Skydiver Felix Baumgartner set to break sound barrier
The Austrian skydiver Felix Baumgartner will attempt to become the first human to break the sound barrier unaided by a vehicle.
He is going to jump out of a balloon at more than 120,000ft (36.5km) above Roswell, New Mexico.
In the near vacuum at that altitude, he should accelerate beyond about 690mph (1,110km/h) within 40 seconds.
If all goes well, he will open a parachute near the ground to land softly in the desert, 10 minutes later.
The 43-year-old adventurer - famous for jumping off skyscrapers - is under no illusions about the dangers he faces.
Where he 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 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."
Difficult wind conditions at Roswell airport mean that lift-off for the balloon will occur no earlier than 1130 local time (1730 GMT; 1830BST).
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 102,800ft (31.3km) in August 1960.
Now an octogenarian, Kittinger is part of 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.
Heated sun visor
Oxygen supply hose
Main parachute handle
HD camera on each leg
Suit made of layered material
Mirror to check parachute
High altitude balloon: expands with altitude
Balloon made of plastic film 0.002cm thick
Frame attaches capsule to balloon
Sliding door to exit capsule
Foam insulated shell
Engineers have done everything possible to limit the risks. They have built the Austrian a special pressurised capsule to carry him under the helium balloon.
He 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 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.
Baumgartner will be instrumented to acquire this new data.
The concern is that he might be destabilised 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 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 stabilisation 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 Baumgartner will succeed in his quest. He has already completed practice jumps from 71,600ft (21.8km) and 97,100ft (29.6km).
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 realise you've accomplished something really big."
A suite of high-definition cameras will follow the action. Some of these will be attached to Baumgartner himself.
But wary of broadcasting a tragedy to worldwide TV audiences, the organisers 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," Brian Utley, who will file the official report to the FAI after the jump, told BBC News.
"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.
Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter: @BBCAmos