US Air Force's X37B in orbit for more than 469 days

One of the US Air Force's most mysterious projects - an unmanned space-faring plane - has broken its own longevity record by staying in orbit for more than 469 days.

There aren't many details floating around in public regarding the X-37B - a classified project that's seen three launches into space thus far - but a new report by the IDG News Service suggests the plane is being used to test out the Air Force's new space capabilities while also boosting the intelligence community's surveillance capacity.

Launched back in December 2012, the Boeing-designed X-37B has been floating above the Earth ever since.

The plane is only 8.8m long and looks like a smaller cousin to NASA's past space shuttle models, It has been designed with quick turn-around times in mind, so that it can easily be re-launched after landing.

Although few people know exactly what the vehicle is intended for, astrophysicist Jonathan McDowell told IDG it seems clear one of the big features is the plane's ability to stay in space for extended periods of time.

The US Air Force has two of the Boeing-built robotic vehicles. One of them was launched into space on top of an Atlas 5 rocket on December 11, 2012. It's been up there ever since - but exactly where or why is unknown.

The space plane is able to sustain itself on such a long flight through the solar cells built into its wings which charge internal banks of lithium-ion batteries.

It's only the second time such a craft has been sent into space.

What the reusable space plane carries in its 2.1m by 1.2m cargo bay is classified. So is its mission. So is its course.

This is because it falls under the command of the US Air Force Rapid Capabilities Office - an organisation which, among other things, tests experimental equipment on real operational deployments.

"The Air Force now has a policy of acquiring capabilities rather than missions, so some general somewhere probably thinks it would be spiffy to have a space plane that can launch at short notice," he said.

"It's worthwhile learning lessons from the shuttle and how to do turn-arounds cheaper."

The fact that the project is clouded in secrecy hasn't stopped people from speculating on its goals, however.

Before the X-37B's first launch, officials told the Christian Science Monitor that it would conduct various experiments" involving the transport of "satellite sensors, subsystems, components, and associated technology" into space and back to Earth.

"I believe it's testing some kind of experimental sensor for the National Reconnaissance Office; for example, a hyperspectral imager, or some new kind of signals intelligence package,"McDowell said to IDG.

"The sensor was more successful than expected, so the payload customer asked the X-37 folks to keep the spacecraft in orbit longer."