US Military Wants Sky Stations

The US Defense Advanced Research Projects Agency, DARPA, is soliciting proposals for research into aircraft that can stay aloft for more than 5 years.
The project, code-named VULTURE, is for heavier-than-air station that does not need to land. This is a request for early information but there are some restrictions on the design:
"The Government is not interested in approaches that use either radioactive energy sources or employs any form of buoyant flight for this application."
So, blimps need not apply.

What kind of system is allowed?
"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."

Automatic refueling, long-life batteries, solar powered, wind powered, very long-term glider, floating cities... lots of things would work.

Is the lack of such a platform due to missing technology or because no one has tried it before?

Get your ideas together quickly. DARPA has scheduled an Industry Day for June 7 to explain their goals in detail.

Solicitation Number: SN07-38