"The concept is to have an airship that flies above 65,000 feet and is geostationary, which means it stays in the same position over the Earth," said Oleg Wasynczuk, a professor of electrical and computer engineering at Purdue. Wasynczuk is leading the project with John Sullivan, a professor of aeronautics and astronautics.

Engineers and scientists are working to develop solar cells and an advanced fuel cell to power the craft. That means developing mathematical models to simulate what the fuel cell's performance could be at such high altitudes. The system is supposed to generate 500 kW, which is about 10 times the electricity necessary for the needs of an average home, according to Associate Professor of Nuclear Engineering Shripad Revankar.

"No such fuel cell system has yet been developed for space applications," he said. "We create mathematical models so that we can predict the fuel cell's behavior while it is interacting with a grid that contains all of these different systems. Say the wind suddenly picks up and the motors have to be turned on to keep the airship from drifting. How will this impact power to other systems on the ship? We need to know how the fuel cell will perform given the dynamic nature of this grid."

They also are developing the aerodynamic design and a control system to help keep the airship steady amid high winds and computer simulations to show how the craft would perform given specific design characteristics.

The airship is intended to fly well above commercial aircraft, which cruise as high as 40,000 feet, and conventional blimps, which reach around 5,000 feet. But, it would have better surveillance capabilities than satellites because it would be closer to the ground. Another advantage is that the ship could hover over one location anywhere in the world, whereas satellites are geostationary only over the equator, Sullivan said.

Because the unmanned ship might have to remain in flight for as long as a year at a time, it would have to be equipped with innovative power systems to provide electricity. So a portion of its skin would be covered with photovoltaic cells, which convert sunlight into electricity. The photovoltaic cells would provide electricity during the day to power the ship, and also run equipment that turns water into hydrogen and oxygen, which would be used in the fuel cell during the night. As the fuel cell generated electricity, it would produce water, which would be returned to the system to begin the cycle over again during daytime hours.

Researchers must simulate how the fuel cell would interact with the ship's various systems, including those for communications, surveillance and the electric motors for thrust.

Equipping the airship with both PV and a fuel cell should allow the ship to run continually without refueling.

Engineers are not yet certain how large the airship might be, but it may be as long as 300 meters, or around 900 feet - roughly four times the length of the Goodyear blimp. A major challenge will be finding materials for the airship's skin that are capable of withstanding the extreme ultraviolet radiation at such high altitudes for extended periods, Sullivan said.