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.