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The US air force is about to launch its X-37B mini-space shuttle back into space, with the purpose of the top-secret mission still the subject of wild theories.

US authorities insist the solar-powered robotic vessel is nothing but a test vehicle, and its only purpose is to carry out a few experiments in orbit. However a leading anti-nuclear activist believes the 29-foot craft  is in fact carrying an atomic weapon.

Dave Webb, chairman of the Global Network Against Weapons & Nuclear Power in Space, told USA Today that the X-37B "is part of the Pentagon's effort to develop the capability to strike anywhere in the world with a conventional warhead in less than an hour."

Others believe the ship, which has no crew and is able to land unaided, is a spy-satellite or a satellite-killer. Meanwhile China and Russia are reportedly concerned the X-37B is some kind of space-weapon.

The X-37B was first launched in April 2010 and this is its third mission into space. Although the US air force says the vehicle is designed to stay in orbit for 270 days, it returned from its second mission in August 2012 after 15 months (469 days) in orbit.

What the mini-shuttle did during its two previous space cruises remains strictly confidential.

John Pike, director of news site GlobalSecurity.org, attempted to scotch the conspiracy theories, telling USA Today: "To the extent that it [X-37B] does have a purpose, I think its purpose is to keep the Chinese guessing as to what the purpose is.

"Inquiring minds want to know, right? But posing this question presumes that [the mini-shuttle] does serve some specific purpose. And I think that might be imposing greater rationality on the whole thing than is warranted."