Built by drone manufacturer AeroVironment, the Global Observer is a 70-foot-long jumbo drone with a wingspan nearly as long as one of the Air Force’s B-52 bombers. Powered by liquid-hydrogen fuel cells, it was billed as a persistent eye-in-the-sky capable of loitering at 65,000 feet for a week a time without spewing carbon dioxide into the atmosphere. The Pentagon also envisioned many missions. The drone’s 380-pound payload of spy cameras and sensors could stare at a diameter of 600 miles of earth at once, while doubling as a communications relay. It could patrol the oceans and possibly track hurricanes — the Department of Homeland Security was interested in it too.