What happens when a human is exposed to a vacuum?

According to popular science fiction accounts, on a space station orbiting Saturn, a man inside a punctured spacesuit swells to monstrous proportions and explodes. On Mars, the eyes of a man exposed to the near-vacuum of the martian atmosphere, pop out of his head and dangle by their optic nerves on the sides of his face. En route to Jupiter on the Discovery spacecraft, Astronaut Dave Bowman space walks for 15 seconds with no helmet, and in no apparent pain, succeeds in reentering the Discovery through an open hatch. Fortunately, only in science fiction stories do humans ever come into direct contact with the vacuum of space, but these contacts are often portrayed as having horrific consequences.

To experience the vacuum is to die, but not quite in the gristly manner portrayed in the popular movies Total Recall and Outland. The truth of the matter seems to be closer to what Stanley Kubrik had in mind in 2001: A Space Odyssey.

According to the McGraw/Hill Encyclopedia of Space, when animals are subjected to explosive decompression to a vacuum-like state, they do not suddenly balloon-up or have their eyes pop out of their heads. It is, in fact, virtually impossible to compress or expand organic tissues in this way.

Instead, death arises from the response of the free gasses trapped within the tissues. When the ambient pressure falls below 47 millimeters of mercury, about 1/20 the atmospheric pressure at sea level, the water inside all tissues passes into a vapor state beginning at the skin surface. This causes the collapse of surface cells and the loss of huge amounts of body heat via evaporation. After 15 seconds, mental confusion sets-in, and after 20 seconds you become unconscious. You can survive this for about 80 seconds if a pressure higher than about 47 millimeters of mercury is then reestablished.

There have been instances of accidental exposure to a hard vacuum during space suit tests in vacuum chambers, and by pilots flying military aircraft at 100,000 feet. The experience was not fatal, or even exceptionally uncomfortable, for the typically 10 to 15 seconds or so that it was experienced.

The decompression incident on Kittinger's balloon jump is discussed further in Shayler's Disasters and Accidents in Manned Spaceflight: [When Kittinger reached his peak altitude] "his right hand was twice the normal size... He tried to release some of his equipment prior to landing, but was not able to as his right hand was still in great pain. He hit the ground 13 min. 45 sec. after leaving Excelsior. Three hours after landing his swollen hand and his circulation were back to normal."

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