WASHINGTON – The next Air Force spy craft is likely to be a giant, unmanned dirigible that can remain aloft at high altitudes, keeping an unblinking watch on vehicles, planes and even people.

The dirigible is the brainchild of the U.S. Air Force and the Pentagon’s research arm, which together will spend $400 million to develop a prototype that could pave the way for a fleet of spy airships, military officials said Thursday.

The plans represent the final stage of work to develop a giant airborne radar system capable of providing ground operators with intricate detail over vast expanses, even if the dirigible is hundreds of miles from its target.

The project reflects a recent shift in Pentagon planning and spending priorities under Defense Secretary Robert Gates, who has urged the military services to improve intelligence and surveillance operations, while cutting high-tech weaponry costs.

However, it marks the return to a form of flight that has stirred anxiety and doubt since the deadly 1937 disaster involving the Hindenburg. In Iraq, the military has used less-sophisticated tethered blimps called aerostats to conduct surveillance around military bases.

Unlike other surveillance platforms, the proposed airship will stay aloft for 10 years and provide a constant watch over an area, Air Force officials said.

“It is absolutely revolutionary,” said Werner Dahm, chief scientist for the Air Force. “It is a cross between a satellite and a Global Hawk (spy plane).”

The airship will fly at 65,000 feet, or 12 miles, beyond the range of any handheld missile, and safe from most fighter planes.

At that height, it would be nearly impossible to see. But the dirigible could be vulnerable to some surface-to-air missiles, and would be unable to maneuver out of the way. Nonetheless, the airship’s range will allow it to operate at distant edges of any military theater, likely out of the range of many missiles.

The airship would provide the military a much better understanding of an adversary’s movements, habits and tactics, officials said. The ability to constantly monitor small movements in a wide area – like the Afghanistan-Pakistan border, for example – will dramatically improve military intelligence, officials said.

“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. When you only have a short-time view – whether it is a few hours or a few days – that is not enough to put the picture together.”

The dirigible will be filled with helium and powered by an innovative system that uses solar panels to recharge hydrogen fuel cells. Military officials said those underlying technologies, including a very light hull and low power transmitters, were critical to making the project work.

“The things we had to do here were not trivial, they were revolutionary,” said Jan Walker, a spokeswoman for the Defense Advanced Research Projects Agency.

The final version of the airship will be about 450 feet long. However, the prototype will be only a third of that size. The craft is known to military planners as ISIS, or Integrated Sensor Is the Structure, referring to the radar system built into the structure of the craft.

The ISIS has a hull made of a lightweight, thick skin. Zeppelins – like the Hindenburg – have a rigid external structure. Blimps are not rigid, and are given their shape from the pressure of the helium gas. According to military, the ISIS is closer to a blimp than a zeppelin, but officials most frequently call it an airship. “Airship,” like “dirigible,” is a broader term.

The Air Force has signed an agreement to develop a demonstration dirigible along with the defense research agency. Due to be finished by 2014, the Air Force will begin to use the prototype after an initial three-month testing period. The military has not yet designated a contractor. The Air Force’s intelligence, surveillance and reconnaissance – or ISR – abilities have improved dramatically in the last five years with the expanded use of Predators and other drones. Although Air Force drones can linger over an area for a long time, they do not watch constantly.

The radar system is what gives the new airship its value to military planners.

“Being able to observe threats with a very large radar in the sky we have the ability to see things much better,” Dahm said. “Being able to watch those things, understand what is happening, is really the game-changing piece here.”