Boeing revealed this week it has successfully tested a prototype of an unmanned aerial aircraft powered by hydrogen that can linger at high altitude for as long as four days without refueling. The twin-engine unmanned aerial vehicle, called the Phantom Eye, took off from a California desert test site and flew for 28 minutes in its first test. The plane suffered a minor problem when its landing gear dug into the dry lake bed where it landed and broke. The prototype plane has wings 150 feet long to allow it to linger above the weather and turbulence at 65,000 feet. By burning hydrogen, it produces little if any emissions. A production version of the aircraft will have wings more than 200 feet long and the capability of lingering for up to 10 days. The test version of the plane, Boeing said, can carry some 450 pounds of cameras and detection equipment aloft.