United State spy in sky, 12 miles high

The US military hopes the blimp, floating 12 miles above a surveillance area in near space, will give it a better understanding of events on the ground. It will be equipped with a radar system able to provide unprecedented detail over a wide area from hundreds of miles away.

"It is constant surveillance, uninterrupted," Dahm said. "To be able to observe over a long period of time, you get a much better understanding of how an adversary operates."

The craft - called Isis - will be powered by hydrogen fuel cells recharged by its own solar panels, and will be filled with helium, which will give the craft its shape. The deployment of a blimp will raise memories in the US of the Hindenburg disaster in 1937, when three people died after the airship went down in flames in New Jersey.

Despite falling out of favour in subsequent years, airships have made a recent comeback. A Silicon Valley company plans to offer passenger sightseeing tours on the 246ft-long zeppelin it is developing at Moffett Fields, a historic airfield that was home to a 785ft dirigible operated by the Army Air Corps, the precursor of the US Air Force. That craft crashed in 1935, ending the army's first experiment with airships.