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).

Cross-section of a high-tech balloon pressurised capsule

Since all the pilot can do is control the balloon's altitude, he needs to hitch a ride with the winds if he is to make it round the world.

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.

Steve Fossett explains how he landed safely in Russia earlier this year(0'19")

The American Steve Fossett managed to get half way round the world last year in the Solo Spirit, which carried an unpressurised capsule. But he had to endure temperatures of minus 20 Celsius.

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 Pil�tre de Rozier.

It was revived and improved 20 years ago by Don Cameron, a leading British expert in hot-air ballooning.

The Rozier: a hybrid of classic and hydrogen balloons

His Bristol-based company has designed all the balloons in the race - except that of the British team, Virgin Global Challenger. Its balloon has been built and designed by Lindstrand Balloons.

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.

Dick Rutan talks to reporters just after bailing out of Global Challenger (0'16")

If it fell faster than that, e.g. because of a helium leak, the crew members would strap on parachutes and leap out of the capsule.

This is what happened when Dick Rutan and Dave Melton bailed out of the Global Hilton earlier this month.

Technical Data

Solo Spirit
Capsule: composite gondola - unpressurised. Steve Fossett had to breathe oxygen through a mask for most of the flight.
Creature comforts: Single bunk with sleeping bag, bucket for a toilet.
Balloon: Rozier containing 270,000 cubic feet (7645.563 cubic metres) of helium and an 80,000 cubic foot (2265.352 cubic metre) hot-air cone
Power: propane burners

J Renee
Capsule: Cuboid gondola with six-inch thick walls - unpressurised.
Balloon: Rozier with 200,000 cu ft (5663.38 cu m) helium cell and 100,000 cu ft (2831.69 cu m) hot-air cone
Creature comforts: special heater, custom-designed to maintain temperature within the gondola at a minimum of 45 F when the outside temperature is -50 F
Power: propane burners

Breitling Orbiter II
Capsule: cylindrical capsule
Balloon: Rozier balloon, envelope volume 529,000 cu ft (14979.6401 cu m)
Power: kerosene burners and solar panels to supply batteries that power an on-board computer which controls the life-support system

Global Hilton
Capsule: carbon-fibre sphere - pressurised.
Balloon: 500,000 cu ft (14158.45 cu m) Rozier
Power: propane burners

Virgin Global Challenger
Capsule: aluminium cylinder - pressurised.
Creature comforts: Two decks, three pilot stations, sleeping compartment
Balloon: Rozier-type with 1.1 million cu ft (31148.59 cu m) helium cell
Power: propane burners