Editing Outer space (section)

== Human access ==
=== 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/>

=== Boundary ===
{{For|the furthest reaches of space|observable universe}}
[[File:Earth's atmosphere.svg|thumb|upright=1.35|Illustration of Earth's atmosphere gradual transition into outer space]]

The transition between Earth's atmosphere and outer space lacks a well-defined physical boundary, with the air pressure steadily decreasing with altitude until it mixes with the [[solar wind]]. Various definitions for a practical boundary have been proposed, ranging from {{Convert|30|km|mi|abbr=on}} out to {{Convert|1600000|km|mi|abbr=on}}.<ref name="Betz"/> In 2009, measurements of the direction and speed of ions in the atmosphere were made from a [[sounding rocket]]. The altitude of {{Convert|118|km|mi|sigfig=3|abbr=on}} above Earth was the midpoint for charged particles transitioning from the gentle winds of the Earth's atmosphere to the more extreme flows of outer space. The latter can reach velocities well over {{Convert|268|m/s|ft/s|abbr=on}}.<ref name=thompton20090409/><ref name=jgr114/> 

High-altitude [[aircraft]], such as [[high-altitude balloon]]s have reached altitudes above Earth of up to 50 km.<ref name="Grush"/> Up until 2021, the United States designated people who travel above an altitude of {{convert|50|mi|km|abbr=on}} as astronauts.{{sfn|Wong|Fergusson|2010|p=16}} [[United States Astronaut Badge|Astronaut wings]] are now only awarded to spacecraft crew members that "demonstrated activities during flight that were essential to public safety, or contributed to human space flight safety".<ref name=FAA_2021/>

The region between airspace and outer space is termed "near space". There is no legal definition for this extent, but typically this is the altitude range from {{cvt|20|to|100|km|mi}}.<ref name=Hao_Fabio_2019/> For safety reasons, [[commercial aircraft]] are typically limited to altitudes of {{Cvt|12|km|mi}}, and air navigation services only extend to {{cvt|18|to|20|km|mi}}.<ref name=Hao_Fabio_2019/> The upper limit of the range is the [[Kármán line]], where [[astrodynamics]] must take over from [[aerodynamics]] in order to achieve flight.<ref name="j253"/> This range includes the [[stratosphere]], [[mesosphere]] and lower [[thermosphere]] layers of the Earth's atmosphere.<ref name="f457"/>

Larger ranges for ''near space'' are used by some authors, such as {{cvt|18|to|160|km|mi}}.<ref name="k364"/> These extend to the altitudes where [[orbital flight]] in [[very low Earth orbit]]s becomes practical.<ref name="k364"/> Spacecraft have entered into a highly elliptical [[orbital flight|orbit]] with a perigee as low as {{Convert|80|to|90|km|mi|abbr=on}}, surviving for multiple orbits.<ref name=McDowell_2018/> At an altitude of {{Convert|120|km|mi|abbr=on}},<ref name=McDowell_2018/> descending spacecraft begin [[atmospheric entry]] as [[atmospheric drag]] becomes noticeable. For [[spaceplane]]s such as [[NASA]]'s [[Space Shuttle]], this begins the process of switching from steering with thrusters to maneuvering with [[Flight control surfaces|aerodynamic control surfaces]].<ref name=petty20030213/>

The Kármán line, established by the [[Fédération Aéronautique Internationale]], and used internationally by the [[United Nations]],<ref name="Betz"/> is set at an altitude of {{convert|100|km|mi|abbr=on}} as a working definition for the boundary between aeronautics and astronautics. This line is named after [[Theodore von Kármán]], who argued for an altitude where a vehicle would have to travel faster than [[Orbital speed|orbital velocity]] to derive sufficient [[aerodynamic lift]] from the atmosphere to support itself,{{sfn|O'Leary|2009|p=84}}<ref name=space_begin/> which he calculated to be at an altitude of about {{Convert|83.8|km|mi|abbr=on}}.<ref name="Grush"/> This distinguishes altitudes below as the region of [[aerodynamics]] and [[airspace]], and above as the space of [[astronautics]] and ''free space''.<ref name="Betz"/>

There is no internationally recognized legal altitude limit on national airspace, although the Kármán line is the most frequently used for this purpose. Objections have been made to setting this limit too high, as it could inhibit space activities due to concerns about airspace violations.<ref name=McDowell_2018/> It has been argued for setting no specified singular altitude in international law, instead applying different limits depending on the case, in particular based on the craft and its purpose. Increased commercial and military sub-orbital spaceflight has raised the issue of where to apply laws of airspace and outer space.<ref name="k364"/><ref name="j253"/> Spacecraft have flown over foreign countries as low as {{Convert|30|km|mi|abbr=on}}, as in the example of the Space Shuttle.<ref name="Grush"/>

=== Legal status ===
{{Main|Space law}}
[[File:SM-3 launch to destroy the NRO-L 21 satellite.jpg|thumb|upright|Conventional anti-satellite weapons such as the [[RIM-161 Standard Missile 3|SM-3 missile]] remain legal under the [[law of armed conflict]], even though they create hazardous [[space debris]]]]
The [[Outer Space Treaty]] provides the basic framework for international space law. It covers the legal use of outer space by nation states, and includes in its definition of ''outer space'', the Moon, and other celestial bodies. The treaty states that outer space is free for all nation states to explore and is not subject to claims of national sovereignty, calling outer space the "province of all mankind". This status as a [[common heritage of mankind]] has been used, though not without opposition, to enforce the right to access and shared use of outer space for all nations equally, particularly non-spacefaring nations.<ref name="Durrani"/> It prohibits the deployment of [[nuclear weapon]]s in outer space. The treaty was passed by the [[United Nations General Assembly]] in 1963 and signed in 1967 by the Union of Soviet Socialist Republics (USSR), the United States of America (USA), and the United Kingdom (UK). As of 2017, 105 state parties have either ratified or acceded to the treaty. An additional 25 states signed the treaty, without ratifying it.<ref name="unoosa2" /><ref name=unoosa/>

Since 1958, outer space has been the subject of multiple United Nations resolutions. Of these, more than 50 have been concerning the international co-operation in the peaceful uses of outer space and preventing an arms race in space.<ref name=garros/> Four additional [[space law]] treaties have been negotiated and drafted by the UN's [[United Nations Committee on the Peaceful Uses of Outer Space|Committee on the Peaceful Uses of Outer Space]]. Still, there remains no legal prohibition against deploying conventional weapons in space, and [[anti-satellite weapon]]s have been successfully tested by the USA, USSR, China,{{sfn|Wong|Fergusson|2010|p=4}} and in 2019, India.<ref name=Solanki2019/> The 1979 [[Moon Treaty]] turned the jurisdiction of all heavenly bodies (including the orbits around such bodies) over to the international community. The treaty has not been ratified by any nation that currently practices human spaceflight.<ref name=esf20071105/>

In 1976, eight equatorial states (Ecuador, Colombia, Brazil, The Republic of the Congo, Zaire, Uganda, Kenya, and Indonesia) met in Bogotá, Colombia: with their "Declaration of the First Meeting of Equatorial Countries", or the [[Bogotá Declaration]], they claimed control of the segment of the geosynchronous orbital path corresponding to each country.<ref name=bogota1976/> These claims are not internationally accepted.<ref name=aasl31_2006/>

An increasing issue of international space law and regulation has been the dangers of the growing number of [[space debris]].<ref name="European Society of International Law 2023 p580"/>

=== Earth orbit ===
{{main|Geocentric orbit|Orbital decay}}
[[File:Newton Cannon.svg|thumb|240px|[[Newton's cannonball]], an illustration of how objects can "fall" in a curve around the planet]]
When a rocket is launched to achieve orbit, its thrust must both counter gravity and accelerate it to [[orbital speed]]. After the rocket terminates its thrust, it follows an arc-like [[trajectory]] back toward the ground under the influence of the Earth's [[gravitational force]]. In a [[closed orbit]], this arc will turn into an [[ellipse|elliptical]] loop around the planet. That is, a spacecraft successfully enters Earth orbit when its [[centripetal acceleration|acceleration due to gravity]] pulls the craft down just enough to prevent its momentum from carrying it off into outer space.<ref name=NESDIS_2025/>

For a [[low Earth orbit]], orbital speed is about {{Convert|7.8|km/s |mph|-2|abbr=on}};<ref name=hill1999/> by contrast, the fastest piloted airplane speed ever achieved (excluding speeds achieved by deorbiting spacecraft) was {{Convert|2.2|km/s|mph|-2|abbr=on}} in 1967 by the [[North American X-15]].<ref name=shiner20071101/> The upper limit of orbital speed at {{Convert|11.2|km/s|mph|-2|abbr=on}} is the [[escape velocity|velocity required to pull free]] from Earth altogether and enter into a [[heliocentric orbit]].<ref name=williams2010/> The energy required to reach Earth orbital speed at an altitude of {{Convert|600|km|mi|abbr=on}} is about 36&nbsp;[[Megajoule|MJ]]/kg, which is six times the energy needed merely to climb to the corresponding altitude.<ref name=dimotakis1999/>

Very low Earth orbit (VLEO) has been defined as orbits that have a mean altitude below 450 km (280 mi), which can be better suited for Earth observation with small satellites.<ref name=Llop_et_al_2014/> Low Earth orbits in general range in altitude from {{cvt|180|to|2000|km|mi}} and are used for scientific satellites. [[Medium Earth orbit]]s extends from {{cvt|2000|to|35780|km|mi}}, which are favorable orbits for navigation and specialized satellites. Above {{cvt|35780|km|mi}} are the [[high Earth orbit]]s used for weather and some communication satellites.<ref name=Riebeck_2009/>

Spacecraft in orbit with a [[Apsis|perigee]] below about {{Convert|2000|km|mi|abbr=on}} (low Earth orbit) are subject to drag from the Earth's atmosphere,{{sfn|Ghosh|2000|pp=47–48}} which decreases the orbital altitude. The rate of orbital decay depends on the satellite's cross-sectional area and mass, as well as variations in the air density of the upper atmosphere, which is significantly effected by [[space weather]].<ref name="z356"/> At altitudes above {{cvt|800|km|mi|abbr=on}}, orbital lifetime is measured in centuries.<ref name=NASA_FAQ/> Below about {{cvt|300|km|mi|abbr=on}}, decay becomes more rapid with lifetimes measured in days. Once a satellite descends to {{cvt|180|km|mi|abbr=on}}, it has only hours before it vaporizes in the atmosphere.<ref name=slsa/> 

Radiation in orbit around Earth is concentrated in [[Van Allen radiation belt]]s, which trap [[cosmic radiation|solar and galactic radiation]]. Radiation is a threat to astronauts and space systems. It is difficult to shield against and space weather makes the radiation environment variable. The radiation belts are equatorial [[toroid]]al regions, which are bent towards Earth's poles, with the [[South Atlantic Anomaly]] being the region where charged particles approach Earth closest.<ref name=Baker_et_al_2018/><ref name="u460"/> The innermost radiation belt, the inner Van Allen belt, has its intensity peak at altitudes above the equator of half an Earth radius,<ref name="e494"/> centered at about 3000 km,<ref name="a298"/> increasing from the upper edge of low Earth orbit which it overlaps.<ref name=Irfan_et_al_2002/><ref name=Koteskey_2024/><ref name=Kovar_et_al_2020/>