A pair of big, blimp-like craft, moored to the ground and flying as high as 10,000 feet (3,000 metres), are to be added to a high-tech shield designed to protect the Washington D.C. area from air attack, at least for a while.

The bulbous, helium-filled "aerostats" - each more than three quarters the length of a football field at 243 feet (74 meters) - are to be stitched into existing defenses as part of an exercise of new technology ordered by the Defense Department.