Boeing's giant flying tadpole has taken to the skies once again. That's right, Boeing's Phantom Eye UAV took its second flight yesterday for a 66-minute test flight out of Edwards Air Force Base in California. The hydrogen-fueled drone climbed to 8,000 feet above the Mojave Desert and reached a top speed of 71 miles per hour, according to a Boeing press release.

So you may be saying to yourself, "71 mph, no stealth; what's the big deal?" Well, Phantom Eye is one of a relatively new crop of concept UAVs designed to operate at very high altitudes and stay there for a long time -- four days, carrying a 450-pound payload at 65,000 feet in Phantom Eye's case.

The so-called High Altitude Long Endurance (HALE) aircraft are meant to loiter in airspace not used by other planes, watching targets on the ground, scanning swaths of air or land, or passing data and messages over long distances or mountain ranges that would normally interrupt communications. It has even been suggested that fleets of HALE drones could serve as an atmospheric back-up of U.S. satellites and ground-based communications gear if those were ever destroyed.