37B was first launched in 2010.

So what is the X-37B being used for?

The spacecraft is under the control of the US Air Force Rapid Capabilities Office, which has been known to test experimental equipment for operational deployments. As such, the space plane's mission and its flight route are classified.

Amateur space watchers, as well as the Chinese and Russian governments, are convinced that the X-37B is either some kind of space weapon or it is being used as a spy satellite. It was spotted in the vicinity of the Tiangong-1 space station in 2012.

The spy plane theory does have merit: unlike traditional satellites, the X-37B has a small engine that allows it to move on its own and reach orbits as high as 500 miles over the Earth.

Some observers claim it could be some form of space weapon or weapons carrier.

The X-37B is soon due to return to Earth and Boeing is planning to house and maintain it in a disused garage at the Kennedy Space Center in Florida. That could be good news for jobs, hit badly when Nasa retired the space shuttle in 2011.

"This signals another increase in the number of space activities that will be taking place at or near Kennedy Space Center," said Sen Bill Nelson. "I expect it will mean several hundred jobs as well."