ACCORDING to the old adage, what goes up must come down. But in the case of balloons, the descent sometimes comes sooner than expected. Although helium-filled weather balloons regularly launch instruments high into the stratosphere, at altitudes of 20km (65,000 feet) the air is 15 times less dense, causing the balloons to expand and ultimately burst.

This problem has long vexed the American military, which would like to use lighter-than-air dirigibles as atmospheric satellites, or stratellites. From 20km a blimp would be able to continuously survey an area the size of Texas for months at a time, but in greater detail and at much lower cost than geostationary satellites or those moving in low Earth orbit.

Despite the problems, dirigibles are coming back into service. Global Telesat Corporation, an American supplier of unmanned aerial vehicles, recently purchased its first SkySat. This 38 metre-long remotely operated airship is designed to carry communications and monitoring equipment for intelligence, surveillance and reconnaissance missions. With its familiar cigar-shaped design, however, it will rise only so far—14km, to be precise. And at this altitude ferocious winds require a SkySat to burn through its fuel supply just to stay still. As a result it can remain aloft for only a few days at a time.

The ultimate goal is to find a way to slip above the jet stream and into the virtually windless stratosphere for an even better vantage point, says Dan Erdberg of Sanswire, a Florida company which developed SkySat. Over the years the company has looked at various solutions, and it is now working with Global Telesat to develop a novel, snakelike airship called the STS-111. It is made up of four connected and articulated inflatable sections.

Each section of the STS-111 contains two gas envelopes—one inside the other. The inner envelope holds helium in the head section of the blimp and an ethane fuel gas in the three tail sections. The outer envelope of all four sections contains air maintained at atmospheric pressure. Although the airship starts off flaccid, the helium and the ethane expand as it gains altitude. As it does so, the air in the outer envelope is released to allow these gases to fill the void left behind. This approach gives the gases more room to expand without stretching the envelope of the airship too much.

Venting gases is not a new idea, says Mr Erdberg, but usually it involves releasing helium, which reduces the buoyancy of the craft, so that making a safe descent or maintaining a steady altitude becomes extremely difficult. What makes Sanswire's design unique is that it allows the airship to have the same weight on landing as it did when it took off.

This is because air is pumped back into the tail envelopes as the ethane fuel is burned by the craft's thrusters, and is also pumped into the tail and head during descent as the inner envelopes contract. The serpentine aerofoil shape is designed to face into what little wind there is, generating a small amount of lift. And since ethane has roughly the same density as air, the fuel in effect weighs nothing, further reducing the amount of helium needed to create buoyancy.

That is the theory, at least. So far, apart from some low-altitude test flights, the only stratospheric tests have involved a scaled-down version tethered to a weather balloon. Tests of a full-sized STS-111 at high altitude are not expected to begin until next year. So the military will have to wait a bit longer to see if Sanswire's ideas hold up—or its claims are inflated.