View source for Outer space

{{Short description|Void between celestial bodies}}
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{{About|the space between celestial bodies|the general concept|Space}}
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[[File:Webb%27s_First_Deep_Field.jpg|thumb|upright=1.4|Being essentially empty, outer space allows the earliest (redder) galaxies to be viewed without obstruction, as in the [[Webb's First Deep Field]] image.]]

'''Outer space''', or simply '''space''', is the expanse that exists beyond [[Atmosphere of Earth|Earth's atmosphere]] and between [[astronomical object|celestial bodies]].<ref name=Merriam-Webster/> It contains ultra-low [[Orders of magnitude (pressure)|levels of particle densities]], constituting a [[ultra-high vacuum|near-perfect vacuum]]<ref name=Roth_2012/> of predominantly [[hydrogen]] and [[helium]] [[plasma (physics)|plasma]], permeated by [[electromagnetic radiation]], [[cosmic ray]]s, [[cosmic neutrino background|neutrinos]], [[magnetic field]]s and [[cosmic dust|dust]]. The baseline [[temperature]] of outer space, as set by the [[cosmic background radiation|background radiation]] from the [[Big Bang]], is {{convert|2.7255|K|C F|adj=ri1|sigfig=3|abbr=out}}.<ref name="CBE2008" /> 

The [[warm–hot intergalactic medium|plasma between galaxies]] is thought to account for about half of the [[baryonic matter|baryonic (ordinary) matter]] in the universe, having a [[number density]] of less than one [[hydrogen atom]] per cubic metre and a [[kinetic temperature]] of millions of [[kelvins]].<ref name=baas41_908/> Local concentrations of matter have condensed into [[star]]s and [[galaxy|galaxies]]. Intergalactic space takes up most of the volume of the [[universe]], but even galaxies and [[star system]]s consist almost entirely of empty space. Most of the remaining [[mass–energy equivalence|mass-energy]] in the [[observable universe]] is made up of an unknown form, dubbed [[dark matter]] and [[dark energy]].{{sfn|Freedman|Kaufmann|2005|pp=573, 599–601, 650-653}}<ref name="Trimble 1987" /><ref name="nasa_darkenergy" />

Outer space does not begin at a definite altitude above Earth's surface. The [[Kármán line]], an altitude of {{cvt|100|km|mi}} above [[sea level]],{{sfn|O'Leary|2009|p=84}}<ref name=space_begin/> is conventionally used as the start of outer space in space treaties and for aerospace records keeping. Certain portions of the upper [[stratosphere]] and the [[mesosphere]] are sometimes referred to as "[[near space]]". The framework for international [[space law]] was established by the [[Outer Space Treaty]], which entered into force on 10 October 1967. This treaty precludes any claims of [[national sovereignty]] and permits all states to freely [[space exploration|explore outer space]]. Despite the drafting of [[UN resolution]]s for the peaceful uses of outer space, [[anti-satellite weapon]]s have been tested in [[Geocentric orbit|Earth orbit]].

The concept that the space between the Earth and the Moon must be a vacuum was first proposed in the 17th century after scientists discovered that [[air pressure]] decreased with altitude. The immense scale of outer space was grasped in the 20th century when the distance to the [[Andromeda Galaxy]] was first measured. Humans began the physical exploration of space later in the same century with the advent of high-altitude [[Balloon (aircraft)|balloon flights]]. This was followed by crewed [[rocket launch|rocket flight]]s and, then, crewed Earth orbit, first achieved by [[Yuri Gagarin]] of the [[Soviet Union]] in 1961. The economic cost of putting objects, including humans, into space is very high, limiting human [[spaceflight]] to [[low Earth orbit]] and the [[Moon]]. On the other hand, [[uncrewed spacecraft]] have reached all of the known [[planet]]s in the [[Solar System]]. Outer space represents a challenging environment for [[exploration|human exploration]] because of the hazards of [[vacuum]] and [[radiation]]. [[Microgravity]] has a negative effect on human [[physiology]] that causes both [[muscle atrophy]] and [[Spaceflight osteopenia|bone loss]].

== Terminology ==
The use of the short version ''space'', as meaning "the region beyond Earth's sky", predates the use of full term "outer space", with the earliest recorded use of this meaning in an epic poem by [[John Milton]] called ''[[Paradise Lost]]'', published in 1667.<ref name=harper2001/><ref name=Brady2007/>

The term ''outward space'' existed in a poem from 1842 by the English poet Lady [[Emmeline Stuart-Wortley]] called "The Maiden of Moscow",{{sfn|Stuart Wortley|1841|p=410}} but in astronomy the term ''outer space'' found its application for the first time in 1845 by [[Alexander von Humboldt]].{{sfn|Von Humboldt|1845|p=39}} The term was eventually popularized through the writings of [[H.&nbsp;G.&nbsp;Wells]] after 1901.<ref name="entymonline"/> [[Theodore von Kármán]] used the term of ''free space'' to name the space of altitudes above Earth where spacecrafts reach conditions sufficiently free from atmospheric drag, differentiating it from [[airspace]], identifying a legal space above territories free from the [[sovereign]] jurisdiction of countries. This definition of the boundary to outer space became known as the [[Kármán line]].<ref name="Betz"/> 

"[[:wikt:spaceborne|Spaceborne]]" denotes existing in outer space, especially if carried by a [[spacecraft]];<ref name="Merriam-Webster 2022"/><ref name="Fall Cao Hong Eymard 2022"/> similarly, "[[:wikt:space-based|space-based]]" means based in outer space or on a planet or moon.<ref name=based/>

== Formation and state ==
{{Main|Big Bang}}
[[File:CMB Timeline300 no WMAP.jpg|upright=1.5|thumb|alt=An artist's concept of the expanding universe opening up from the viewer's left, facing the viewer in a 3/4 pose.|Timeline of the [[expansion of the universe]], where space is represented schematically at each time by circular sections. On the left, the dramatic expansion of [[cosmic inflation|inflation]]; at the center, the expansion [[accelerating expansion of the universe|accelerates]] (artist's concept; neither time nor size are to scale)]]

The size of the whole universe is unknown, and it might be [[Infinity|infinite]] in extent.{{sfn|Liddle|2015|pp=33}} According to the Big Bang theory, the very early universe was an extremely hot and dense state about [[age of the universe|13.8&nbsp;billion years ago]]<ref name=planck_2013 /> which rapidly [[expansion of the universe|expanded]]. About 380,000 years later the universe had cooled sufficiently to allow protons and electrons to combine and form hydrogen—the so-called [[Recombination (cosmology)|recombination epoch]]. When this happened, matter and energy became decoupled, allowing photons to travel freely through the continually expanding space.<ref name="SciAm301_1_36"/> Matter that remained following the initial expansion has since undergone gravitational collapse to create stars, galaxies and other astronomical objects, leaving behind a deep [[vacuum]] that forms what is now called outer space.{{sfn|Silk|2000|pp=105–308}} As light has a finite velocity, this theory constrains the size of the directly observable universe.<ref name="SciAm301_1_36"/>

The present day [[shape of the universe]] has been determined from measurements of the [[Cosmic microwave background radiation|cosmic microwave background]] using satellites like the [[Wilkinson Microwave Anisotropy Probe]]. These observations indicate that the [[spatial geometry]] of the observable universe is "[[Flatness (cosmology)|flat]]", meaning that photons on parallel paths at one point remain parallel as they travel through space to the limit of the observable universe, except for local gravity.<ref name="WMAP"/> The flat universe, combined with the measured mass density of the universe and the accelerating [[Hubble's law|expansion of the universe]], indicates that space has a non-zero [[vacuum energy]], which is called [[dark energy]].{{sfn|Sparke|Gallagher|2007|pp=329–330}}

Estimates put the average [[energy density]] of the present day universe at the equivalent of 5.9 protons per cubic meter, including dark energy, dark matter, and baryonic matter (ordinary matter composed of atoms). The atoms account for only 4.6% of the total energy density, or a density of one proton per four cubic meters.<ref name=nasa_wmap/> The density of the universe is clearly not uniform; it ranges from relatively high density in galaxies—including very high density in structures within galaxies, such as planets, stars, and [[black hole]]s—to conditions in vast [[Void (astronomy)|voids]] that have much lower density, at least in terms of visible matter.<ref name=aj89_1461/> Unlike matter and dark matter, dark energy seems not to be concentrated in galaxies: although dark energy may account for a majority of the mass-energy in the universe, dark energy's influence is 5 [[Order of magnitude|orders of magnitude]] smaller than the influence of gravity from matter and dark matter within the Milky Way.<ref name=rvmphys_75_559 />

== Environment ==
{{Main|Space environment|Space weather|Space weathering}}
[[File:Night Sky from Hawai‘i and Chile (iotw2225c).jpg|thumb|upright=2|A wide field view of outer space as seen from Earth's surface at night. The [[interplanetary dust cloud]] is visible as the horizontal band of [[zodiacal light]], including the ''false dawn''<ref name=eso_2017/> (edges) and ''[[gegenschein]]'' (center), which is visually crossed by the [[Milky Way]]]]
Outer space is the closest known approximation to a [[perfect vacuum]]. It has effectively no [[friction]], allowing stars, [[planets]], and [[moons]] to move freely along their [[orbit]]s. The deep vacuum of [[#Intergalactic space|intergalactic space]] is not devoid of [[matter]], as it contains a few [[hydrogen atoms]] per cubic meter.<ref name=pasj20_230/> By comparison, the air humans breathe contains about 10<sup>25</sup> molecules per cubic meter.{{sfn|Borowitz|Beiser|1971}}<ref name=patrick/> The low density of matter in outer space means that electromagnetic radiation can travel great distances without being scattered: the [[mean free path]] of a [[photon]] in intergalactic space is about 10<sup>23</sup>&nbsp;km, or 10&nbsp;billion light years.{{sfn|Davies|1977|p=93}} In spite of this, [[Extinction (astronomy)|extinction]], which is the [[Absorption (electromagnetic radiation)|absorption]] and [[scattering]] of photons by dust and gas, is an important factor in galactic and intergalactic [[astronomy]].<ref name=fitzpatrick2004/>

Stars, planets, and moons retain their [[atmosphere]]s by gravitational attraction. Atmospheres have no clearly delineated upper boundary: the density of atmospheric gas gradually decreases with distance from the object until it becomes indistinguishable from outer space.{{sfn|Chamberlain|1978|p=2}} The Earth's atmospheric [[pressure]] drops to about {{nowrap|0.032 [[Pascal (unit)|Pa]]}} at {{Convert|100|km|mi|abbr=off}} of altitude,<ref name=squire2000/> compared to 100,000&nbsp;Pa for the [[International Union of Pure and Applied Chemistry]] (IUPAC) definition of [[Standard temperature and pressure|standard pressure]]. Above this altitude, [[Isotropy|isotropic]] gas pressure rapidly becomes insignificant when compared to [[radiation pressure]] from the [[Sun]] and the [[dynamic pressure]] of the [[solar wind]]. The [[thermosphere]] in this range has large gradients of pressure, temperature and composition, and varies greatly due to [[space weather]].<ref name=jmsj_85B_193/>

The temperature of outer space is measured in terms of the [[kinetic theory of gases|kinetic]] activity of the gas,<ref name=Spitzer_1948/> as it is on Earth. The radiation of outer space has a different temperature than the kinetic temperature of the gas, meaning that the gas and radiation are not in [[thermodynamic equilibrium]].{{sfn|Prialnik|2000|pp=195–196}}{{sfn|Spitzer|1978|p=28–30}} All of the observable universe is filled with photons that were created during the Big Bang, which is known as the [[cosmic microwave background radiation]] (CMB). (There is quite likely a correspondingly large number of [[neutrino]]s called the [[cosmic neutrino background]].<ref name="fp2_30"/>) The current [[black body]] temperature of the background radiation is about {{convert|2.7|K|C F|0}}.<ref name=apj707_2_916/> The gas temperatures in outer space can vary widely. For example, the temperature in the [[Boomerang Nebula]] is {{convert|1|K|C F|0}},<ref name=ALMA2013/> while the [[solar corona]] reaches temperatures over {{convert|1,200,000|-|2,600,000|K|F|-5}}.<ref name=apj325_442/>

Magnetic fields have been detected in the space around many classes of celestial objects. Star formation in spiral galaxies can generate small-scale [[dynamo]]s, creating turbulent magnetic field strengths of around 5–10&nbsp;μ[[Gauss (unit)|G]]. The [[Davis–Greenstein effect]] causes elongated [[Cosmic dust|dust grains]] to align themselves with a galaxy's magnetic field, resulting in weak optical [[Polarization (waves)|polarization]]. This has been used to show ordered magnetic fields that exist in several nearby galaxies. [[Magnetohydrodynamics|Magneto-hydrodynamic]] processes in [[Active galactic nucleus|active]] [[Elliptical galaxy|elliptical galaxies]] produce their characteristic [[Astrophysical jet|jets]] and [[radio lobe]]s. Non-thermal [[Astronomical radio source|radio sources]] have been detected even among the most distant [[redshift|high-z]] sources, indicating the presence of magnetic fields.<ref name=WielebinskiBeck2010/>

Outside a protective atmosphere and magnetic field, there are few obstacles to the passage through space of energetic [[subatomic particle]]s known as [[cosmic ray]]s. These particles have energies ranging from about 10<sup>6</sup>&nbsp;[[Electronvolt|eV]] up to an extreme 10<sup>20</sup>&nbsp;eV of [[ultra-high-energy cosmic ray]]s.<ref name=rmp83_3_907/> The peak flux of cosmic rays occurs at energies of about 10<sup>9</sup>&nbsp;eV, with approximately 87% protons, 12% helium nuclei and 1% heavier nuclei. In the high energy range, the flux of [[electron]]s is only about 1% of that of protons.{{sfn|Lang|1999|p=462}} Cosmic rays can damage electronic components and pose a [[Health threat from cosmic rays|health threat]] to space travelers.{{sfn|Lide|1993|p=11{{hyphen}}217<!-- Note: this is not a page range -->}}

Scents retained from low Earth orbit, when returning from [[extravehicular activity]], have a burned, metallic odor, similar to the scent of [[arc welding]] fumes. This results from [[oxygen]] in low Earth orbit, which clings to suits and equipment.<ref name=ls2012/><ref name="PopSicSmell"/><ref name="a981"/> Other regions of space could have very different odors, like that of different alcohols in [[molecular cloud]]s.<ref name="m655"/>

== 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"/> Because bacteria can survive for millions of years, it is at least theoretically possible for Galactic-scale panspermia to occur.<ref name=Ginsburg_et_al_2018/>

====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/>

== Regions ==
{{Also|Location of Earth}}
===Regions near the Earth===
The outermost layer of the Earth's atmosphere is termed the [[exosphere]]. It extends outward from the [[thermopause]], which lies at an altitude that varies from {{convert|250|to|500|km|mi}}, depending on the incidence of solar radiation. Beyond this altitude, collisions between molecules are negligible and the atmosphere joins with interplanetary space.<ref name=Catling_Kasting_2017/> The region in proximity to the Earth is home to a multitude of Earth–orbiting satellites and has been subject to extensive studies. For identification purposes, this volume is divided into overlapping regions of space.<ref name="uscode.house.gov 2022"/>{{sfn|Schrijver|Siscoe|2010|p=363, 379}}<ref name=sdc20150426/><ref name=sr2165/>

'''{{Visible anchor|Near-Earth space}}''' is the region of space extending from low Earth orbits out to [[geostationary orbit]]s.<ref name="uscode.house.gov 2022"/> This region includes the major orbits for [[artificial satellite]]s and is the site of most of humanity's space activity. The region has seen high levels of space debris, sometimes dubbed [[space pollution]], threatening nearby space activity.<ref name="uscode.house.gov 2022"/> Some of this debris re-enters Earth's atmosphere periodically.<ref name=portree_loftus1999/> Although it meets the definition of outer space, the atmospheric density inside low-Earth orbital space, the first few hundred kilometers above the Kármán line, is still sufficient to produce significant [[Drag (physics)|drag]] on satellites.<ref name=slsa/>

[[File:Debris-GEO1280.jpg|thumb|A computer-generated map of objects orbiting Earth, as of 2005. About 95% are debris, not working artificial satellites<ref name=ARES/>]]

{{anchor|Geospace}}'''Geospace''' is a region of space that includes Earth's [[upper atmosphere]] and [[magnetosphere]].{{sfn|Schrijver|Siscoe|2010|p=363, 379}} The Van Allen radiation belts lie within the geospace. The outer boundary of geospace is the [[magnetopause]], which forms an interface between the Earth's magnetosphere and the solar wind. The inner boundary is the [[ionosphere]].<ref name=geospace/>{{sfn|Schrijver|Siscoe|2010|p=379}}

The variable space-weather conditions of geospace are affected by the behavior of the Sun and the solar wind; the subject of geospace is interlinked with [[heliophysics]]—the study of the Sun and its impact on the planets of the Solar System.{{sfn|Fichtner|Liu|2011|pp=341–345}} The day-side magnetopause is compressed by solar-wind pressure—the subsolar distance from the center of the Earth is typically 10 Earth radii. On the night side, the solar wind stretches the magnetosphere to form a [[magnetotail]] that sometimes extends out to more than 100–200 Earth radii.{{sfn|Koskinen|2010|pp=32, 42}}<ref name=HonesJr1986/> For roughly four days of each month, the lunar surface is shielded from the solar wind as the Moon passes through the magnetotail.{{sfn|Mendillo|2000|p=275}}

Geospace is populated by electrically charged particles at very low densities, the motions of which are controlled by the [[Earth's magnetic field]]. These plasmas form a medium from which storm-like disturbances powered by the solar wind can drive electrical currents into the Earth's upper atmosphere. [[Geomagnetic storm]]s can disturb two regions of geospace, the radiation belts and the ionosphere. These storms increase fluxes of energetic electrons that can permanently damage satellite electronics, interfering with shortwave radio communication and [[Global Positioning System|GPS]] location and timing.{{sfn|Goodman|2006|p=244}} Magnetic storms can be a hazard to astronauts, even in low Earth orbit. They create [[aurora (astronomy)|aurorae]] seen at high latitudes in an oval surrounding the [[geomagnetic pole]]s.<ref name=oecd/>

[[File:Artemis 1 at maximum distance from Earth.jpg|thumb|Earth and the Moon as seen from cislunar space on the 2022 [[Artemis 1]] mission]]
XGEO space is a concept used by the USA to refer to the space of high Earth orbits, with the 'X' being some multiple of [[geosynchronous orbit]] (GEO) at approximately {{convert|35786|km|mi|0|abbr=on}}.<ref name=sdc20150426/> Hence, the [[Lagrange point#Earth–Moon|L2 Earth-Moon Lagrange point]] at {{convert|448900|km|mi|0|abbr=on}} is approximately 10.67 XGEO.<ref name=Cunio_et_al_2021/> Translunar space is the region of lunar [[transfer orbit]]s, between the Moon and Earth.<ref name="NASA 2013"/> 

{{anchor|Cislunar space}}{{anchor|cislunar space}}
'''Cislunar space''' is a region outside of Earth that includes [[lunar orbit]]s, the [[orbit of the Moon|Moon's orbital space around Earth]] and the Earth-Moon [[Lagrange point]]s.<ref name=sr2165/> The region where a body's [[gravitational potential]] remains dominant against gravitational potentials from other bodies, is the body's [[Sphere of influence (astrodynamics)|sphere of influence]] or gravity well, mostly described with the [[Hill sphere]] model.<ref name=yoder1995/> In the case of Earth this includes all space from the Earth to a distance of roughly 1% of the mean distance from Earth to the Sun,{{sfn|Barbieri|2006|p=253}} or {{convert|1.5|e6km|e6mi|abbr=unit}}. Beyond Earth's Hill sphere extends along [[Earth's orbit]]al path its orbital and [[co-orbital]] space. This space is co-populated by groups of co-orbital [[Near-Earth Object]]s (NEOs), such as [[Horseshoe orbit|horseshoe librator]]s and [[Earth trojan]]s, with some NEOs at times becoming [[temporary satellite]]s and [[Quasi-satellite|quasi-moon]]s to Earth.<ref name=Granvik_et_al_2012/>

{{anchor|Deep space}}
'''Deep space''' is defined by the United States government as all of outer space which lies further from Earth than a typical low-Earth-orbit, thus assigning the Moon to deep-space.<ref name=USC_10101/> Other definitions vary the starting point of deep-space from, "That which lies beyond the orbit of the moon," to "That which lies beyond the farthest reaches of the Solar System itself."{{sfn|Dickson|2010|p=57}}{{sfn|Williamson|2006|p=97}}<ref name=Collins/> The [[ITU-R|International Telecommunication Union responsible for radio communication]], including with satellites, defines deep-space as, "distances from the Earth equal to, or greater than, {{convert|2|e6km|e6mi|abbr=unit}},"<ref name=ITU/> which is about five times the [[Lunar distance (astronomy)|Moon's orbital distance]], but which distance is also far less than the distance between Earth and any adjacent planet.<ref name=Williams_2021/>
[[File:Orbitalaltitudes.svg|center|700px|thumb|Near-Earth space showing the low-Earth (blue), medium Earth (green), and high Earth (red) orbits. The last extends beyond the radius of geosynchronous orbits]]

=== Interplanetary space ===
{{main|Interplanetary medium}}
[[File:Comet Hale-Bopp 1995O1.jpg|right|thumb|The sparse plasma (blue) and dust (white) in the tail of [[comet Hale–Bopp]] are being shaped by pressure from [[Sunlight|solar radiation]] and the solar wind, respectively.|alt=At lower left, a white coma stands out against a black background. Nebulous material streams away to the top and left, slowly fading with distance.]]

Interplanetary space within the [[Solar System]] is dominated by the gravitation of the Sun, outside the gravitational spheres of influence of the planets.<ref name="j999"/> Interplanetary space extends well beyond the orbit of the outermost planet [[Neptune]], all the way out to where the influence of the galactic environment starts to dominate over the Sun and its solar wind producing the [[Heliopause (astronomy)|heliopause]] at 110 to 160 AU.<ref name=universetoday_interplanetaryspace /> The heliopause deflects away low-energy galactic cosmic rays, and its distance and strength varies depending on the activity level of the solar wind.<ref name=Kohler2017/><ref name=phillips2009/> The solar wind is a continuous stream of charged particles emanating from the Sun which creates a very tenuous atmosphere (the [[heliosphere]]) for billions of kilometers into space. This wind has a particle density of 5–10 [[proton]]s/cm<sup>3</sup> and is moving at a velocity of {{Convert|350|-|400|km/s|mph|abbr=on}}.{{sfn|Papagiannis|1972|pp=12–149}}

The region of interplanetary space is a nearly total vacuum, with a mean free path of about one [[astronomical unit]] at the orbital distance of the Earth. This space is not completely empty, but is sparsely filled with cosmic rays, which include [[ion]]ized [[atomic nucleus|atomic nuclei]] and various subatomic particles. There is gas, plasma and dust,<ref name="EA-20190312"/> small [[meteor]]s, and several dozen types of [[organic chemistry|organic]] molecules discovered to date by [[rotational spectroscopy|microwave spectroscopy]].<ref name=asp2003/> Collectively, this matter is termed the [[interplanetary medium]].<ref name=universetoday_interplanetaryspace /> A cloud of interplanetary dust is visible at night as a faint band called the [[zodiacal light]].<ref name="leinert_grun_1990"/>

Interplanetary space contains the magnetic field generated by the Sun.{{sfn|Papagiannis|1972|pp=12–149}} There are magnetospheres generated by planets such as Jupiter, Saturn, [[Mercury (planet)|Mercury]] and the Earth that have their own magnetic fields. These are shaped by the influence of the solar wind into the approximation of a teardrop shape, with the long tail extending outward behind the planet. These magnetic fields can trap particles from the solar wind and other sources, creating belts of charged particles such as the Van Allen radiation belts. Planets without magnetic fields, such as Mars, have their atmospheres gradually eroded by the solar wind.<ref name=ssr69_3_215/>

=== Interstellar space ===
{{Redirect|Interstellar space|the album|Interstellar Space}}
{{Main|Interstellar medium}}
[[File:52706main hstorion lg.jpg|right|thumb|[[Bow shock]] formed by the [[magnetosphere]] of the young star [[LL Orionis]] (center) as it collides with the [[Orion Nebula]] flow|alt=Patchy orange and blue nebulosity against a black background, with a curved orange arc wrapping around a star at the center.]]

Interstellar space is the physical space outside of the bubbles of plasma known as [[astrosphere]]s, formed by [[stellar wind]]s originating from individual stars.<ref name=jpl_interstellarspace /> It is the space between the stars or [[stellar systems]] within a nebula or galaxy.<ref name=Cooper_2023/> Interstellar space contains an [[interstellar medium]] of sparse matter and radiation. The boundary between an astrosphere and interstellar space is known as an [[astropause]]. For the Sun, the astrosphere and astropause are called the heliosphere and heliopause, respectively.<ref name=Garcia-Sage_et_al_2023/>

Approximately 70% of the mass of the interstellar medium consists of lone hydrogen atoms; most of the remainder consists of helium atoms. This is enriched with trace amounts of heavier atoms formed through [[stellar nucleosynthesis]]. These atoms are ejected into the interstellar medium by stellar winds or when evolved stars begin to shed their outer envelopes such as during the formation of a [[planetary nebula]].<ref name="Ferrière2001"/> The cataclysmic explosion of a [[supernova]] propagates [[shock waves]] of stellar ejecta outward, distributing it throughout the interstellar medium, including the heavy elements previously formed within the star's core.<ref name="witt2001"/> The density of matter in the interstellar medium can vary considerably: the average is around 10<sup>6</sup> particles per m<sup>3</sup>,<ref name=Boulares1990/> but cold [[molecular clouds]] can hold 10<sup>8</sup>–10<sup>12</sup> per m<sup>3</sup>.{{sfn|Prialnik|2000|pp=195–196}}<ref name="Ferrière2001"/>

A [[List of interstellar and circumstellar molecules|number of molecules]] exist in interstellar space, which can form dust particles as tiny as 0.1&nbsp;[[Micrometre|μm]].{{sfn|Rauchfuss|2008|pp=72–81}} The tally of molecules discovered through [[radio astronomy]] is steadily increasing at the rate of about four new species per year. Large regions of higher density matter known as molecular clouds allow chemical reactions to occur, including the formation of organic polyatomic species. Much of this chemistry is driven by collisions. Energetic cosmic rays penetrate the cold, dense clouds and ionize hydrogen and helium, resulting, for example, in the [[trihydrogen cation]]. An ionized helium atom can then split relatively abundant [[carbon monoxide]] to produce ionized carbon, which in turn can lead to organic chemical reactions.<ref name="PNAS103_33_12232"/>

The local interstellar medium is a region of space within 100&nbsp;[[parsec|pc]] of the Sun, which is of interest both for its proximity and for its interaction with the Solar System. This volume nearly coincides with a region of space known as the [[Local Bubble]], which is characterized by a lack of dense, cold clouds. It forms a cavity in the [[Orion Arm]] of the Milky Way Galaxy, with dense molecular clouds lying along the borders, such as those in the [[constellation]]s of [[Ophiuchus]] and [[Taurus (constellation)|Taurus]]. The actual distance to the border of this cavity varies from 60 to 250&nbsp;pc or more. This volume contains about 10<sup>4</sup>–10<sup>5</sup> stars and the local interstellar gas counterbalances the [[Stellar-wind bubble|astrospheres]] that surround these stars, with the volume of each sphere varying depending on the local density of the interstellar medium. The Local Bubble contains dozens of warm interstellar clouds with temperatures of up to 7,000&nbsp;K and radii of 0.5–5&nbsp;pc.<ref name=redfield2006/>

When stars are moving at sufficiently high [[peculiar velocities]], their astrospheres can generate [[bow shock]]s as they collide with the interstellar medium. For decades it was assumed that the Sun had a bow shock. In 2012, data from [[Interstellar Boundary Explorer|Interstellar Boundary Explorer (IBEX)]] and NASA's [[Voyager program|Voyager]] probes showed that the Sun's bow shock does not exist. Instead, these authors argue that a [[Mach number|subsonic]] bow wave defines the transition from the solar wind flow to the interstellar medium.<ref name=bow_science/><ref name=bow/> A bow shock is a third boundary characteristic of an astrosphere, lying outside the [[heliosphere#Termination shock|termination shock]] and the astropause.<ref name=bow/>

=== Intergalactic space ===
{{Main|Warm–hot intergalactic medium|Intracluster medium|Intergalactic dust}}
[[File:Structure of the Universe.jpg|thumb|alt=Structure of the Universe|Distribution of Matter in a cubic section of the universe. The blue fiber-like structures represent matter, while the empty regions show the [[cosmic void]]s]]

Intergalactic space is the physical space between galaxies. Studies of the large-scale distribution of galaxies show that the universe has a foam-like structure, with [[Galaxy groups and clusters|groups and clusters of galaxies]] lying along filaments that occupy about a tenth of the total space. The remainder forms [[cosmic void]]s that are mostly empty of galaxies. Typically, a void spans a distance of 7–30 megaparsecs.{{sfn|Wszolek|2013|p=67}}

Surrounding and stretching between galaxies is the [[intergalactic medium]] (IGM). This [[rarefaction|rarefied]] plasma<ref name=jafelice_opher1992/> is organized in a [[galaxy filament|galactic filamentary]] structure.<ref name=wadsley2002/> The diffuse photoionized gas contains filaments of higher density, about one atom per cubic meter,<ref name="Harvard & Smithsonian 2022 k265"/> which is 5–200 times the average density of the universe.<ref name="apj_714_1715"/> The IGM is inferred to be mostly primordial in composition, with 76% hydrogen by mass, and enriched with higher mass elements from high-velocity galactic outflows.<ref name="Oppenheimer_Davé_2006"/>

As gas falls into the intergalactic medium from the voids, it heats up to temperatures of 10<sup>5</sup>&nbsp;K to 10<sup>7</sup>&nbsp;K.<ref name=baas41_908/> At these temperatures, it is called the [[warm–hot intergalactic medium]] (WHIM). Although the plasma is very hot by terrestrial standards, 10<sup>5</sup> K is often called "warm" in astrophysics. Computer simulations and observations indicate that up to half of the atomic matter in the universe might exist in this warm–hot, rarefied state.<ref name="apj_714_1715" /><ref name=ssr134_1_141/><ref name="apjs_182_378"/> When gas falls from the filamentary structures of the WHIM into the galaxy clusters at the intersections of the cosmic filaments, it can heat up even more, reaching temperatures of 10<sup>8</sup>&nbsp;K and above in the so-called [[intracluster medium]] (ICM).<ref name="apj546_100"/>

{{multiple image
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| caption_align     = center
| header_background = 
| header_align      = center
| header            = Overview of different scales of space as [[Location of Earth|regions around Earth]]
| image1            = L2 rendering.jpg
| width1            = 93
| caption1          = [[Earth#Earth–Moon system|Earth-Moon System]]
| image2            = Inner solar system objects top view for wiki.png
| width2            = 93
| caption2          = [[Inner Solar System]] with [[Near-Earth object]]s
| image3            = Oort cloud Sedna orbit.svg
| width3            = 93
| caption3          = [[Solar&nbsp;System]] and [[Oort cloud]]
| image4            = Angular map of fusors around Sol within 9ly (large).png
| width4            = 93
| caption4          = [[List of nearest stars and brown dwarfs|Nearest stars]]
| image5            = Local Interstellar Clouds with motion arrows.jpg
| width5            = 93
| caption5          = [[Local Interstellar Cloud]] and neighbouring [[interstellar medium]]
| image6            = Galaxymap.com, map 100 parsecs (2022).png
| width6            = 93
| caption6          = [[List of nearby stellar associations and moving groups|Star association]]s and interstellar medium map of the [[Local Bubble]]
| image7            = Galaxymap.com, map 1000 parsecs (2022).png
| width7            = 93
| caption7          = Molecular clouds around the Sun inside the [[Orion-Cygnus Arm]]
| image8            = OrionSpur.png
| width8            = 93
| caption8          = Orion-Cygnus Arm and neighbouring arms
| image9            = Milky Way Arms ssc2008-10.svg
| width9            = 93
| caption9          = Orion-Cygnus Arm inside the [[Milky&nbsp;Way]]
| image10            = Milky Way side view.png
| width10            = 93
| caption10          = The Sun within the structure of the Milky&nbsp;Way
| image11           = 06-Local Group (LofE06240).png
| width11           = 93
| caption11         = [[Satellite galaxies of the Milky Way]] in [[Local Group]]
| image12           = 07-Laniakea (LofE07240).png
| width12           = 93
| caption12         = [[Virgo Supercluster|Virgo SCl]] in [[Laniakea Supercluster|Laniakea SCl]]
| image13           = Laniakea.gif
| width13           = 93
| caption13         = Laniakea SCl in [[Pisces–Cetus Supercluster Complex]]
| image14           = Observable Universe with Measurements 01.png
| width14           = 93
| caption14         = [[Observable Universe]] of the [[Universe]]
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}}

== History of discovery ==
{{Further|Timeline of knowledge about galaxies, clusters of galaxies, and large-scale structure}}
In 350 BCE, Greek philosopher [[Aristotle]] suggested that ''nature abhors a vacuum'', a principle that became known as the ''[[Horror vacui (physics)|horror vacui]]''. This concept built upon a 5th-century BCE [[Ontology|ontological]] argument by the Greek philosopher [[Parmenides]], who denied the possible existence of a void in space.{{sfn|Grant|1981|p=10}} Based on this idea that a vacuum could not exist, in the West it was widely held for many centuries that space could not be empty.{{sfn|Porter|Park|Daston|2006|p=27}} As late as the 17th century, the French philosopher [[René Descartes]] argued that the entirety of space must be filled.{{sfn|Eckert|2006|p=5}}

In [[ancient China]], the 2nd-century astronomer [[Zhang Heng]] became convinced that space must be infinite, extending well beyond the mechanism that supported the Sun and the stars. The surviving books of the Hsüan Yeh school said that the heavens were boundless, "empty and void of substance". Likewise, the "sun, moon, and the company of stars float in the empty space, moving or standing still".{{sfn|Needham|Ronan|1985|pp=82–87}}

The Italian scientist [[Galileo Galilei]] knew that air has mass and so was subject to gravity. In 1640, he demonstrated that an established force resisted the formation of a vacuum. It would remain for his pupil [[Evangelista Torricelli]] to create an apparatus that would produce a partial vacuum in 1643. This experiment resulted in the first mercury [[barometer]] and created a scientific sensation in Europe. Torricelli suggested that since air has weight, then [[air pressure]] should decrease with altitude.<ref name=West_2013/> The French mathematician [[Blaise Pascal]] proposed an experiment to test this hypothesis.{{sfn|Holton|Brush|2001|pp=267–268}} In 1648, his brother-in-law, Florin Périer, repeated the experiment on the [[Puy de Dôme]] mountain in central France and found that the column was shorter by three inches. This decrease in pressure was further demonstrated by carrying a half-full balloon up a mountain and watching it gradually expand, then contract upon descent.{{sfn|Cajori|1917|pp=64–66}}

[[File:Magedurger Halbkugeln Luftpumpe Deutsches Museum.jpg|thumb|upright|left|The original [[Magdeburg hemispheres]] (left) used to demonstrate Otto von Guericke's vacuum pump (right)|alt=A glass display case holds a mechanical device with a lever arm, plus two metal hemispheres attached to draw ropes.]]

In 1650, German scientist [[Otto von Guericke]] constructed the first [[vacuum pump]]: a device that would further refute the principle of ''horror vacui''. He correctly noted that the atmosphere of the Earth surrounds the planet like a shell, with the density gradually declining with altitude. He concluded that there must be a vacuum between the Earth and the Moon.{{sfn|Genz|2001|pp=127–128}}

In the 15th century, German theologian [[Nicolaus Cusanus]] speculated that the universe lacked a center and a circumference. He believed that the universe, while not infinite, could not be held as finite as it lacked any bounds within which it could be contained.{{sfn|Tassoul|Tassoul|2004|p=22}} These ideas led to speculations as to the infinite dimension of space by the Italian philosopher [[Giordano Bruno]] in the 16th century. He extended the Copernican [[heliocentric]] cosmology to the concept of an infinite universe filled with a substance he called [[Aether (classical element)|aether]], which did not resist the motion of heavenly bodies.{{sfn|Gatti|2002|pp=99–104}} English philosopher [[William Gilbert (astronomer)|William Gilbert]] arrived at a similar conclusion, arguing that the stars are visible to us only because they are surrounded by a thin aether or a void.{{sfn|Kelly|1965|pp=97–107}} This concept of an aether originated with ancient Greek philosophers, including Aristotle, who conceived of it as the medium through which the heavenly bodies move.{{sfn|Olenick|Apostol|Goodstein|1986|p=356}}

The concept of a universe filled with a [[luminiferous aether]] retained support among some scientists until the early 20th century. This form of aether was viewed as the medium through which light could propagate.{{sfn|Hariharan|2003|p=2}} In 1887, the [[Michelson–Morley experiment]] tried to detect the Earth's motion through this medium by looking for changes in the [[speed of light]] depending on the direction of the planet's motion. The [[null result]] indicated something was wrong with the concept. The idea of the luminiferous aether was then abandoned. It was replaced by [[Albert Einstein]]'s theory of [[special relativity]], which holds that the speed of light in a vacuum is a fixed constant, independent of the observer's motion or [[frame of reference]].{{sfn|Olenick|Apostol|Goodstein|1986|pp=357–365}}{{sfn|Thagard|1992|pp=206–209}}

The first professional astronomer to support the concept of an infinite universe was the Englishman [[Thomas Digges]] in 1576.{{sfn|Maor|1991|p=195}} But the scale of the universe remained unknown until the [[List of the most distant astronomical objects#Timeline of most distant astronomical object recordholders|first successful measurement of the distance]] to a nearby star in 1838 by the German astronomer [[Friedrich Bessel]]. He showed that the star system [[61 Cygni]] had a [[stellar parallax|parallax]] of just 0.31&nbsp;[[arcsecond]]s (compared to the modern value of 0.287″). This corresponds to a distance of over 10 [[light year]]s.{{sfn|Webb|1999|pp=71–73}} In 1917, [[Heber Doust Curtis|Heber Curtis]] noted that [[nova]]e in spiral nebulae were, on average, 10 magnitudes fainter than galactic novae, suggesting that the former are 100 times further away.<ref name=Curtis1988/> The distance to the [[Andromeda Galaxy]] was determined in 1923 by American astronomer [[Edwin Hubble]] by measuring the brightness of [[cepheid variable]]s in that galaxy, a new technique discovered by [[Henrietta Leavitt]].<ref name=csiro_20041025/> This established that the Andromeda Galaxy, and by extension all galaxies, lay well outside the Milky Way.{{sfn|Tyson|Goldsmith|2004|pp=114–115}} With this Hubble formulated the [[Hubble constant]], which allowed for the first time a calculation of the age of the Universe and size of the Observable Universe, starting at 2 billion years and 280 million light-years. This became increasingly precise with better measurements, until 2006 when data of the [[Hubble Space Telescope]] allowed a very accurate calculation of the age of the Universe and size of the Observable Universe.<ref name="p537"/>

The modern concept of outer space is based on the [[Big Bang cosmology|"Big Bang" cosmology]], first proposed in 1931 by the Belgian physicist [[Georges Lemaître]].<ref name=nature127_3210_706/> This theory holds that the universe originated from a state of extreme energy density that has since undergone [[Hubble's law|continuous expansion]].<ref name=Big_Bang/>

The earliest known estimate of the temperature of outer space was by the Swiss physicist [[Charles Édouard Guillaume|Charles É. Guillaume]] in 1896. Using the estimated radiation of the background stars, he concluded that space must be heated to a temperature of 5–6&nbsp;K. British physicist [[Arthur Eddington]] made a similar calculation to derive a temperature of 3.18&nbsp;K in 1926. German physicist [[Erich Regener]] used the total measured energy of [[cosmic ray]]s to estimate an intergalactic temperature of 2.8&nbsp;K in 1933.<ref name="Apeiron2_3_79"/> American physicists [[Ralph Alpher]] and [[Robert Herman]] predicted 5&nbsp;K for the temperature of space in 1948, based on the gradual decrease in background energy following the then-new [[Big Bang]] theory.<ref name="Apeiron2_3_79"/>

== Exploration ==
{{Main|Space exploration|Human presence in space}}
{{See also|Astronautics|Spaceflight|Human spaceflight}}
[[File:As08-16-2593.jpg|thumb|left|South is up in the [[first image of Earth]] taken by a person,<ref name="Apollo8FlightJournalDay1"/> probably by [[Bill Anders]] (during the 1968 [[Apollo 8]] mission)]]

For most of human history, space was explored by observations made from the Earth's surface—initially with the unaided eye and then with the telescope. Before reliable rocket technology, the closest that humans had come to reaching outer space was through balloon flights. In 1935, the American ''[[Explorer II]]'' crewed balloon flight reached an altitude of {{Convert|22|km|mi|abbr=on}}.<ref name=ssr13_2_199/> This was greatly exceeded in 1942 when the third launch of the German [[V-2 rocket|A-4 rocket]] climbed to an altitude of about {{Convert|80|km|mi|abbr=on}}. In 1957, the uncrewed satellite [[Sputnik 1]] was launched by a Russian [[R-7 Semyorka|R-7 rocket]], achieving Earth orbit at an altitude of {{Convert|215|-|939|km|mi}}.{{sfn|O'Leary|2009|pp=209–224}} This was followed by the first human spaceflight in 1961, when [[Yuri Gagarin]] was sent into orbit on [[Vostok 1]]. The first humans to escape low Earth orbit were [[Frank Borman]], [[Jim Lovell]] and [[William Anders]] in 1968 on board the American [[Apollo 8]], which achieved lunar orbit{{sfn|Harrison|2002|pp=60–63}} and reached a maximum distance of {{Convert|377349|km|mi|abbr=on}} from the Earth.{{sfn|Orloff|2001}}

The first spacecraft to reach escape velocity was the Soviet ''[[Luna 1]]'', which performed a fly-by of the Moon in 1959.{{sfn|Hardesty|Eisman|Krushchev|2008|pp=89–90}} In 1961, ''[[Venera 1]]'' became the first planetary probe. It revealed the presence of the solar wind and performed the first fly-by of [[Venus]], although contact was lost before reaching Venus. The first successful planetary mission was the 1962 fly-by of Venus by [[Mariner 2]].{{sfn|Collins|2007|p=86}} The first fly-by of Mars was by [[Mariner 4]] in 1964. Since that time, uncrewed spacecraft have successfully examined each of the Solar System's planets, as well their moons and many [[minor planet]]s and comets. They remain a fundamental tool for the exploration of outer space, as well as for observation of the Earth.{{sfn|Harris|2008|pp=7, 68–69}} In August 2012, ''[[Voyager 1]]'' became the first man-made object to leave the Solar System and enter [[interstellar space]].<ref name="mike_wall"/>

==Application==
{{See also|Space science|Benefits of space exploration|Earth observation|Commercialization of space|Space habitation}}
[[File:ISS-44 Milky Way.jpg|thumb|300px|View from [[International Space Station]], showing the yellow-green [[airglow]] of Earth's [[ionosphere]] with the Milky Way in the background.]]
Outer space has become an important element of global society. It provides multiple applications that are beneficial to the economy and scientific research.

The placing of artificial satellites in Earth orbit has produced numerous benefits and has become the dominating sector of the [[space economy]]. They allow relay of [[Communications satellite|long-range communications]] like television, provide a means of [[Satellite navigation|precise navigation]], and permit direct monitoring of [[Weather satellite|weather conditions]] and [[remote sensing]] of the Earth. The latter role serves a variety of purposes, including tracking soil moisture for agriculture, prediction of water outflow from seasonal snow packs, detection of diseases in plants and trees, and [[Spy satellite|surveillance]] of military activities.{{sfn|Razani|2012|pp=97–99}} They facilitate the discovery and monitoring of [[climate change]] influences.<ref name="Space Foundation 2023 k582"/> Satellites make use of the significantly reduced drag in space to stay in stable orbits, allowing them to efficiently span the whole globe, compared to for example [[stratospheric balloon]]s or [[high-altitude platform station]]s, which have other benefits.<ref name="Bisset 2023 s159"/>

The absence of air makes outer space an ideal location for astronomy at all wavelengths of the [[electromagnetic spectrum]]. This is evidenced by the pictures sent back by the Hubble Space Telescope, allowing light from more than 13&nbsp;billion years ago—almost to the time of the Big Bang—to be observed.<ref name=hubblesite_cosmicdawn /> Not every location in space is ideal for a telescope. The [[Interplanetary dust cloud|interplanetary zodiacal dust]] emits a diffuse near-infrared radiation that can mask the emission of faint sources such as extrasolar planets. Moving an [[infrared telescope]] out past the dust increases its effectiveness.<ref name=esa105/> Likewise, a site like the [[Daedalus (crater)|Daedalus crater]] on the [[far side of the Moon]] could shield a [[radio telescope]] from the [[Electromagnetic interference|radio frequency interference]] that hampers Earth-based observations.<ref name=maccone2001/>

[[File:Solardisk.jpg|right|thumb|Concept for a [[space-based solar power]] system to beam energy down to Earth<ref name=BBC_2020/>]]
The deep vacuum of space could make it an attractive environment for certain industrial processes, such as those requiring ultraclean surfaces.<ref name=chapman1991/> Like [[asteroid mining]], [[space manufacturing]] would require a large financial investment with little prospect of immediate return.<ref name="IJA10_307"/> An important factor in the total expense is the high cost of placing mass into Earth orbit: ${{Inflation|US|6000|2006|r=-3|fmt=c}}–${{Inflation|US|20000|2006|r=-3|fmt=c}} per kg, according to a 2006 estimate (allowing for inflation since then).<ref name="jsr43_3_696"/> The cost of access to space has declined since 2013. Partially reusable rockets such as the [[Falcon 9]] have lowered access to space below $3,500 per kg. With these new rockets the cost to send materials into space remains prohibitively high for many industries. Proposed concepts for addressing this issue include, fully [[reusable launch system]]s, [[non-rocket spacelaunch]], [[momentum exchange tether]]s, and [[space elevators]].{{sfn|Bolonkin|2010|p=xv}}

[[Interstellar travel]] for a human crew remains at present only a theoretical possibility. The distances to the nearest stars mean it would require new technological developments and the ability to safely sustain crews for journeys lasting several decades. For example, the [[Project Daedalus|Daedalus Project]] study, which proposed a spacecraft powered by the [[nuclear fusion|fusion]] of [[deuterium]] and [[helium-3]], would require 36 years to reach the "nearby" [[Alpha Centauri]] system. Other proposed interstellar propulsion systems include [[light sail]]s, [[Bussard ramjet|ramjets]], and [[beam-powered propulsion]]. More advanced propulsion systems could use [[antimatter]] as a fuel, potentially reaching [[relativistic speed|relativistic velocities]].<ref name=Crawford1990/>

From the Earth's surface, the ultracold temperature of outer space can be used as a [[Renewable energy|renewable]] cooling technology for various applications on Earth through [[passive daytime radiative cooling]].<ref name=Yu_et_al_2022/><ref name=Ma_2021/> This enhances [[Long-wave infrared|longwave infrared]] (LWIR) [[thermal radiation]] heat transfer through the atmosphere's [[infrared window]] into outer space, lowering ambient temperatures.<ref name="Zevenhovena-2018"/><ref name="Wang-2021"/> [[Photonic metamaterial]]s can be used to suppress solar heating.<ref name="Heo-2022"/>

== See also ==
{{div col|colwidth=20em}}
* [[Absolute space and time]]
* [[Artemis Accords]]
* [[List of government space agencies]]
* [[List of topics in space]]
* [[Olbers' paradox]]
* [[Outline of space science]]
* [[Panspermia]]
* [[Space art]]
* [[Space and survival]]
* [[Space race]]
* [[Space station]]
* [[Space technology]]
* [[Timeline of knowledge about the interstellar and intergalactic medium]]
* [[Timeline of Solar System exploration]]
* [[Timeline of spaceflight]]
{{div col end}}

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{{refend}}

== External links ==
{{sister project links|voy=Space|v=Category:Astronomy|d=Q4169|n=Category:Space|s=Category:Astronomy|b=Category:Subject:Astronomy}}

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