There is even the risk of being shot down - in 1995 the balloon of two Americans was shot down and they were killed when they entered Belorussian airspace.

So what drives them? Money? Richard Branson and Steve Fossett are already millionaires.

A place in the history books? Or, perhaps, the simple pleasure of floating away and above it all?

Steve Fossett nearly made the journey in the Solo Spirit low-tech gondola

Branson described his fascination with this gentle sport: "There were definitely times where I said to myself, if I get out of this alive I will definitely thank someone for getting me out of this fix, and call it a day.

"Then about three hours later you start seeing the magnificence of the Sahara and the beautiful mountains, the dawn coming up, being somewhere which most people in the world have never seen... It's difficult to resist the whole fascination of that." In the last year three men have nearly lost their lives trying to fly round the world in a balloon.

The prize

The winner of this exclusive race could win up to $1.5 million (£919,664).

The US brewery Anheuser-Bush has set aside a one million-dollar prize: $500,000 (£307,000) go to the winners, and a further $500,000 to the charity of their choice.

But most of these men are millionaires already, so it can't be the money that drives them on.

One can only assume that the challenge and the hope of glory keeps the competition running.

Rules of the game

The deadline for the great ballooning challenge is December 31, 1999. The International Aviation Federation, which officiates over record-breaking flights, has set strict rules:

• The balloon must begin and end its circumnavigation on the same longitudinal line.

• There is a minimum distance that must be covered which is equivalent to about two thirds of the "Great Circle", the circle around the Earth at its largest diameter. The balloon must be picked up on radar or by aircraft at specified "waypoints" chosen by the pilot on the voyage.

• The balloon must not land until it has completed its journey. The FAI has installed sealed altimeters that record the height of the craft. After landing, the "spy in the cab" is checked to make sure the balloon has not landed at any point.