The US 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 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 programs at Air Force Space Command at Peterson Air Force Base, Colorado.
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. Near-space has been a cultural blind spot -- too high up for aircraft, but too low for satellites," the air force's chief of staff, General John Jumper, wrote earlier this year in a foreword to a paper on the subject.
One simple prototype, dubbed "Combat SkySat," was tested in the skies over Arizona in January through March with a series of 12 test launches.
The Air Force is considering seeking up to US$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.
For the idea to work, the Air Force will have to overcome a series of potential problems.
Winds are relatively low between 20,000m 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.
In addition, the 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.
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 Lieutenant-Colonel Edward Tomme of the Air Force Space Command's Space Warfare Center
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