A US Air Force secret X-37B space plane has been in space for more than 400 days. Why?

A SPACE plane has been orbiting Earth for more than 400 days, on a mission kept shrouded in secrecy by US military chiefs. Why?

IT'S the US Air Force's most "hush-hush" project: A space-plane that has been quietly orbiting the Earth for more than 400 days. It's still up there. Just what is it, and what is it up to?

The unmanned X-37B is reminiscent of NASA's retired space shuttle. It has the familiar black tiles on its undersides. The upper surface is white - and there's a set of large doors set in its back.

There the similarities end.

This secretive craft is only the size of a bus at 8.8m long. And it carries no crew.

And its purpose is unknown - although theories range from surveillance to weapons deilvery.

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 project seems set for a major expansion: The disused facilities at NASA's Kennedy Space Centre in Florida - previously the home of the iconic Space Shuttle - are being eyed as a future site from which to base the X-37 project.

It would be no minor renovation. The Kennedy Space Centre will likely get an upgraded runway, new radars and guidance hardware and refurbished workshops.

If the Kennedy project goes ahead, it would represent a significant leap forward in the US military's space-based operation.

So what would such an expensive project deliver?

Pet project ... US Air Force personnel examine an X-37B after it returned from its space deployment.

SPACE SURVEILLANCE?

This seems to be the most likely explanation. In fact, amateur space watchers claimed in January 2012 that an X-37B was lurking in the vicinity of China's Tiangong-1 space station test module. This was neither confirmed nor denied by US authorities. A spacecraft capable of switching out different types of sensor packages to suit particular spying missions would be a very effective addition to any military. On one mission it could carry cloud-penetrating radar. The next an infra-red camera to delve deeply into a jungle. The next it could be an ultra-sensitive electronic listening device.

Robot school ... the X-37B Orbital Test Vehicle undergoes testing at the Astrotech facility in Titusville, Florida, in 2010.

WEAPONS TEST?

If true, testing the X-37B as a space-based weapons platform would be in contravention of several international treaties. The Pentagon has denied the allegation. Analysts tend to agree: They say the craft is unlikely to be a serious weapons delivery system. It appears to have only a low-power orbital manoeuvring engine - reducing its ability to move quickly to any crisis scene. Nevertheless - the simple fact that it has a multipurpose cargo bay means the craft can carry whatever fits in that space. As a result it has the potential to carry anything from small missiles through to nuclear weapons into orbit.

Shadowy mission ... while we know the X-37B exists, we don't know what it does.

ROBOTICS TEST?

It's also possible that the X-37B is simply undergoing intensive endurance testing. A similar robotic aircraft - the US Navy's X47B - last year took off and landed on an aircraft carrier for the first time. This was only after it was put through several evaluation stages including simply moving about the flight decks and hangar spaces. The challenges - and capabilities - of such semi-autonomous robotic aircraft are yet to be fully understood. Only extensive exposure to "real world" conditions will define exactly what they can - and cannot - do.

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