Theories fly about US space drone

Many think the OTV is spying on China. Photo / AP

It arrived back at a California air base after dark. Only the eagle-eyed would have spotted the snub-nosed spacecraft gliding out of the black sky.

Officially, the unmanned Boeing-built X-37B Orbital Test Vehicle (OTV) had just completed its longest-ever mission, spending almost two years circling the Earth conducting experiments.

But its secretive history has sparked countless theories about what it was really doing in space.

One is that the United States Air Force has developed a drone spy ship, which it uses to shadow Chinese satellites. Another claim is that it has been developed to engage in targeting rival spy satellites. Iran has described it as America's space warplane. There were few clues in an official press release.

"The landing of OTV-3 marks a hallmark event for the programme," said an unidentified programme manager quoted in the USAF statement.

"The mission is our longest to date and we're pleased with the incremental progress we've seen in our testing of the reusable space plane."

The first of the X-37B spaceplanes flew in April 2010 and returned after eight months. A second mission followed a year later. The next flight is planned for 2015.

At 9m long it is thought to carry only a control system and fuel for its thrusters, leaving a cavity roughly the size of a truck bed.

The project began with Nasa before being handed to the military seven years later amid great secrecy, heightening speculation.

The USAF insists it was never designed to carry weapons. Instead, spying of some kind is one of the more plausible possibilities.

On a previous mission, amateur space trackers spotted that its path mirrored that of China's space lab, named Tiangong 1.

Dr David Baker, editor of Spaceflight, told the BBC: "Space-to-space surveillance is a whole new ball game made possible by a finessed group of sensors and sensor suites, which we think the X-37B may be using to maintain a close watch on China's nascent space station."