The experimental aircraft are being called "atmospheric satellites," because they will be able to perform many of the functions of orbiting satellites, only at lower altitudes. Project engineers aspire to build a plane that will essentially be able to park 55,000 to 60,000 feet above a particular location on Earth and remain there for at least six months.

"That would provide you with a tower 55,000 to 60,000 feet above a city to provide something like telecommunications," said John DelFrate, a project manager for the program's solar-powered aircraft.

While NASA itself is not interested in providing telecommunications services, DelFrate said, the agency seeks to share its technology with private industry, so that commercial ventures can help defray the cost of scientific research performed with these high-altitude aircraft.

The ERAST program is based at NASA's Dryden Flight Research Center in Edwards, Calif. It will test prototypes of four aircraft designs during the next two months, aiming to reach several goals.

The most ambitious design of the four experimental aircraft is the solar-powered Helios Prototype, an enormous flying wing that spans 248 feet (74 meters), but weighs just 1,600 pounds (720 kilograms). Wider than a Boeing 747, the top of the Helios Prototype is clad with solar panels, which, its designers hope, will power the plane to rise above 100,000 feet. The target year for reaching that milestone is 2001. Two years later, project engineers hope to fly the Helios to 60,000 feet and keep it there for four days, using batteries to provide flying power through the nights.

Most commercial airliners fly below 40,000 feet.

Already the Helios prototype has demonstrated that it can cope with the high altitudes and low temperatures of flight at extreme altitude, DelFrate said, adding that a predecessor plane has flown to more than 80,000 feet.

Project developers at NASA cite several scientific uses for such airplanes. They can carry instruments to provide scientists with long-term data about the atmosphere. Current observations taken from traditional aircraft are limited both by altitude and the relatively short duration of flights. With efficient high-flying planes, scientists could measure atmospheric changes not just over hours, but over months and seasons.

Such aircraft could be used for remote sensing to provide data about agriculture, weather, or natural disasters. For example, instruments could be flown above forest fires to help firefighters battle troublesome blazes. New planes could fly at safe altitudes above hurricanes to help forecasters determine storm severity and predict landfall.

The chief advantages of an atmospheric satellite compared to its space-based counterpart would be its low cost and versatility, DelFrate said.

Instead of the massive rocket required to launch a payload into space, a high-altitude airplane simply needs a runway. Plus, DelFrate adds, "If you have a problem with your payload package, you can bring it back down and fix it. You can't do that with the space-based systems quite so easily."

Eventually, engineers foresee an airplane that can stay up indefinitely. AeroEnvironment, the private company that is working with NASA to develop the Helios Prototype, is calling its craft the "eternal airplane."

"You don't want the airplanes on the ground," DelFrate said. "You want them in the air. You lose money when the airplane is sitting on the ground."