A daredevil skydiver will today attempt a record-breaking jump from the edge of space.
Felix Baumgartner will leap from a balloon at an altitude of 120,000ft (36,576m), breaking the sound barrier.
He will be in freefall for five and half minutes above the New Mexico desert.
If he succeeds he will become the first person to reach supersonic speeds without the protection of an aircraft.
Weather forecasters are optimistic that calm conditions will allow the 55-storey-high balloon to launch at dawn, around 1.30pm BST.
"I am ready," said Baumgartner, who has been planning the jump for more than five years. "I feel like a tiger in a cage waiting to get out."
The delicate balloon is made of a plastic film just 1/10th the thickness of a sandwich bag.
It holds 850,000 cubic metres of helium gas and will dramatically expand as the balloon rises towards the stratosphere.
Baumgartner will be carried aloft in a small space capsule. The ascent is expected to take three hours.
He will wear a specially-designed suit that is pressurised and heavily insulated to protect him against the extreme conditions at high altitude.
He will breath pure oxygen throughout the flight. The moments immediately after he jumps will be critical.
"The problem is that for around 30 seconds I will have no air cushion whatsoever, meaning that I won't be able to control the way my body spins," he said.
"I have to get myself into a stable position before I reach the speed of sound."
Scientists will monitor the effects of his descent on his body. They hope to develop life-saving technology that could be used by high-altitude pilots, astronauts and space tourists.
"Proving that a human can break the speed of sound in the stratosphere and return to Earth would be a step towards creating near-space bailout procedures that don't exist," said Mr Baumgartner.
During a test jump earlier this year, from an altitude of 97,145ft (29,610m), Mr Baumgartner reached 537mph, the speed of a commercial jetliner.
"At 90,000ft in freefall, I could see the capsule, the sky was completely black, I could see the curvature of the earth. It was amazing," he said.
The current freefall record is held by Colonel Joe Kittinger, who jumped at an altitude of 102,800ft (31,333m) in 1960.