Later today, Austrian daredevil Felix Baumgartner is hoping to become the first human being to break the sound barrier without any mechanical help.
The former special forces soldier is planning to make the world's highest skydive from 23 miles above the Earth's surface. On any normal dive, jumpers rarely descend above a velocity of around 120mph thanks to air resistance. At 23 miles up, where the air is much thinner, Mr Baumgartner expects to break the 768mph sound barrier and will know whether he's successful when his body emits a sonic boom.
Failure could be fatal. Should his body go into a spin greater than 180 rotations per minute, his brain is likely to be liquidised by super-pressurised blood rushing to his head. His suit should emit a corrective parachute if that happens, but no one is entirely sure it will work.
The jump, which will take place above Roswell, New Mexico, is the product of more than five years of planning and has consumed Mr Baumgartner. He has made two practice runs from the Roswell area, from 15 miles high in March and 18 miles in July. But this is the first time he has tried to break the current world record of 19.44 miles, set by the American pilot Joseph Kittinger in 1960.
While the 43-year-old's jump may seem like madness, Nasa is watching his progress closely. Since the disintegration of the Space Shuttle Columbia in 2003 with the loss of all those on board, Nasa has been looking for ways to create a system where astronauts could survive a high-altitude crash. The jump can be made only if wind speeds are less than two miles per hour.
Watch a live video feed of the jump from the edge of space, starting at 2pm at www.independent.co.uk