NEXT YEAR, Rodd Millner, an Australian ex-commando, plans to one-up the American hero Chuck Yeager, the first person to travel faster than the speed of sound. In a stunt that will either redefine the boundaries of science or the boundaries of idiocy, Millner intends to don a space suit and ride a balloon 130,000 feet up to the edge of space. Once he gets there, he will jump. An experienced skydiver, speedboat racer, scuba diver, and, before that, an insurance salesman, Millner believes that he will reach a speed of between 700 and 900 miles per hour within one minute of leaping from the balloon. If he is successful, he will be the first human to break the sound barrier sans vehicle.

Why? "Because it can be done," said Millner in an interview on Tuesday. The idea occurred to him two years ago. "I was reading a Richard Branson book, and he stated that the last aeronautical challenge was to go around the world in one go. But I always thought balloons were meant to go up, and then the questions just started coming, like how high can a balloon go? And then, being a skydiver, I thought, 'I wonder if I could skydive down!'" The thirty-six-year-old, who sounds both good-humored and sane, will soon embark on a nine-month training program in which he'll undergo hundreds of practice skydives. Starting with "clean skin" jumps, Millner and the "Space Jump" team will gradually increase the equipment that he carries. Ideally, any problems will be identified before the final ascent -- which should take around two and a half hours -- and descent -- which should take less than ten minutes.

There is much that can go wrong, both on the way up and on the way down. Millner, who will depart from a purpose-built Australian Defense Force (ADF) launch facility in central Australia, will ride up in a small gondola attached to a helium balloon made of expandable plastic. Though lighter than air, the balloon is a behemoth (imagine ten helium-filled Washington Monuments side by side). With a volume of twelve million cubic feet, it will be visible to the naked eye from ground level even when it is at maximum altitude.

Maximum altitude will be higher than four Everests stacked one on top of the other, and Millner will pass through various regions of the atmosphere to reach it. The most difficult region may be the tropopause (between 40,000 and 52,000 feet). With a temperature of approximately -80 C, it doesn't get colder anywhere on earth. At 130,000 feet, what little atmosphere is left will be slightly warmer at -30 C. Why stop at 130,000? The balloon can't float much higher. The dissipating atmosphere means there is nothing for it to float through. Left alone, the balloon would bob at that altitude indefinitely. However, once Millner has exited, it will be deflated by remote control, allowing the gondola to return to earth by parachute.

The Space Jump team refuse to discuss funding sources for their multi-million dollar project, but they are keen to promote the giant format film that will result from it. Millner will be wearing a specially designed suit equipped with a number of cameras, and in addition to the falling-man-cam, his big dive will be filmed by remote controlled cameras in the balloon gondola, a high-altitude Lear Jet at 40,000 feet, low-altitude craft at 10,000 feet, a team of skydivers (themselves falling from 40,000 feet), and by cameras on the ground.

Of course, cameras are the least important part of the suit. In order to protect its occupant from extreme cold and pressure variation, the outfit will be three-layered and pressurized. At this stage, Space Jump will not release any more details about it, but common sense dictates that Millner will require an oxygen supply. One thing is certain, if the suit fails him, he will die.

U. S. Air Force Captain Joe W. Kittinger wore a pressurized suit when he successfully jumped 102,000 feet from a balloon in 1960. Kittinger did not break the sound barrier, but there is a good chance that Millner will. Because the speed of sound varies with altitude and temperature, it will initially decrease as Millner accelerates towards the earth, reaching an approximate minimum of 660 miles per hour at around 80,000 feet (it's about 760 miles per hour at ground level). Will he outrace sound? "Yes," says Millner. "It should be a really smooth transition. Because the atmosphere is so thin up there, there's no resistance to slow me down so I'll fall at those speeds. As I get into the thicker atmosphere it will gradually slow me down to normal speed." Other effects, like the physiological outcome of accelerating from zero to possibly 900 miles an hour in less than a minute and exceeding the sound threshold (and then going back through it), are yet to be determined. "We're taking the human body where it's never been before," said Millner, "and ultimately, we don't know."

It's extremely unlikely that when Millner breaks the sound barrier he will generate a sonic boom -- the explosive and audible shock wave that occurs when an object begins to move faster than the sound it propagates. Because a sonic boom is a function of the size and shape of an object, Millner's small size (relative to a plane) means his fall may be marked not by a boom but a pop. The intrepid Aussie expects to have full radio communication with the team all the way down. He will steer with his body, like a normal skydiver, and he hopes to open his parachute -- a normal skydiving rig (with one backup) -- at about 5,000 feet. Millner expects the thickening atmosphere to slow him down to a little over 120 miles per hour by the time he pulls the cord.

If he succeeds, Millner will set a number of records. In addition to being the first human being to break the sound barrier wearing little more than a jumpsuit, he'll be the highest balloonist (the previous record for a manned balloon flight is 121,390 feet) and the furthest free faller. The Space Jump team claims that Millner's dive will advance knowledge and perhaps help develop escape strategies for astronauts. How? "That's a question for the scientists," says Millner, whose main issue is: "Can I do this?" As for where he'll land, the official guess is within 50 kilometers of the launch site. "But I'll be basically aiming for the planet Earth," Millner said, "and trying to hit it." If he reaches 130,000 feet, he will certainly hit it. The only question is: how hard?