The Department of Defense wants to develop spy satellites that can detect a military force mobilizing halfway around the world, enabling it to immediately assess possible threats to national security. An unmanned surveillance aircraft packed into the nose of a missile would be launched over suspicious areas to gather more intelligence; if the threat were confirmed, it would be replaced by another aircraft that could perform low-flying surveillance for up to five years without returning to Earth to refuel.

Toward that end, the Pentagon has commissioned development of the Rapid Eye and the Vulture—two new unmanned, high-altitude aircraft better able, it says, to meet today's needs for gathering information about nimbler threats.

"We're not talking about big, thick structures that we want to keep an eye on like in the Cold War," says Wade Pulliam, program manager with the Defense Advanced Research Project Agency's (DARPA) Tactical Technology Office, an arm of the Pentagon charged with researching and developing new technology for the defense department. "Threats today are more fluid right now, and military responses are more likely to be low-level and long-term, rather than fast and sharp. So endurance of all the assets involved is important."

The goal is to have demos of the technology for both aircraft within three years and working models ready to go within five years. DARPA plans to spend $12 million developing the Rapid Eye and $7.9 million on the Vulture through the end of fiscal year 2009.

The idea behind the Rapid Eye is to create an aircraft that can be stored on board a ballistic missile able to deliver it anywhere in the world within an hour. The Rapid Eye would travel inside the missile—with its wings folded or deflated, depending on the design—and be released over a designated spot. The Rapid Eye program will research and develop technologies and systems that would let the military deliver a 500-pound, high-altitude unmanned aerial vehicle via a ballistic missile to an approximate location, decelerate the missile so that its payload can be deployed, then launch the spy plane and start its propulsion system. Once aloft, it would provide intelligence, surveillance and reconnaissance capabilities at high altitudes for at least seven hours without refueling.

The Vulture project, which stands for Very high altitude, Ultraendurance, Loitering Theater Unmanned Reconnaissance Element, seeks to deliver and maintain an aircraft that can remain above a surveillance target for at least five years. Weighing in at an anticipated 1,000 pounds, the vehicle is being designed to collect its power from its environment—via solar or some other source—to store and use energy efficiently, and include a robotic refueling capability. With a wingspan of between 300 and 500 feet, the Vulture would function like a low-orbit satellite as much as like an aircraft, staying aloft far longer than any surveillance plane can today, says Jamey Jacob, an Oklahoma State University associate professor of mechanical and aerospace engineering, adding, "It would provide a persistent presence for the military." Jacobs is working as a consultant to defense contractors developing designs for the aircraft.

The two biggest hurdles in developing the Vulture are choosing the best energy source to power the vehicle and well-constructed sturdy components that will last at least five years. One option is to make the aircraft modular, so that components can break off and fly home via remote control when necessary and new modules can be flown up and remotely attached. Another option is to use a second aircraft to refuel the Vulture and repair it while in flight.

Several improvements in technology over the past decade—including solar cells and more efficient engines—make these aircraft more of a reality than they have been in the past. "The aircraft have to be light and run extremely efficiently," Jacob says. "You can't run off of today's fuels for five years. Ideally, the aircraft would have solar cells that can alternately charge and operate while in flight."

The projects are complementary. "The Rapid Eye is for when you don't know where you want to be but you need to get there quickly," Pulliam says. "The Vulture is for when you want to be over an area for a long time."

It's unclear whether the Rapid Eye will be able to be retrieved after it is used; DARPA is leaving that question to the firms that submit designs for the plane. "We would prefer to be able to recover the aircraft," Pulliam says, "but the cost of the aircraft will not be such that it must be recovered."

U.K. defense firm QinetiQ claims to hold the record in keeping an unmanned aircraft aloft. In 2005 its Zephyr high-altitude, solar-powered vehicle stayed aloft continuously for 54 hours during a test flight. The previous unmanned endurance record was set in 2001 by a jet-powered U.S. Air Force Global Hawk surveillance aircraft, which flew for more than 30 hours. The Zephyr is a carbon-fiber aircraft weighing 66 pounds and with a wingspan of about 59 feet that by day flies on solar power generated by amorphous silicon arrays that cover its wings; it is powered at night by lithium-sulfur batteries recharged during the day using solar power.