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.”
Weather permitting, lift-off from Roswell airport should occur about 0700 local time (1300 GMT; 1400BST).