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What would happen if a human were exposed to space without wearing a spacesuit?

If a human were to be in space without a spacesuit (i.e., Extravehicular Mobility Unit: EMU) of some sort, what would happen and how long would the person remain conscious?

The key concerns for a human without protective clothing beyond Earth’s atmosphere are the following, listed in the order of mortal significance: 1. Ebullism 2. Hypoxia 3. Hypocapnia 4. Decompression sickness 5. Localized temperature differences 6. Cellular and organ damage from high energy photons and (sub-atomic) particles

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 ~19 km (12 miles) or pressures less than 6.3 kPa (47 mm Hg). 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 tissues (anoxia) and multiplicative increase of body volume occur within ~10 s, followed by circulatory failure and flaccid paralysis in ~30 s. The lungs also collapse (atelectasis) in this process, but will continue to release water vapor leading to cooling and ice formation in the respiratory tract.

A rough estimate is that a human will have about 90 s to be recompressed, after which death may be unavoidable. The absence of O2 outside the body causes rapid de-oxygenation of the blood (hypoxia) and is the primary reason for unconsciousness within ~ 14 s.

If a person is exposed to low pressures more slowly, hypoxia causes gradual loss of cognitive functions starting at about 3 km (2 miles) altitude equivalent. Less severe effects include the formation of N2 gas bubbles and consequent interference with organ function (decompression sickness), which is actually less severe in space than in diving. Meanwhile, reduction of blood CO2 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. Human lungs cannot recover from such injury.

Few humans have experienced all the conditions described above, but more limited exposures have ocurred. Joseph W. Kittinger Jr. experienced localized ebullism during a 31 km (19 miles) ascent in a He-driven gondola. His right-hand glove failed to pressurize and his hand expanded to roughly twice its normal volume accompanied by disabling pain. His hand took about 3 h to recover after his return to the ground.

Two other people were decompressed accidentally during space mission training programs on the ground, but both survived after exposures less than 5 min in duration. International Space Station (ISS) and Space Shuttle astronauts regularly work in Extravehicular Mobility Units (EMUs, i.e., 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.

Extreme temperature variations are a problem in space, because heat exchange occurs primarily via (infrared) photons (i.e., radiative heat exchange dominates). While the absence of convection and conduction insulates the body and prevents rapid loss of heat, localized heating can occur where the body is exposed to starlight at distances comparable to the Earth-Sun distance and localized cooling can occur in parts of the body in the shade.

Direct exposure to high energy photons (ultraviolet, X-ray, and gamma) and energized subatomic particles (mostly protons) would cause much more severe effects than localized temperature fluctuations. These can irreversibly damage 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.

As described by some researchers, decompression is a serious concern during the ExtraVehicular Activities (EVAs) of astronauts. Current EMU designs take decompression and other space-exposure 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 minimizing decompression risk. Some investigators have considered pressurizing 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) (for reference, by definition, the atmospheric pressure at sea level is 101.4 kPa or 14.7 psi). In such a design, pressurization of the torso and extremeties could be achieved mechanically, avoiding the mobility reductions of pneumatic pressurization.

November 2007, Suniti Karunatillake (more by Suniti Karunatillake) (Like this Answer)

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