Felix Baumgartner will attempt to become the first human to break the sound barrier unaided by a vehicle.
The Austrian skydiver is going to jump out of a balloon at more than 120,000ft (36.5 km) above Roswell, New Mexico.
In the near vacuum at that altitude, Felix Baumgartner should accelerate beyond about 690 mph (1,110 km/h) within 40 seconds.
If all goes well, Felix Baumgartner, 43, will open a parachute near the ground to land softly in the desert, 10 minutes later.
The adventurer – famous for jumping off skyscrapers – is under no illusions about the dangers he faces.
Where Felix Baumgartner is going, the air pressure is less than 2% of what it is at sea level, and it is impossible to breathe without an oxygen supply.
Others who have tried to break the existing records for the highest, fastest and longest freefalls have lost their lives in the process.
“If something goes wrong, the only thing that might help you is God,” says Felix Baumgartner.
“Because if you run out of luck, if you run out of skills, there is nothing left and you have to really hope he is not going to let you down.”
Weather permitting, lift-off from Roswell airport should occur about 07:00 local time (13:00 GMT).
The absolute mark for the highest skydive is held by retired US Air Force Col. Joe Kittinger.
He leapt from a balloon at an altitude of 102,800 ft (31.3km) in August 1960.
Now an octogenarian, Joe Kittinger is part of Felix Baumgartner’s team and will be the only voice talking to him over the radio during the two-and-a-half hour ascent and the 10-minute descent.
Engineers have done everything possible to limit the risks. They have built the Austrian a special pressurized capsule to carry him under the helium balloon.
Felix Baumgartner will also be wearing a next-generation, full-pressure suit, an evolution of the orange protective clothing worn by shuttle astronauts on launch.
Although the jump has the appearance of another Felix Baumgartner stunt, his team prefers to stress its high scientific relevance.
The researchers on the Red Bull Stratos project believe it will inform the development of new systems for emergency evacuation from high-performance, high-altitude vehicles. NASA and its spacecraft manufacturers have asked to be kept informed.
There are a few examples of pilots being ejected in supersonic airflows when their planes broke apart in the sky, but there is no detailed data on what happens to the human body as it goes supersonic and then, as it slows, goes subsonic again.
Felix Baumgartner will be instrumented to acquire this new data.
The concern is that he might be destabilized by shockwaves passing over his body, and that these might throw him into an uncontrolled spin.
“It’s very important he gets into a delta position,” said Felix Baumgartner’s trainer, Luke Aikins.
“This is hands at his side and his head low, ripping through the sky. This will be crucial to breaking the speed of sound and remaining stable.”
Engineers have incorporated an automatic device in his gear that would deploy a drogue stabilization chute if he gets into trouble. But the team’s medical director, former shuttle flight surgeon Dr. Jon Clark, hopes the stiffness of the pressure suit itself will suffice.
“We know that pressure suits limit mobility which we often consider as a bad thing, but in this scenario of going through the sound barrier, it actually adds a protection because it acts like an exoskeleton,” he explained.
“We don’t know what the human will endure accelerating through the sound barrier in coming back down without the aid of aircraft. And that is really the essence of the scientific goal of this mission.”
There is high confidence Felix Baumgartner will succeed in his quest. He has already completed practice jumps from 71,600 ft (21.8 km) and 97,100ft (29.6 km).
The second of these jumps he described as an extraordinary experience.
“It’s almost overwhelming,” he said.
“When you’re standing there in a pressure suit, the only thing that you hear is yourself breathing, and you can see the curvature of the Earth; you can see the sky’s totally black. It’s kind of an awkward view because you’ve never seen a black sky. And at that moment, you realize you’ve accomplished something really big.”
A suite of high-definition cameras will follow the action. Some of these will be attached to Felix Baumgartner himself.
But wary of broadcasting a tragedy to worldwide TV audiences, the organizers will be putting a 20-second delay on the live video feed.
Four GPS systems in the suit will gather the dive data required to satisfy the Federation Aeronautique Internationale (FAI) that records have indeed been broken.
“The data is recorded on an SD microcard in his chest pack,” said Brian Utley, who will file the official report to the FAI after the jump.
“I insert that card into the equipment. From that moment on, I have control over the equipment. I’m with it until Felix goes into the capsule, and when he lands I am the first person to approach him so I can take possession of that card again.”
A BBC/National Geographic documentary is being made about the project and will probably be aired in November.