On Thursday, Pentagon officials unveiled a $400 million project to develop a prototype of an unmanned spy aircraft that would resemble a high-tech dirigible. Dirigibles have been decidedly out of fashion since the infamous Hindenburg accident on May 6, 1937. The new airship would hover about 12 miles above the ground, beyond the range of hand-held missiles, but within the range of some surface-to-air missiles. The most noteworthy practical application of the airship would be monitoring small movements on the Afghanistan-Pakistan border. The surveillance dirigible would be filled with helium and use solar panels to recharge hydrogen fuel cells. The prototype will be 150-feet long, with the goal of building production models three times as big. The program is known as Integrated Sensor Is the Structure (ISIS). No contracts have yet been signed, but the leading contenders are assumed to be Northrop Grumman and Lockheed Martin.