Article on BBC News Website
The Austrian extreme sportsman Felix Baumgartner says his next goal is to try to break the long-standing record for the highest ever parachute jump.
It is 50 years since the American Joe Kittinger made history by leaping from a balloon at 102,800ft (31km).
Many have sought to repeat the feat down the decades but all have failed.
Baumgartner, who is famous for stunts such as jumping off the Petronas Towers, aims to skydive from a balloon sent to at least 120,000 ft (37km).
It is likely that in his long freefall of more than five minutes, he will exceed the speed of sound - the first person to do so without the aid of a machine.
"No-one really knows what that will be like," he said.
"The fact is you have a lot of different airflows coming around your body; and some parts of your body are in supersonic flow and some parts are in transonic flow. What kind of reaction that creates, I can't tell you," he told BBC News.
Rio base jump
Felix Baumgartner's base-jumping has not always pleased the authorities
Baumgartner and his supporters claim the project will gather scientific data also about the stratosphere and how the body copes with the extreme conditions so high above the Earth's surface.
The most recent attempt to try to better Kittinger's mark was made in 2008 by the Frenchman Michel Fournier.
Joe Kittinger (USAF)
Joe Kittinger made his leap before the first American went into space
The former paratrooper and adventurer had spent years preparing for "Le Grand Saut", or Big Jump, only to see his balloon break free and float off into the sky just as he was about to climb inside the ascent capsule.
Baumgartner has frequently incurred the ire of the authorities because of his base-jumping - the highly dangerous practice of parachuting from buildings. He also made headlines in 2003 when he crossed the English Channel on a carbon wing strapped to his back.
His assault on Kittinger's record is likely to take place later this year over an as yet unnamed location in North America. He will ascend to the stratosphere in a pressurized capsule attached to a 450ft-high (140m) helium balloon, and then jump out at an altitude he hopes will exceed 120,000ft. .
He will be wearing a specially modified full-pressure suit and helmet.
Altitude graphic (BBC)
The organisers of the project called Red Bull Stratos say, if all goes well, he should break the speed of sound about 35 seconds into his descent.
Joe Kittinger's 16 August 1960 jump was an extraordinary achievement. It was made nine months before Alan Shepard was even launched on the first American sub-orbital space trip.
Kittinger experienced intense swelling in his right hand as his glove malfunctioned and his body reacted to the low pressure at high altitude.
"I was headed back down to a friendly Earth," he recalls. "It's extremely hostile up there and the further you fall, the friendlier it is," the retired USAF colonel told the BBC.
He is now supporting the Austrian in his endeavour.
As well as coping with freezing temperatures and ultra-thin air, a key objective for Baumgartner must be to try to maintain a good attitude during the descent and prevent his body from going into a spin and blacking out.
Felix Baumgartner (Red Bull Stratos)
Baumgartner acknowledges the risks of breaking the sound barrier
If he does go into a spin, it is unlikely, he says, he will be able to correct it.
In any case, his chute will be automatically deployed if he is unconscious.
Baumgartner has an eye on the benefits he believes can accrue to space exploration, making it possible to bring astronauts back to Earth alive if their vehicle malfunctions.
"We want to prove a human person - if they have to bail out of a capsule from 120,000ft - can come back safely to Earth," he explained.
Michel Fournier has promised to make another attempt in 2010 also, if he can secure the funding.
A BBC/National Geographic Channel documentary is being made about Baumgartner's project. The 90-minute film will be transmitted on BBC Two in the UK shortly after the jump.