Editing Outer space (section)

=== Effect on biology and human bodies ===
{{main|Effect of spaceflight on the human body|Space medicine|Bioastronautics}}
{{See also|Astrobiology|Astrobotany|Plants in space|Animals in space}}
[[File:Bruce McCandless II during EVA in 1984.jpg|upright|thumb|Because of the hazards of a vacuum, astronauts must wear a pressurized [[space suit]] while outside their spacecraft.|alt=The lower half shows a blue planet with patchy white clouds. The upper half has a man in a white spacesuit and maneuvering unit against a black background.]]

Despite the harsh environment, several life forms have been found that can withstand extreme space conditions for extended periods. Species of lichen carried on the ESA [[BIOPAN]] facility survived exposure for ten days in 2007.<ref name="Astrobiology_11_4_281"/> Seeds of ''[[Arabidopsis thaliana]]'' and ''[[Nicotiana tabacum]]'' germinated after being exposed to space for 1.5 years.<ref name="Astrobiology_12_5_517"/> A strain of ''[[Bacillus subtilis]]'' has survived 559 days when exposed to low Earth orbit or a simulated Martian environment.<ref name="Astrobiology_12_5_498"/> The [[Panspermia|lithopanspermia]] hypothesis suggests that rocks ejected into outer space from life-harboring planets may successfully transport life forms to another habitable world. A conjecture is that just such a scenario occurred early in the history of the Solar System, with potentially [[microorganism]]-bearing rocks being exchanged between Venus, Earth, and Mars.<ref name="Nicholson2010"/>

====Vacuum====
{{main|Uncontrolled decompression}}
The lack of pressure in space is the most immediate dangerous characteristic of space to humans. Pressure decreases above Earth, reaching a level at an altitude of around {{convert|19.14|km|mi|abbr=on}} that matches the [[vapor pressure of water]] at the [[Human body temperature|temperature of the human body]]. This pressure level is called the [[Armstrong line]], named after American physician [[Harry G. Armstrong]].<ref name=Tarver_et_al_2022/> At or above the Armstrong line, fluids in the throat and lungs boil away. More specifically, exposed bodily liquids such as saliva, tears, and liquids in the lungs boil away. Hence, at this altitude, human survival requires a pressure suit, or a pressurized capsule.{{sfn|Piantadosi|2003|pp=188–189}}

Out in space, sudden exposure of an unprotected human to very low [[Atmospheric pressure|pressure]], such as during a rapid decompression, can cause [[pulmonary barotrauma]]—a rupture of the lungs, due to the large pressure differential between inside and outside the chest.<ref name=Battisti_et_al_2022/> Even if the subject's airway is fully open, the flow of air through the windpipe may be too slow to prevent the rupture.<ref name=krebs_pilmanis1996/> Rapid decompression can rupture eardrums and sinuses, bruising and blood seep can occur in soft tissues, and shock can cause an increase in oxygen consumption that leads to [[Hypoxia (medical)|hypoxia]].<ref name=Busby_1967/>

As a consequence of rapid decompression, oxygen dissolved in the blood empties into the lungs to try to equalize the [[partial pressure]] gradient. Once the deoxygenated blood arrives at the brain, humans lose consciousness after a few seconds and die of hypoxia within minutes.<ref name=bmj286/> Blood and other body fluids boil when the pressure drops below {{convert|6.3|kPa|psi|0}}, and this condition is called [[ebullism]].<ref name=jramc157_1_85/> The steam may bloat the body to twice its normal size and slow circulation, but tissues are elastic and porous enough to prevent rupture. Ebullism is slowed by the pressure containment of blood vessels, so some blood remains liquid.{{sfn|Billings|1973|pp=1–34}}<ref name=landis20070807/>

Swelling and ebullism can be reduced by containment in a [[pressure suit]]. The Crew Altitude Protection Suit (CAPS), a fitted elastic garment designed in the 1960s for astronauts, prevents ebullism at pressures as low as {{convert|2|kPa|psi|1}}.<ref name=am39_376/> Supplemental oxygen is needed at {{Convert|8|km|mi|0|abbr=on}} to provide enough oxygen for breathing and to prevent water loss, while above {{Convert|20|km|mi|abbr=on}} pressure suits are essential to prevent ebullism.{{sfn|Ellery|2000|p=68}} Most space suits use around {{convert|30|-|39|kPa|psi|0}} of pure oxygen, about the same as the partial pressure of oxygen at the Earth's surface. This pressure is high enough to prevent ebullism, but evaporation of nitrogen dissolved in the blood could still cause [[decompression sickness]] and [[air embolism|gas embolisms]] if not managed.{{sfn|Davis|Johnson|Stepanek|2008|pp=270–271}}

====Weightlessness and radiation====
{{Main|Weightlessness|Radiobiology}}
[[Human evolution|Humans evolved]] for life in Earth [[Gravitation|gravity]], and exposure to weightlessness has been shown to have deleterious effects on human health. Initially, more than 50% of astronauts experience [[space motion sickness]]. This can cause nausea and vomiting, [[Vertigo (medical)|vertigo]], headaches, [[lethargy]], and overall malaise. The duration of space sickness varies, but it typically lasts for 1–3 days, after which the body adjusts to the new environment. Longer-term exposure to weightlessness results in [[muscle atrophy]] and deterioration of the skeleton, or [[spaceflight osteopenia]]. These effects can be minimized through a regimen of exercise.{{sfn|Kanas|Manzey|2008|pp=15–48}} Other effects include fluid redistribution, slowing of the [[cardiovascular system]], decreased production of [[red blood cell]]s, balance disorders, and a weakening of the [[immune system]]. Lesser symptoms include loss of body mass, nasal congestion, sleep disturbance, and puffiness of the face.<ref name=cmaj180_13_1317/>

During long-duration space travel, radiation can pose an [[acute health hazard]]. Exposure to high-energy, ionizing [[cosmic rays]] can result in fatigue, nausea, vomiting, as well as damage to the immune system and changes to the [[white blood cell]] count. Over longer durations, symptoms include an increased risk of cancer, plus damage to the eyes, [[nervous system]], lungs and the [[Human gastrointestinal tract|gastrointestinal tract]].<ref name=nsbri_radiation/> On a round-trip [[Mars]] mission lasting three years, a large fraction of the cells in an astronaut's body would be traversed and potentially damaged by high energy nuclei.<ref name=curtis_and_Letaw/> The energy of such particles is significantly diminished by the shielding provided by the walls of a spacecraft and can be further diminished by water containers and other barriers. The impact of the cosmic rays upon the shielding produces additional radiation that can affect the crew. Further research is needed to assess the radiation hazards and determine suitable countermeasures.<ref name=sas4_11_1013/>