Up, up and around the world.

Thud! Landing his huge balloon on a frozen lake in Canada wasn't Steve Fossett's idea of glory. His goal was to be the first person to pilot a balloon around the globe nonstop - and do it solo. But just 48 hours into his voyage, and thousands of miles short of his goal, the electric generator on board his airship broke down. Without electricity, Fossett couldn't communicate with his team of weather forecasters, navigators, and other balloon specialists on the ground. He was forced to call off his adventure.

But at least Fossett was alive to tell his tale. Friendly winds had carried him and his airship - Solo Challenger - over land, instead of forcing him to crash into frigid Atlantic Ocean waters. Now, says Timothy "Bo" Kemper, the project manager for Solo Challenger, Fossett is determined to try again.

STIFF COMPETITION

Fossett will have to act fast, though. At least three other contenders are attempting to beat him to the punch: Unicef Flyer (piloted (by three balloonists from Holland); Global Challenger, (with three flyers from England); and Odyssey (a two-member team from New Mexico).

Few balloonists have risked a whirl around the globe. After all, says Kemper, the 32,200km (20,000mi) flight is "a challenge both mentally and physically.". You have to be willing to have for weeks at a time in a cramped capsule, with military-style meals for food, and in Fossett's case, a bucket for a toilet. Without an engine, steering wheel, or brakes, the crews are at the mercy of Earth's winds. Fossett and his competitors must fly their balloons up or down to different altitudes, seeking out favorable winds.

DESIGN CHALLENGE

To achieve their goal, the competitors are relying on balloons that are much more sophisticated than traditional hot-air balloons. In those balloons, burners heat air molecules, causing them to zip around, spread apart, and "puff up" the balloon. The hot air - less dense than the same volume of cooler, outside air - lifts the balloon.

By contrast, Solo Challenger, Unicef Flyer, and Global Challenger use a combination of hot air and helium gas. Molecules of helium weigh less - and are less dense - than air (see SW 11/18/94, p. 12). That's what enables helium-filled balloons to rise.

"But at night, the helium gets cold and contracts. You lose your lift and descend," says Kemper. That's where the hot air comes in. Each night, the balloonists plan to use propane gas tanks to heat air molecules in the balloon's outer shell, which in turn will warm the helium. As the warmed helium molecules spread out, the balloons will float higher into the sky.

But just how high should they go? To navigate an engineless balloon around the globe, you don't just need upward lift; you also need the forward push of Earth's winds. And wind speeds and directions vary at different altitudes.

FORWARD FLYING

Lou Billones, a meteorologist (weather scientist) who advises Fossett on wind direction and weather conditions, came up with a strategy for Solo Challenger: By flying at a relatively low altitude - from 5,600 to 7,500m (18,000 to 24,000ft) above sea level - Fossett would catch the 80kph (50mph) winds just below the jet stream, a band of strong atmospheric winds (see diagram, left ). Earth's rotation creates a force known as the Coriolis effect, which moves the jet stream - and any balloonist hitching a ride - from west to east around the globe.

By contrast, Unicef Flyer and Global Challenger (expected to take off as SW goes to press) plan to climb higher than Solo Challenger - to 9,000 to 12,500m (30,000 to 40,000ft). That would take them directly into the jet stream's 16lkph (100mph) winds.

Odyssey is setting even loftier goals. That team, expected to launch in June, plans to cruise some 39km (24mi) high in the stratosphere, virtually at the edge of space, where winds blow 89 to 97kph (55 to 60mph).

But the higher you climb, explains Billones, the "thinner" the air. That's because the air at high altitudes is under less pressure; there's less air pushing down from above. So the molecules that make up air - mainly nitrogen and oxygen - spread out. Result: With each breath, you take in less life-giving oxygen.

So Fossett wears an oxygen mask once his balloon climbs to about 5,600m (18,000ft). And because they're flying even higher, "the other teams will be traveling in pressurized capsules," says Billones. These capsules pump in extra air to keep internal pressure - and oxygen levels - similar to those at ground level.

Yet despite all the technical wizardry, the months of training, and the life-threatening risks that go into this 'round-the-world endeavor, there's no prize waiting at the finish line. So what's the big draw.?

"We have an opportunity to capture the last great aviation challenge," says Bob Martin, a pilot on the Odyssey. "It's a great adventure."