Felix Baumgartner's world skydive record attempt under way
Austrian skydiver Felix Baumgartner is diving to Earth on his mission to break a series of freefall records.
His giant helium balloon left the ground at Roswell, New Mexico, about three hours ago and climbed through the sky to more than 128,000ft (39km).
Baumgartner then jumped out.
The near absence of air at this high altitude means he should break the speed of sound as he falls - a velocity in excess of 690mph (1,110km/h).
The journey down should take 10 minutes, about half of it in freefall.
No-one has ever gone so high in a balloon, nor attempted to make such a high skydive.
The current record for the biggest jump of all time is now 52 years old. It was set by US Air Force Col Joe Kittinger when he leapt from a helium envelope at an altitude of 102,800ft (31.3km).
There are immense risks involved in what the Austrian is trying to do.
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.
Engineers have done everything possible to limit the risks. They have built the Austrian a special pressurised capsule to carry him aloft 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.
Engineers have incorporated an automatic device in his gear that would deploy a drogue stabilisation chute if he gets into trouble.
There is, however, high confidence in Baumgartner's team that he will complete the task ahead of him. He has been buoyed by the success of two practice jumps that have taken him progressively higher into the stratosphere - from 71,600ft (21.8km) and 97,100ft (29.6km).
The official lift-off time for the balloon was 0931MDT (1631GMT; 1731BST). Mission control at Roswell airport is following every moment of what was a more than two-hour ascent to the jump altitude.
Baumgartner is in video and radio contact throughout. The only person who will speak to him, however, is Col Kittinger, who was brought into the team early to advise the Austrian how best to beat the octogenarian's records.
"We are going to get your goal and your dream accomplished Felix," Kittinger told Baumgartner just before lift-off.