The air at 20,000m 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 Lieutenant Colonel Toby Volz, who oversees near-space programmes at Air Force Space Command at Peterson Air Force Base, Colorado.

An 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, General John Jumper, wrote earlier this year in a foreword 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.
 
Potential problems

The air force is considering seeking up to $15 million for 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.

Scientists hope to see the plan
in place within five years

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

Winds are relatively low between 20,000 and 24,000m, (usually less than 32km 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.

The US Air Force regards near-space altitudes as 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 US was at war with that nation.

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 Earth. Still others would have some capability to manoeuvre and be able to stay over their target longer.

Other proposals
 
More expensive proposals, such as a massive blimp would cost tens of millions of dollars and stay aloft for years.

Such a design could carry bombs or other weapons to drop on ground targets, according to Lieutenant Colonel Edward Tomme of the Air Force Space Command's Space Warfare Centre.

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.

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