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Felix Baumgartner is going from base jumping to space jumping. He is preparing to jump from a balloon 120,000 feet (36.5 km) above the surface of the Earth, at the edge of space. Baumgartner is famous for a record base-jump from the Petronas Towers, in Kaula Lumpur. Base jumping involves jumping off a fixed object, liked a building, and opening a parachute on the way down – a risky business but not as risky as jumping from space.

Jumping from a high altitude balloon is not new. The ESERO collection on the eLibrary shows footage of Joe Kittinger’s jump, in 1960. Kittinger had a leak in his right glove, which caused him to temporarily lose the use of that hand. The edge of space is a hostile place for humans: the low atmospheric pressure and low temperature being the primary concerns. At the height that Baumgartner is planning to jump, there is virtually no atmosphere: which means that the pressure is so low that he must wear a sealed pressurised suit.

You can’t get a decent cup of tea on Everest. Not only is there a serious lack of tea shops, but the low atmospheric pressure means that water boils at 69 degrees Celsius, instead of the usual 100. As you go further up, and the pressure drops, the boiling point of liquids will drop. This means that in space, the blood in a human body will boil, if the body is not protected by a pressurised suit.

As Baumgartner falls, he will rapidly accelerate towards the Earth. The pressure (or density) of the atmosphere is so low that he will actually break the sound barrier before he starts to slow down. This has never been done before. Let’s look at some of the maths behind it:

The drag force on a body moving through a fluid is given by:

The force due to gravity F = mg, where m is the mass of the body and g is the gravitational acceleration. This is just your weight.

When these two forces are equal, the body stops accelerating – it has reached a constant speed, often called the terminal velocity. The atmosphere is a fluid (gases are fluids as well as liquids). In the stratosphere the density is very low and therefore the drag force is comparatively very low. So a falling body can reach a very high speed before the drag force equals the weight of the body and stops accelerating. The density is so low at 120000 feet that it is virtually zero.

Once Baumgartner enters the higher density atmosphere he will slow down, eventually to a speed at which it will be safe to deploy his parachute. Going back to our equation for drag, with a parachute the area has become much larger, so the terminal velocity is much lower. If the chute did not open, then Baumgartner would reach a terminal velocity of around 120 miles per hour (about 200 km/hr) – which shows why the parachute is so important.

An Institute of Physics resource on the eLibrary looks at forces and terminal velocity - useful for secondary and post-16 students. You can learn more about the mission, the technology and the science at the Red Bull Stratos homepage.