Space exposure

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

Explanation and history of space exposure

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.

Ebullism, hypoxia, hypocapnia and decompression sickness

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) and multiplicative increase of body volume occur within 10 seconds, 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. The absence of oxygen outside the body causing rapid de-oxygenation of the blood (hypoxia) is the primary reason for unconsciousness within 14 seconds. 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 actually 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.

Few humans have experienced these four conditions. Joseph Kittinger experienced localised ebullism during a 31 kilometres (19 mi) ascent in a helium-driven gondola. His right-hand glove failed to pressurise and his hand expanded to roughly twice its normal volume accompanied by disabling pain. His hand took about 3 hours to recover after his return to the ground. Two other people were decompressed accidentally during space mission training programs on the ground, but both incidents were less than 5 minutes in duration, and both victims survived. International Space Station and Space Shuttle astronauts regularly work in Extravehicular Mobility Units (EMUs or space suits) that are at pressures less than 30% of the spacecraft to facilitate mobility, without experiencing noticeable decompression sickness. Nevertheless, the decompression accident of Soyuz 11 killed all three cosmonauts on board.

Decompression is a serious concern during the extra-vehicular activities (EVAs) of astronauts. Current EMU designs take this and other issues into consideration, and have evolved over time. A key challenge has been the competing interests of increasing astronaut mobility (which is reduced by high-pressure EMUs, analogous to the difficulty of deforming an inflated balloon relative to a deflated one) and minimising decompression risk. Investigators have considered pressurising a separate head unit to the regular 71 kPa (10.3 psi) cabin pressure as opposed to the current whole-EMU pressure of 29.6 kPa (4.3 psi)). In such a design, pressurisation of the torso could be achieved mechanically, avoiding mobility reduction associated with pneumatic pressurisation.

Extreme temperature variations

Extreme temperature variations are a problem in space, because heat exchange occurs primarily via infrared radiation. While the absence of convection and conduction causes an insulating effect preventing rapid dissipation of body heat, localized heating can occur if exposed to starlight at distances comparable to the Earth-Sun distance.

Cellular mutation and destruction from high energy photons and (sub-atomic) particles

A more severe long-term effect would be the direct exposure to high energy photons (ultraviolet, X-ray, and gamma) and energized subatomic particles (primarily protons). These can permanently denature DNA and other cellular molecules through atomic and nuclear interactions. Prolonged exposure and the ability of X and gamma photons to penetrate the entire body may cause death from organ failure, while even short-term exposure may cause cancer.

In science fiction

Spacing is a staple of science fiction, where it usually occurs as a method of execution (or other sort of killing) by vacuum exposure in space - usually accomplished by ejecting the subject through the airlock of a spacecraft or space station without a space suit. The primary cause of death would be asphyxia. Many films show people exposed to vacuum in unrealistic ways.

• Ellen Ripley of the Aliens series is a particularly adept spacer, having defeated xenomorph adversaries with at least 3 different forms of spacing.
• Movies such as Outland show people exploding in a vacuum.
• 2001: A Space Odyssey, A Trip to the Moon, From the Earth to the Moon, and several episodes of Doctor Who show people surviving a vacuum with no suit on.
• Battlestar Galactica and the film Sunshine, show some people suffering after a short exposure, and others dying plus freezing from longer exposure.
• In one episode of Farscape, John Crichton leaps through space from one ship to another without a suit. His exposure only lasts several seconds and he survives without any permanent damage, but is shown to be in considerable pain afterwards.
• In Total Recall, the character of Douglas Quaid can survive near-vacuum conditions just by holding his breath.
• Several recent films, such as Mission to Mars, show people freezing instantly in a vacuum.
• In the episode Tangent, on the TV show Stargate SG1, characters are perfectly fine even after being exposed to space for about five seconds.
• "Spacing" is a common topic in Babylon 5, used as a joke and referenced as a punishment for Mutiny and Treason (the only law left in earth that still permits execution). In the season 2 episode And Now For a Word Dr. Franklin describes an experience of witnessing someone die as a result of exposure to space.
• Event Horizon shows it in a very graphic example.

See also

• Human adaptation to space
• Proton exposure