AROUND THE WORLD IN A BALLOON.

IT'S AVIATION'S LAST FEAT--CIRCLING THE GLOBE IN A BALLOON. DO THE DANGERS OUTWEIGH THE PRIZE?

On a windless morning just after Christmas, the largest manned balloon ever launched will lift off from Alice Springs, Australia, and vanish into the dawn sky. The morning has to be windless--the riskiest part of any ballooning venture is takeoff.

Within two hours, the `Re/Max' balloon (named for its sponsor, a real estate company) and its three-man crew plans to climb 39 kilometers (24 miles) above Earth--shattering the manned-balloon altitude record of 34.4 km (21.4 mi). "That's as high as you can go short of outer space," says team spokesman Kevin Roark. (See diagram.) But the Re/Max crew cares less about the altitude than distance. Their historic mission: to be the first ever to circle the globe in a balloon. That's a giant first. Not one of the other 21 attempts since 1783 has even come close!

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The 32,200-km (20,000-mi) around-the-world balloon flight is aviation's last challenge. The enormous Re/Max balloon will soar to the stratosphere, the atmospheric layer that stretches from 10 km (6.2 mi) to 48 km (30 mi) above the Earth's surface. Cruising at that altitude, the Re/Max team will view the dark of deep space above them and Earth's curvature below. Barring disaster, the balloon crew should land back in Australia 18 or so days later--having floated around the globe.

Why fly so high? To avoid the turbulence that has doomed all past efforts. Chicago businessman Steve Fossett set the balloon-distance record last August by flying 24,457 km (15,200 mi)--from Argentina to the Coral Sea off Queensland, Australia--before a fierce thunderstorm flung his shredded balloon into the ocean. The longest balloon trip in history still fell 7,723 km (4,800 mi) short of a full lap around the Earth. Now, as an added incentive, the Anheuser-Busch company will award $1 million to the first team to clinch the perilous exploit.

DANGERS IN SPACE

The earliest form of air travel--the balloon--is still the most hazardous. Balloons are delicate by nature. To make matters worse, they're subject to atmospheric dangers like ice, lightning, and wind. The Re/Max crew will strive to bypass such dangers by literally floating above the weather, which subsides at about 5,240 meters (50,000 feet). But the stratosphere poses many of the same risks as space travel. "Any sudden structural failure," says Roark, "and our three crew members will likely perish."

Clearly, long-distance balloonists must carefully study the atmosphere--their lives depend on it. Balloons lack engines or steering wheels. Pilots navigate without them by climbing (with helium, the gas inside balloons and descending--the balloon literally vents out helium, spilling it into the air. A pilot's constant aim: to catch hold of a favorable wind to propel the balloon in the desired direction.

Steve Fossett will launch another low-altitude flight from Morocco this winter. He may find the skies overly crowded as he shoots to circle the globe in a fifth try. A half-dozen contenders from the U.S., Switzerland, and South Africa will join him in launching from the northern hemisphere--and floating in the troposphere, the atmospheric layer nearest Earth. The contenders do so in order to drift eastward on the jet stream, a wind current at about 9,114 m (30,000 ft), created by the Earth's rotation.

Blowing at speeds up to 322 km/h (200 mph), the jet stream is the fast lane around the globe. But it can be fickle. During its aborted journey, a Swiss challenger called the Breitling Orbiter crept along at just 8 km/h (5 mph) last year while consuming precious fuel.

While Fossett and other low flyers drift on the jet stream, the Re/Max team will hitch a ride high above them on a stratospheric wind that blows westerly for five or so weeks each year. Moving at just 113 km/in (70 mph), wind up there is slower than the jet stream. It's also smoother and more predictable--too high to be ruffled by weather changes like violent thunderstorms, which form only as high as 20 km (13 mi). "We're getting rid of all those dangers in going to a higher altitude," says pilot Dave Liniger.

SUPER BALLOON

The first challenge, of course, is to get a manned balloon off the ground. To make the balloon as light as possible, Re/Max made its balloon from polyethylene, a plastic film thinner than a sandwich bag that NASA developed for unmanned balloons. The pilots' lives hinge on a material just 20 microns (0.0008 in) thick--about half the thickness of a human hair! The Re/Max balloon relies on the element helium for its lift. Helium rises because it's lighter than air (next to hydrogen, helium is the lightest gas of all). Team scientists calculated how much helium it will take to keep 4,536 kilograms (10,000 lbs) of gear aloft for more than two weeks. Just before launching, the limp balloon will be inflated with 5,100 cubic meters (170,000 cubic feet) of helium.

The helium will expand continuously as the balloon floats upward. The reason: the higher one climbs, the thinner the air. That's because high-altitude air is under less pressure from the atmosphere pressing down from above. So there's more room for helium to expand as the balloon ascends.

By the time the balloon levels off in the stratosphere, the helium will have expanded to 11.8 million cu. m (39 million cu. ft)--229 times its original volume. The fully inflated balloon will reach awesome proportions: At 140 m (460 ft) wide and nearly 213 m (700 ft) tall, it will be taller than the Empire State Building and big enough to contain the entire Houston Astrodome!

During daylight hours, the sun warms the balloon to about 13 [degrees] F (- 25 [degrees] C), making the helium gas inside less dense and therefore more buoyant, or inclined to rise. But as the sun sets, the gas cools, reversing the balloon's lift. By the time the balloon sinks to about 24,000 m (80,000 ft), the sun rises to warm the balloon all over again.

After takeoff, the most dangerous part of a ballooning feat is landing on solid ground. The Re/Max crew will begin a cautious descent as much as two days in advance by releasing helium from the balloon, thereby reducing buoyancy. If the balloon drops too fast, the crew can throw out some or all of the 585 kg (1,300 lbs) of tiny glass beads carried as ballast, a weight that counteracts buoyancy. In an emergency, they can dump empty oxygen tanks. Worst case scenario: a crew member can parachute off wearing a pressurized Russian space suit.

What will the Re/Max team do to celebrate if they successfully circle the globe? Maybe fly in a more conventional aircraft to every country they flew over--a global victory lap. "It's going to be a wonderful adventure," Liniger says. No kidding.

RELATED ARTICLE: DEADLY AIR: The Re/Max Solution

The stratosphere is a deadly place for humans. High-altitude air is so thin that its molecules--mainly nitrogen and oxygen--spread out. Result: each human breath inhales less life-giving oxygen. For this reason, most balloonists wear oxygen masks above 5,600 m (18,000 ft.). Luckily, the Re/Max crew doesn't need masks. They'll travel in a pressurized cabin. Artificial air pumped inside from external canisters will restore normal oxygen levels.

The cabin will also simulate the atmospheric pressure of sea level. You're not aware of it as you walk around every day, but the weight of the atmosphere squeezes your body with a force of 6.6 kg (14.7 lbs) per square inch. If a window blows out, the Re/Max cabin would lose its pressure and the crew would die instantly. "At that altitude your body literally explodes," says meteorologist Bob Rice.