Pressures of life in the jet stream

The crew aim to take the balloon up into the jet stream, a tube of air at 9,000 metres flowing from west to east at speeds of up to 200 miles per hour, writes Charles Arthur.

If the airstream runs favourably, it should carry the unpowered balloon along at an average of 50mph. But it is possible to forecast the jet stream's patterns only five days ahead; if it breaks up in the next week, the voyage may become impossible. If necessary, the crew can parachute out. Their finishing point will depend heavily on local prevailing winds.

The challenge of the task does not rest solely with the weather. Life inside the capsule of the Virgin Global Challenger will be cramped, smelly and disorientating. The three men have only 23 cubic metres in which to sleep, eat and use the toilet. There are no washing facilities; the tonne of water they are carrying is intended solely for drinking, because they will dehydrate rapidly in the pressurised air they will be breathing. This will be equivalent to living at an altitude of 2,500m. In order to avoid kidney damage, each man will have to drink about five litres of water per day.

Their meals will consist of "wet microwave" dehydrated foods, complemented with fresh fruit and vegetables, amino acid tablets, and various daily "treats" - including caviar, chocolate and a miniature of vodka.

There is no room to stretch the legs. The crew risk atrophy and might be unable to walk after 18 days.

There will also be problems of sleep and orientation. There will always be one man taking eight hours rest in the sleeping compartment under the floorboards while the other two work at the controls.

As they cross time zones, the trio will find it hard to avoid some sort of jet lag, because of the body's natural tendency in isolation to run on a 25-hour day.

The men will have a plentiful supply of human contact though, with a "chase plane" following their progress around the world, and radio links and satellite telephones allowing them to talk to family and friends at any time.