The Air Force is eyeing a seldom-used region of the Earth's atmosphere called "near space" for communications and intelligence-gathering with one of the oldest types of aircraft — balloons.

The air at 65,000 feet and higher is too thin for most traditional airplanes, so military officials are testing unmanned helium balloons at those altitudes. This frigid part of the atmosphere is above most weather but well below low Earth orbit, where the far costlier space station and satellites operate.

"It's a region of the atmosphere that historically has really not been exploited," said Lt. Col. Toby Volz (search), who oversees near-space programs at Air Force Space Command (search) at Peterson Air Force Base, Colo.

A key advantage of balloons and blimps is they may be able to stay aloft much longer than an airplane, providing a communications or surveillance platform that can last days or even weeks. They are also much cheaper than satellites, and could let ground forces communicate over far greater ranges than the line-of-sight radios they often carry.

"I've been intrigued by near space's potential for persistent space-like effects on the battlefield ever since I first heard about it," the Air Force's chief of staff, Gen. John Jumper, wrote earlier this year in a forward to a paper on the subject. "Near-space has been a cultural blind spot — too high up for aircraft, but too low for satellites."

One simple prototype, dubbed "Combat SkySat," was tested in the skies over Arizona in January through March with a series of 12 test launches. Kirtland Air Force Base (search), N.M., is also involved in testing near-space craft.

The Air Force is considering seeking up to $15 million on near-space operations and research in its 2007 budget, officials said. Volz said he hopes to see operational near-space systems during the next five years.

For the idea to work, the Air Force will have to overcome a series of potential problems.

Winds are relatively low between 65,000 and 80,000 feet, usually less than 20 miles per hour. But levels of corrosive ozone and ultraviolet radiation are much higher than at the Earth's surface.

Another downside is that balloons take many hours to fill with helium and launch, and sometimes require hangars to steady them while they are being filled.

In addition, the Air Force regards near-space altitudes a part of a country's sovereign air space, unlike orbital space that is open to all, according to officials at Air Force Space Command. So the military would be violating internationally accepted practices and law if it sent an intelligence-gathering balloon over another country without permission — except, of course, if the U.S. was at war with that nation.

Proposals for near-space craft vary in complexity. Some free-floating balloons would cost only a few hundred dollars and be expendable if lost to the winds.

Others would launch a glider to carry a payload down to the Earth. Still others would have some capability to maneuver and be able to stay over their target longer.

More expensive proposals, such as a massive blimp called the High Altitude Airship, move beyond the realm of expendable balloons. These would cost tens of millions of dollars and stay aloft for years. Such a design could also carry bombs or other weapons to drop on ground targets, according to Lt. Col. Edward B. Tomme of the Air Force Space Command's Space Warfare Center

Air Force officers pushing near-space systems for intelligence gathering aren't advocating replacing satellites, just freeing them up for other tasks.

Intelligence satellites, because they are so few and expensive, are generally controlled by national authorities and targeted on matters of interest to top military, intelligence and executive branch officials. Balloons could be launched and recovered by commanders on a battlefield, giving them more flexibility to gather information they need quickly.

As spies, near-space craft will take better pictures than satellites because they are 10 to 20 times closer than a camera in orbit, Tomme wrote earlier this year. If outfitted to eavesdrop on communications, a near-space craft would be likelier to pick up low-power transmissions that satellites cannot hear, he wrote.

At the altitudes being studied by the Air Force, the balloons are out of reach of many interceptor aircraft and missiles.

They are hard to destroy even if they are in range. According to Tomme, in August 1998 an out-of-control Canadian weather balloon survived multiple strafing runs from jet fighters as it flew across Canada, the North Atlantic, Norway, Russia and the Arctic Ocean (search). Because the pressure inside these balloons is close to that of the surrounding air, they resist deflating quickly when punctured.

Some commercial concerns already use balloons for similar purposes. Oil and gas producers in west Texas and Oklahoma receive data from wells transmitted through high-altitude communications balloons, which are much more cost-effective than trying to establish a cellular network in such sparsely populated regions, according to Tomme.