The Defense Advanced Research Projects Agency (DARPA) is in the market for an aircraft that can stay aloft for five-years nonstop, according to this announcement, about the VULTURE program:
The VULTURE Air Vehicle Program will research and develop technologies and systems which will enable the Military to deliver and maintain a 1000 lb, 5 kW airborne payload for an uninterrupted period exceeding 5 years with a 99+% on station probability. The architectures selected and the specific approaches taken by the Offerors will determine the range of technical areas that are developed, including, but not limited to, environmental energy collection, high specific energy storage, extremely efficient propulsion systems, precision robotic refueling, autonomous materiel transfer, extremely efficient vehicle structural design, and mitigation of environmentally-induced loads.
Leave your nukes and blimps at home, though; DARPA says that it won't consider "radioactive energy sources or employs any form of buoyant flight for this application."
