An Austrian hopes to break the world freefall skydiving record by jumping from 23 miles above earth, flying faster than the speed of sound - risking a brain clot or his blood boiling in the process.
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Felix Baumgartner, 43, will jump from a pressurised capsule powered by vast helium balloons on Tuesday, after an attempt to jump on Monday was cancelled due to bad winds.
When he jumps, 120,000 feet above the earth, he will be flying faster than the speed of sound.
His body will go from zero to 690 mph in 34 seconds, and he will be supersonic for almost a minute – free-falling for five minutes and 35 seconds.
However, the jump is fraught with even more danger due to the high altitude.
Baumgartner, also known as "Fearless Felix", could go into a spin, haemorrhage his eyes and get a blood clot on his brain – or his blood might "boil" if his suit rips.
The descent will start with the first step out of the capsule on a ledge the size of a skateboard.
Baumgartner will salute his team, count to three, then push off the step in a "bunny hop."
This will stabilise him as his body goes through a dozen barrel rolls while he falls back down to earth, and keep him from going into a flat spin when he crashes through the sound barrier.
He may become unconscious, at which point his emergency parachute will automatically open.
The previous record for the highest freefall was set by US military pilot Joe Kittinger on 16 August 1960.
He leapt from a helium-filled balloon from 19 miles up and reached 600mph on the way down.