It's only by glancing out of the window when sitting in the comfort of an airline seat that you would know you are thousands of feet above the surface of the Earth.
Airlines these days cruise at around 35,000 feet, some higher and some lower, but without protection at these altitudes a human being cannot survive. It's a sobering thought that loss of cabin pressure at a typical cruising altitude of 35,000 feet would mean the pilots have just 30 to 60 seconds of useful consciousness to put on breathing apparatus or descend to below 10,000 feet to be able to operate the aircraft safely.
After that, judgement and ability is significantly and dangerously impaired. On October 8, Felix Baumgartner will skydive from roughly 3.5 times this height at 120,000 feet where the conditions are hostile and the human body isn't designed to work. If unprotected, he will have less than 6 seconds before falling unconscious. Yet thanks to modern technology, the risks to the frail human body can be minimized.
Before Baumgartner can make this historic jump from the edge of space he will ascend in a specially designed capsule suspended underneath a balloon 182 meters in diameter and filled with helium gas. Surprisingly, the most risky part of the flight is below 1,000 feet where, if the fabric of the balloon rips there isn't enough altitude and therefore time for Felix to escape, deploy parachute and land safely. Once he's over 4,000 feet he will be able to escape safely but then the risks become very different.
The capsule is pressurized to maintain the equivalent atmospheric pressure found at 16,000 feet to reduce the chances of decompression sickness during the ascent. Once at the jump altitude of 120,000 feet Baumgartner will open up the door of the capsule and immediately be reliant on his pressurized suit.
These suits, which are effectively a streamlined space suit must be worn for any jump above the so-called Armstrong Line of 62,000 feet, although pilots generally wear them for operations above 50,000 feet for safety. Above the Armstrong Line the atmospheric pressure is so low that water boils at body temperature and, given that we are made up of 70 percent water, this is bad news. It's said that even the blood will "boil," but more accurately it's the production of the gas during this process that does the real damage.
It's not just the low atmospheric pressure he needs to worry about though since not only does he aim to smash the existing record of the highest sky dive currently held by Kittinger who jumped from 102,800 feet, he also aims to become the fastest in free fall plummeting to Earth at 1,100 kilometers per hour (685 miles per hour). In doing this, he hopes to become the first human being to break the sound barrier without the aid of an aircraft.
The actual value for the speed of sound varies according to air temperature and is lower when the air is cooler. Temperature in the atmosphere generally decreases with height (although due to the ozone layer it does increase again in regions of the stratosphere) so the speed of sound is lower the higher up you go.
This idea that Baumgartner is going to "break the sound barrier" is a little misleading. While he hopes to travel faster than sound, there isn't actually a barrier associated with it. The idea comes from historical experiences where aircraft suffered extreme turbulence, even destruction when approaching the speed of sound.
Instead of being caused by some ethereal barrier limiting supersonic travel it was actually the build up of shockwaves as the speed of the craft approaches the speed of sound.
Probably the greatest danger facing Felix during his record breaking jump at supersonic speeds is getting into a flat spin. As its name suggests it's a flight situation where the body rotates horizontally about an axis generally located around the waist. Rotational speeds can reach up to 120 revolutions per minute and at the extremities of the body up to 22 times the force of gravity can be experienced.
When this happens, blood is forced to pool in the head and feet leading to unconsciousness. It happened to Joseph Kittinger when he jumped from 76,400 feet in 1959 and it was only the automatic deployment of his parachute which saved his life.
Baumgartner has a similar device which will deploy if 3.5g is detected for more than 6 seconds, the point at which he is likely to fall unconscious. This situation can be recovered but if a full spin develops the pressure of blood in the head can cause the brain and eyes to be squashed.
When Baumgartner makes his jump on October 8 he will be well prepared against lack of oxygen, extremely low temperature, low atmospheric pressures and the risk of flat spins. But there are countless other risks that are as yet undiscovered and the human body does have its limits.
It takes someone like Baumgartner to leap into the unknown in such a hazardous environment to not only break records but to further science, as he says on his website "Everyone has limits, not everyone accepts them."