Space exposure

Space exposure is the subjection of a human to the conditions of outer space, without protective clothing and beyond the Earth’s atmosphere in a vacuum.

The key concerns for a human without protective clothing beyond Earth’s atmosphere are the following, listed roughly in the descending order of mortal significance: ebullism, hypoxia, hypocapnia, decompression sickness, extreme temperature variations and cellular mutation and destruction from high energy photons and (sub-atomic) particles.

For the effect of rapid decompression to vacuum conditions, see the main article at Uncontrolled decompression.

Ebullism, the formation of bubbles in body fluids due to reduced ambient pressure, is the most severe component of the experience. Technically, ebullism is considered to begin at an elevation of around 19 kilometres (12 mi) or pressures less than 6.3 kPa (47 mm Hg), known as the Armstrong Limit. Experiments with other animals have revealed an array of symptoms that could also apply to humans. The least severe of these is the freezing of bodily secretions due to evaporative cooling. But severe symptoms such as loss of oxygen in tissue (anoxia), followed by circulatory failure and flaccid paralysis in about 30 seconds. The lungs also collapse (atelectasis) in this process, but will continue to release water vapour leading to cooling and ice formation in the respiratory tract.

A rough estimate is that a human will have about 90 seconds to be recompressed, after which death may be unavoidable. Unconsciousness is likely to occur within 14 seconds, primarily due to the much lower pressure outside the body causing rapid de-oxygenation of the blood (hypoxia). In 1966 NASA volunteer test subject Jim LeBlanc lost consciousness after approximately 15 seconds of being accidentally depressurised in a ground-based depressurization chamber. If a person is exposed to low pressures more slowly, hypoxia causes gradual loss of cognitive functions starting at about 3 kilometres (10,000 ft) altitude equivalent. Less severe effects include the formation of nitrogen gas bubbles and consequent interference with organ function (decompression sickness), which is less severe in space than in diving. Meanwhile, reduction of blood carbon dioxide levels (hypocapnia) can alter the blood pH and indirectly contribute to nervous system malfunctions. If the person tries to hold their breath during decompression, the lungs may rupture internally.

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