Wednesday, January 21, 1998 Published at 17:11 GMT
Ballooning: the ins and outs, the ups and downs
Long-distance balloons hitch a lift on powerful eastbound jet streams
The first passenger-carrying balloon, built by the brothers Montgolfiers, landed in a heap in Paris in 1783. Two centuries later, Man has flown to the Moon and back, so why does circumnavigation of the Earth by hot-air balloon remain so elusive?
The main problem for long-haul balloonists is the weather. There is a small window around December and January when the jet streams around the Northern Hemisphere are at their most powerful, reaching speeds of up to 400kph (about 250mph).
To take full advantage of the jet streams, a balloon needs to go up as high as 12km (seven miles).
The high-tech option is to go up in a sealed and pressurised cabin, which allows the crew to breathe and keep warm at extreme altitudes.
The main drawback, aside from the expense and the weight of the capsule, is that this technology is highly experimental and much can go wrong.
This year, he was forced to land in Russia when a burner developed a defect. He is now considering making another attempt in a pressurised cabin.
Going up and staying up
Classic hot-air balloons rise because warm air is lighter than cold air. A propane gas burner is used to heat the air inside the nylon or polyester balloon. The hotter it gets the higher it goes.
The five gobal challengers all use a Rozier design, named after its original inventor, the 18th century French physicist and aerostatics pioneer, Francois Pilâtre de Rozier.
It was revived and improved 20 years ago by Don Cameron, a leading British expert in hot-air ballooning.
The Rozier has a double skin, or envelope, which is filled with both hot air and contains a helium balloon.
The advantage of this type of balloon is that, as the sun goes down and the balloon's air gets cooler, it does not have to jettison ballast to slow its descent, while hot-air burners can be switched on to stabilise the craft's altitude.
Only a tiny quantity of propane or kerosene is required to heat the helium.
At the start of the voyage, the sphere is only half-filled with helium. During the ascent, decreased pressure and rising gas temperature caused by the heat of the sun expand the helium to full "cruising" volume.
The system is designed to vent helium gas automatically if its pressure becomes excessive. So Rozier balloons depend on the heat of the sun by day and the heat of propane gas at night.
Even if the balloon experienced a major loss of helium, it would remain aloft, turning into a classic hot-air balloon.
If all went catastrophically wrong, the balloon fabric could double as a giant parachute, keeping the balloon's descent rate at five metres (about 16ft) per second.
This is what happened when Dick Rutan and Dave Melton bailed out of the Global Hilton earlier this month.
Breitling Orbiter II
Virgin Global Challenger
All the balloons use satellite equipment to communicate with their control centres and transmit vital information about the craft's position and altitude. Communications instruments are powered by batteries.