Mysterious X-37B unmanned space shuttle launched by U.S. ... and they won't say what it's for

A top secret space plane developed by the US military has blasted off from Cape Canaveral on its maiden voyage.

Billed as a small shuttle, the unmanned X-37B heralds the next generation of space exploration. It will be the first craft to carry out an autonomous re-entry in the history of the US programme.

But its mission - and its cost - remain shrouded in secrecy. The Air Force said the launch was a success but would give no further details.

Lift off: The X-37B sits on top of an Atlas V rocket as it's launched at Cape Canaveral Air Force Station in Florida

Ready for launch: The X-37B rocket in Florida tonight

'Well, you can't hide a space launch, so at some point extra security doesn't do you any good,' said Gary Payton, Air Force deputy under-secretary for space systems.

He remained cagey about what exactly the X-37B is to do.

'On this flight the main thing we want to emphasise is the vehicle itself, not really, what's going on in the on-orbit phase because the vehicle itself is the piece of news here,' was all he would say.

The X-37B Orbital Test Vehicle took a decade to develop and will spend up to nine months in orbit. It will re-enter Earth on autopilot and land like an ordinary plane at the Vandenberg Air Force Base, California.

Mystery: A handout shows scientists working on the prototype for the rocket prior to its launch

A computer graphic shows what the X-37B will look like in space. It's wingspan is a mere 4.5 metres with a length of 8.9 metres

When exactly that will happen, however, even the Air Force can't predict.

'In all honesty, we don't know when it's coming back for sure,' Payton said. 'It depends on the progress we make with the on-orbit experiments and the on-orbit demonstrations.'

The spacecraft will conduct classified experiments while in orbit. The military still has not revealed what those experiments will entail.

Payton said the Air Force's main interest is to test the craft's automated flight control system and learn about the cost of turning it around for launch again.

The X-37B is 9m long (29ft) and has a wingspan of 4.5m (15ft), making it a quarter of the size of a normal shuttle.

It is powered by a solar array and lithium-ion batteries, unlike a traditional craft which is powered by a fuel cell system.The spaceplane is also reusable.

Built by Boeing's Phantom Works division, the X-37 program was originally headed by NASA. It was later turned over to the Pentagon's research and development arm and then to a secretive Air Force unit.

Hundreds of millions of dollars have been spent on the project, but the current total has not been released.

'After a tumultuous history of sponsorship, it's great to see the X37 finally get to the launch pad and get into space,' Payton said.

The Air Force has given a very general description of the mission objectives: testing of guidance, navigation, control, thermal protection and autonomous operation in orbit, re-entry and landing.

However, the ultimate purpose of the X-37B and details about the craft have longed remained a mystery, though experts said the spacecraft was intended to speed up development of combat-support systems and weapons systems.

Secret: It is still unclear what kind of experiments will be conducted onboard