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'{{pp-move-indef}} {{otheruses}} [[Image:Atmosphere layers-en.svg|thumb|The boundaries between the Earth's surface and outer space, at the [[Kármán line]], 100 km (62 mi) and [[exosphere]] at 690 km (430 mi).]] '''Outer space''' (often simply called '''space''') comprises the relatively empty regions of the [[universe]] outside the [[atmosphere]]s of [[celestial bodies]]. ''Outer'' space is used to distinguish it from [[airspace]] and terrestrial locations. Contrary to popular understanding, outer space is not completely empty (i.e. a [[vacuum|perfect vacuum]]), but contains a low density of particles, predominantly hydrogen [[plasma (physics)|plasma]], as well as [[electromagnetic radiation]] and [[cosmic neutrino background|neutrinos]]. Hypothetically, it also contains [[dark matter]] and [[dark energy]]. The term ''outer space'' was first recorded by the [[English people|English]] poet Lady [[Emmeline Stuart-Wortley]] in her poem "The Maiden of Moscow" in 1842,<ref>OED[outer space]</ref> and also later attested to the writings of [[HG Wells]] in 1901.<ref name="entymonline">{{cite web|title=Etymonline : Outer|url=http://www.etymonline.com/index.php?search=outer+space&searchmode=none|accessdate=2008-03-24}}</ref>. The shorter term ''space'' is actually older, first used to mean the region beyond Earth's sky in [[John Milton]]'s ''[[Paradise Lost]]'' in 1667.<ref>{{cite web |url=http://www.etymonline.com/index.php?term=space |title=Space |date=November 2001 |author=Douglas Harper |publisher=The Online Etymology Dictionary |accessdate=2009-06-19}}</ref> == Environment == Outer space is the closest approximation of a [[perfect vacuum]]. It has effectively no [[friction]], allowing [[star]]s, [[planet]]s and [[moon]]s to move freely along ideal gravitational trajectories. But no vacuum is truly perfect, not even in [[intergalactic space]] where there are still a few hydrogen atoms per cubic centimeter. (For comparison, the air we breathe contains about 10¹⁹ molecules per cubic centimeter.) The deep vacuum of space could make it an attractive environment for certain industrial processes, for instance those that require ultraclean surfaces. Stars, planets, asteroids, and moons keep their [[atmosphere]]s by gravitational attraction, and as such, atmospheres have no clearly delineated boundary: the density of atmospheric gas simply decreases with distance from the object. The Earth's atmospheric pressure drops to about 1 Pa at {{convert|100|km|mi}} of altitude, the [[Kármán line]] which is a common definition of the boundary with outer space. Beyond this line, isotropic gas pressure rapidly becomes insignificant when compared to [[radiation pressure]] from the [[sun]] and the [[dynamic pressure]] of the [[solar wind]], so the definition of pressure becomes difficult to interpret. The [[thermosphere]] in this range has large gradients of pressure, temperature and composition, and varies greatly due to [[space weather]]. Astrophysicists prefer to use [[number density]] to describe these environments, in units of particles per cubic centimetre. === Temperature === All of the observable [[universe]] is filled with large numbers of [[photon]]s, created during the [[Big Bang]], the so-called [[cosmic background radiation]], and quite likely a correspondingly large number of [[neutrino]]s called the [[cosmic neutrino background]]. The current [[temperature]] of the photon radiation is about {{convert|3|K|C F|2|lk=on}}. === Effect on human bodies === {{seealso|Space exposure}} Contrary to popular belief,<ref>{{cite web |url=http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/970603.html |title=Human Body in a Vacuum |date=June 03, 1997 |publisher=NASA |accessdate=2009-06-19 }}</ref> a person suddenly exposed to the [[vacuum]] would not explode, [[hypothermia|freeze to death]] or die from boiling blood, but would take a short while to die by [[asphyxia]]tion (suffocation). [[Air]] would immediately leave the [[lungs]] due to the enormous [[pressure gradient]]. Any [[oxygen]] dissolved in the blood would empty into the lungs to try to equalize the [[partial pressure]] gradient. Once the deoxygenated blood arrives at the brain, death would quickly follow. Humans and animals exposed to vacuum will lose [[consciousness]] after a few seconds and die of [[hypoxia]] within minutes. [[Blood]] and other body fluids do boil when their pressure drops below 6.3 kPa, the [[vapor pressure]] of water at body temperature.<ref name="harding">{{cite book |author=Harding, Richard M |title=Survival in Space: Medical Problems of Manned Spaceflight |year=1989 |publisher=Routledge |isbn=0-415-00253-2}}</ref> This condition is called [[ebullism]]. 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.<ref>{{cite book |first=Charles E. |last=Billings |editor=edited by James F. Parker and Vita R. West |year=1973 |title=Bioastronautics Data Book |edition=Second |publisher=NASA |id=NASA SP-3006 |chapter=Barometric Pressure}}</ref><ref>{{cite web |url=http://www.geoffreylandis.com/vacuum.html |title=Human Exposure to Vacuum |date=7 August 2007 |author=Geoffrey A. Landis |publisher=www.geoffreylandis.com |accessdate=2009-06-19}}</ref> Swelling and ebullism can be reduced by containment in a [[flight suit]]. [[Space Shuttle program|Shuttle]] astronauts wear a fitted elastic garment called the Crew Altitude Protection Suit (CAPS) which prevents ebullism at pressures as low as 2 kPa.<ref>{{cite journal | author=Webb P. | title= The Space Activity Suit: An Elastic Leotard for Extravehicular Activity | journal=Aerospace Medicine | year=1968 | volume=39 | issue= | pages= 376&ndash;383 | url= }}</ref> [[Water vapor]] would also rapidly [[evaporate]] off from exposed areas such as the lungs, [[cornea]] of the [[eye]] and mouth, cooling the body. Rapid evaporative cooling of the skin will create frost, particularly in the mouth, but this is not a significant hazard. Space may be cold, but it's mostly vacuum and transfers heat ineffectually; as a result the main temperature regulation concern for space suits is how to get rid of naturally generated body heat. Cold or oxygen-rich atmospheres can sustain life at pressures much lower than atmospheric, as long as the density of oxygen is similar to that of standard sea-level atmosphere. The colder air temperatures found at altitudes of up to {{convert|3|km|mi}} generally compensate for the lower pressures there.<ref name="harding" /> Above this altitude, oxygen enrichment is necessary to prevent [[altitude sickness]], and [[spacesuit]]s are necessary to prevent ebullism above {{convert|19|km|mi}}.<ref name="harding" /> Most spacesuits use only 20 kPa of pure oxygen, just enough to sustain full consciousness. This pressure is high enough to prevent ebullism, but simple [[evaporation]] of blood can still cause [[decompression sickness]] and [[air embolism|gas embolisms]] if not managed. Rapid [[decompression]] can be much more dangerous than vacuum exposure itself. Even if the victim does not hold his breath, venting through the windpipe may be too slow to prevent the fatal rupture of the delicate [[alveoli]] of the [[lung]]s.<ref name="harding" /> [[Eardrum]]s and sinuses may be ruptured by rapid decompression, soft tissues may bruise and seep blood, and the stress of shock will accelerate oxygen consumption leading to [[hypoxia]].<ref>{{cite web |url=http://www.geoffreylandis.com/ebullism.html |title=Ebullism at 1 Million Feet: Surviving Rapid/Explosive Decompression |author=Tamarack R. Czarnik |publisher=www.geoffreylandis.com |accessdate=2009-06-19}}</ref> Injuries caused by rapid decompression are called [[barotrauma]]. A pressure drop as small as 13 kPa, which produces no symptoms if it is gradual, may be fatal if it occurs suddenly.<ref name="harding"/> == Boundary == Traditionally, there was no clear boundary between [[Earth's atmosphere]] and space, as the [[density]] of the atmosphere gradually decreases as the [[altitude]] increases. Nevertheless, several boundaries have been set, namely: * The [[Fédération Aéronautique Internationale]] has established the [[Kármán line]] at an altitude of {{convert|100|km|mi}} as a working definition for the boundary between aeronautics and astronautics. This is used because above an altitude of roughly 100 km, as [[Theodore von Kármán]] calculated, a vehicle would have to travel faster than [[orbital velocity]] in order to derive sufficient [[aerodynamic lift]] from the atmosphere to support itself. * The [[United States]] designates people who travel above an altitude of {{convert|50|mi|km}} as [[astronaut]]s. * [[NASA]]'s mission control uses {{convert|76|mi}} as their [[atmospheric reentry|re-entry]] altitude, which roughly marks the boundary where [[atmospheric drag]] becomes noticeable, (depending on the [[ballistic coefficient]] of the vehicle), thus leading shuttles to switch from steering with thrusters to maneuvering with air surfaces. In 2009, scientists at the [[University of Calgary]] reported detailed measurements with an instrument called the Supra-Thermal Ion Imager (an instrument that measures the direction and speed of ions), which allowed them to determine that space begins {{convert|118|km}} above Earth. The boundary represents the midpoint of a gradual transition over tens of kilometers from the relatively gentle winds of the Earth's atmosphere to the more violent flows of charged particles in space, which can reach speeds well over {{convert|600|mph|-3}}.<ref name="Edge of space">{{cite web |url=http://www.space.com/scienceastronomy/090409-edge-space.html |title=Edge of Space Found |date=09 April 2009 |author=Andrea Thompson |publisher=space.com |accessdate=2009-06-19 }}</ref><ref>{{cite journal |author=L. Sangalli, D. J. Knudsen, M. F. Larsen, T. Zhan, R. F. Pfaff, and D. Rowland |date=2009 |title=Rocket-based measurements of ion velocity, neutral wind, and electric field in the collisional transition region of the auroral ionosphere |journal=Journal of Geophysical Research |volume=114 |pages=A04306 |url=http://www.agu.org/pubs/crossref/2009/2008JA013757.shtml |publisher=American Geophysical Union |doi=10.1029/2008JA013757 |accessdate=2009-06-19}}</ref> This was only the second time that direct measurements of charged particle flows have been conducted at this region, which is too high for balloons and too low for satellites. It was however the first study to include all the relevant elements for this kind of determination – for example, the upper atmospheric winds. The instrument was carried by the JOULE-II rocket on January 19, 2007, and traveled to an altitude of about {{convert|124|mi}}. From there it collected data while it was moving through the "edge of space".<ref name="Edge of space" /> == Space versus Orbit == To perform an [[orbital spaceflight|orbit]], a spacecraft must travel faster than a [[sub-orbital spaceflight]]. A spacecraft has not entered [[orbit]] until it is traveling with a sufficiently great horizontal velocity such that the [[acceleration]] due to [[gravity]] on the spacecraft is less than or equal to the [[Centripetal force|centripetal]] acceleration being caused by its horizontal velocity (see [[circular motion]]). So to enter [[orbit]], a spacecraft must not only reach space, but must also achieve a sufficient [[orbital speed]] ([[angular velocity]]). For a low-Earth orbit, this is about {{convert|7900|m/s|km/h mph|2|lk=on|abbr=on}}; by contrast, the fastest airplane speed ever achieved (excluding speeds achieved by deorbiting spacecraft) was {{convert|2200|m/s|km/h mph|2|abbr=on}} in 1967 by the North American [[X-15]].<ref>{{cite web |url=http://www.airspacemag.com/history-of-flight/x-15_walkaround.html |title=X-15 Walkaround |date=November 01, 2007 |author=Linda Shiner |publisher=Air & Space Magazin |accessdate=2009-06-19}}</ref> [[Konstantin Tsiolkovsky]] was the first person to realize that, given the [[energy]] available from any available [[chemistry|chemical]] [[fuel]], a several-stage [[rocket]] would be required. The [[escape velocity]] to pull free of Earth's gravitational field altogether and move into [[interplanetary space]] is about {{convert|11000|m/s|km/h mph|2|abbr=on}} The energy required to reach velocity for low Earth orbit ([[joule|32&nbsp;MJ/kg]]) is about twenty times the energy required simply to climb to the corresponding altitude (10&nbsp;kJ/(km·kg)). There is a major difference between [[sub-orbital spaceflight|sub-orbital]] and [[orbital spaceflight]]s. The minimum altitude for a stable orbit around Earth (that is, one without significant [[atmospheric drag]]) begins at around {{convert|350|km|mi}} above mean sea level. A common misunderstanding about the boundary to space is that orbit occurs simply by reaching this altitude. Achieving orbital speed can theoretically occur at any altitude, although atmospheric drag precludes an orbit that is too low. At sufficient speed, an airplane would need a way to keep it from flying off into space, but at present, this speed is several times greater than anything within reasonable technology.<!-- an airplane can't achieve orbit since there is insufficient oxygen at the altitudes needed to attain an orbit at the speed a plane can travel. To oxidize the fuel, an airplane must carry its own oxidant, and at that point it is no longer an airplane but a "rocket" or a "space plane".--> A common misconception is that people in orbit are outside Earth's [[gravity]] because they are "floating". They are floating because they are in "[[free fall]]": they are accelerating toward Earth, along with their spacecraft, but are simultaneously moving sideways fast enough that the "fall" away from a straight-line path merely keeps them in orbit at a constant distance above Earth's surface. Earth's gravity reaches out far past the [[Van Allen radiation belt|Van Allen belt]] and keeps the Moon in orbit at an average distance of {{convert|384403|km|mi}}. == Regions == Space being not a perfect [[vacuum]], its different regions are defined by the various atmospheres and "winds" that dominate within them, and extend to the point at which those winds give way to those beyond. Geospace extends from Earth's atmosphere to the outer reaches of Earth's magnetic field, whereupon it gives way to the [[solar wind]] of interplanetary space. Interplanetary space extends to the [[heliopause]], whereupon the solar wind gives way to the winds of the interstellar medium. Interstellar space then continues to the edges of the galaxy, where it fades into the intergalactic void. === Geospace === [[Image:Aurora-SpaceShuttle-EO.jpg|thumb|300px|right|[[Aurora (astronomy)|Aurora australis]] observed by [[Space Shuttle Discovery|''Discovery'']], May [[1991]].]] '''Geospace''' is the region of outer space near the Earth. Geospace includes the upper region of the [[Earth's atmosphere|atmosphere]], as well as the [[ionosphere]] and [[magnetosphere]]. The [[Van Allen radiation belt]]s also lie within the geospace. The region between Earth's atmosphere and the [[Moon]] is sometimes referred to as '''cis-lunar space'''. Although it meets the definition of outer space, the atmospheric density within the first few hundred kilometers above the Kármán line is still sufficient to produce significant [[Drag (physics)|drag]] on [[satellite]]s. Most artificial satellites operate in this region called [[low earth orbit]] and must fire their engines every few days to maintain orbit. The drag here is low enough that it could theoretically be overcome by radiation pressure on [[solar sail]]s, a proposed propulsion system for [[interplanetary travel]]. Planets are too massive for their trajectories to be affected by these forces, although their atmospheres are eroded by the solar winds. Geospace is populated at very low densities by electrically charged particles, whose motions 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. During [[geomagnetic storm]]s two regions of geospace, the radiation belts and the ionosphere, can become strongly disturbed. These disturbances interfere with the functioning of satellite communications and navigation ([[GPS]]) technologies. These storms increase fluxes of energetic electrons that can permanently damage satellite electronics, and can also be a hazard to astronauts, even in [[low-Earth orbit]]. Geospace contains material left over from previous manned and unmanned launches that are a potential hazard to [[spacecraft]]. Some of this [[space debris|debris]] re-enters Earth's atmosphere periodically. The absence of [[air]] makes geospace (and the surface of the [[Moon]]) ideal locations for [[astronomy]] at all wavelengths of the [[electromagnetic spectrum]], as evidenced by the spectacular pictures sent back by the [[Hubble Space Telescope]], allowing light from about 13.7&nbsp;billion years ago — almost to the time of the Big Bang — to be observed. The outer boundary of geospace is the interface between the magnetosphere and the solar wind. The inner boundary is the ionosphere.<ref>{{cite web |url=http://www.lws.nasa.gov/documents/geospace/geospace_gmdt_report.pdf |format=PDF |title=Report of the Living With a Star Geospace Mission Definition Team |month=September |year=2002 | publisher=NASA |accessdate=2007-12-19}}</ref> Alternately, geospace is the region of space between the Earth’s upper atmosphere and the outermost reaches of the Earth’s magnetic field.<ref>{{cite web |url=http://www.lws.nasa.gov/missions/geospace/geospace.htm |title=LWS Geospace Missions |publisher=NASA |accessdate=2007-12-19}}</ref> === Interplanetary === {{main|Interplanetary medium}} Outer space within the [[solar system]] is called '''interplanetary space''', which passes over into [[interstellar medium|interstellar space]] at the [[heliopause]]. The [[vacuum]] of outer space is not really empty; it is sparsely filled with [[cosmic ray]]s, which include [[ion]]ized [[atomic nucleus|atomic nuclei]] and various [[subatomic particle]]s. There is also gas, [[Plasma (physics)|plasma]] and dust, small [[meteor]]s, and several dozen types of [[organic chemistry|organic]] [[molecule]]s discovered to date by [[rotational spectroscopy|microwave spectroscopy]]. Interplanetary space is defined by the [[solar wind]], a continuous stream of charged particles emanating from the Sun that creates a very tenuous atmosphere (the [[heliosphere]]) for billions of miles into space. The discovery since 1995 of [[extrasolar planet]]s means that other stars must possess their own interplanetary media. === Interstellar === {{main|Interstellar medium}} '''Interstellar space''' is the physical space within a [[galaxy]] not occupied by [[star]]s or their [[planetary system]]s. The [[interstellar medium]] resides – by definition – in interstellar space. === Intergalactic === {{main|Intracluster medium|Cosmic microwave background}} '''Intergalactic space''' is the physical space between [[galaxy|galaxies]]. Generally free of dust and debris, intergalactic space is very close to a total [[vacuum]]. Certainly, the space between galaxy clusters, called the [[void (astronomy)|voids]], is nearly empty. Some theories put the average density of the [[universe]] as the equivalent of one hydrogen [[atom]] per cubic meter.<ref>Davidson, Keay & Smoot, George. Wrinkles in Time. New York: Avon, 2008: 158-163</ref><ref>Silk, Joseph. Big Bang. New York: Freeman, 1977: 299.</ref> The density of the universe, however, is clearly not uniform; it ranges from relatively high density in galaxies (including very high density in structures within galaxies, such as [[planet]]s, [[star]]s, and [[black hole]]s) to conditions in vast voids that have much lower density than the universe's average. Surrounding and stretching between galaxies, there is a [[rarefaction|rarefied]] plasma<ref>{{cite web |url=http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1992MNRAS.257..135J |title=The origin of intergalactic magnetic fields due to extragalactic jets |date=July 1992 |author=Jafelice, Luiz C. and Opher, Reuven |publisher=Royal Astronomical Society |accessdate=2009-06-19}}</ref><ref>{{cite web |url=http://www.plasma-universe.com/index.php/99.999%25_plasma |title=99.999% plasma |date=25 May 2009 |publisher=The Plasma Universe Wikipedia-like Encyclopedia |accessdate=2009-06-19}}</ref> that is thought to possess a [[Galaxy filament|cosmic filamentary structure]]<ref>{{cite web |url=http://antwrp.gsfc.nasa.gov/apod/ap020820.html |title=The Universe in Hot Gas |date=20 August 2002 |author=James Wadsley et al. |publisher=NASA |accessdate=2009-06-19}}</ref> and that is slightly denser than the average density in the universe. This material is called the ''intergalactic medium (IGM)'' and is mostly [[ionization|ionized]] [[hydrogen]], i.e. a [[Plasma (physics)|plasma]] consisting of equal numbers of [[electron]]s and [[proton]]s. The IGM is thought to exist at a density of 10 to 100 times the average density of the universe (10 to 100 hydrogen atoms per cubic meter). It reaches densities as high as 1000 times the average density of the universe in rich [[galaxy clusters|clusters of galaxies]]. The reason the IGM is thought to be mostly [[Plasma (physics)|ionized gas]] is that its temperature is thought to be quite high by terrestrial standards (though some parts of it are only "warm" by astrophysical standards). As gas falls into the Intergalactic Medium from the voids, it heats up to temperatures of <math>10^5</math> [[kelvin|K]] to <math>10^7</math> K, which is high enough for the bound electrons to escape from the hydrogen nuclei upon collisions. At these temperatures, it is called the Warm-Hot Intergalactic Medium (WHIM). Computer simulations indicate that on the order of half the atomic matter in the universe might exist in this warm-hot, rarefied state. 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 <math>10^8</math> K and above. == References == {{reflist}} == See also == {{portal|Astronomy|Crab Nebula.jpg}} {{portal|Space|Q space.svg}} {{portal|Spaceflight|RocketSunIcon.svg}} <div style="-moz-column-count:2; column-count:2;"> * [[Inner space]] * [[Interplanetary Internet]] * [[Kármán line]] * [[NASA]] * [[Outer Space Treaty]] * [[Private spaceflight]] * [[Solar wind]] * [[Space and survival]] * [[Space colonization]] * [[Space exploration]] * [[Space science]] * [[Space station]] * [[Space technology]] * [[Spaceflight]] * [[Timeline of spaceflight]] </div> == External links == {{Wiktionary|intergalactic}} {{Wikiquotepar|space}} {{commonscat|Space}} {{Wikinewspar|Portal:Space}} * [http://articles.findarticles.com/p/articles/mi_m1134/is_n1_v107/ai_20517887 Intergalactic Space], [[Natural History (magazine)|Natural History]], Feb 1998 * [http://www.cstar.com/ Morgan Freeman's Space Exploration Channel "Our Space" on ClickStar] * [http://money.cnn.com/2006/02/27/technology/business2_guidetospaceintro Profits set to soar in outer space] * [http://www.newscientistspace.com Newscientist Space] * [http://www.xprize.org X PRIZE Foundation] * [http://freeimages.reliable-facts.com/earth_and_space/ Images of Earth and space taken from outer space] * [http://articles.findarticles.com/p/articles/mi_m1134/is_n1_v107/ai_20517887 Intergalactic Space], [[Natural History (magazine)|Natural History]], Feb 1998 [[Category:Astronomical objects]] [[Category:Environments]] [[Category:Extragalactic astronomy]] [[Category:Intergalactic media]] [[Category:Large-scale structure of the cosmos]] [[Category:Plasma physics]] [[Category:Space]] [[Category:Space exploration]] [[Category:Space plasmas]] [[Category:Vacuum]] [[ar:فضاء خارجي]] [[bn:মহাশূন্য]] [[zh-min-nan:Thài-khong]] [[ca:Espai exterior]] [[cs:Kosmický prostor]] [[cy:Gofod rhyngalaethol]] [[da:Ydre rum]] [[de:Intergalaktisches Medium]] [[el:Διάστημα]] [[es:Espacio exterior]] [[es:Espacio intergaláctico]] [[fa:فضای بیرونی]] [[fi:Avaruus]] [[fr:Espace (cosmologie)]] [[he:חלל]] [[hi:अन्तरिक्ष]] [[hu:Világűr]] [[id:Luar angkasa]] [[is:Geimur]] [[it:Spazio (astronomia)]] [[it:Spazio intergalattico]] [[ja:宇宙空間]] [[lo:ອາວະກາດ]] [[lt:Kosminė erdvė]] [[lv:Kosmiskā telpa]] [[nl:Ruimte (astronomie)]] [[oc:Espaci (cosmologia)]] [[pl:Materia międzygalaktyczna]] [[pl:Przestrzeń kosmiczna]] [[pt:Espaço sideral]] [[qu:Ch'usaq pacha]] [[ru:Космическое пространство]] [[ru:Межгалактическое пространство]] [[sco:Ooter space]] [[se:Gomuvuohta]] [[simple:Outer space]] [[sk:Medzigalaktický priestor]] [[sl:Vesoljski prostor]] [[sr:Међугалактички простор]] [[sv:Rymden]] [[th:ช่องว่างระหว่างดาราจักร]] [[th:อวกาศ]] [[uk:Космічний простір]] [[ur:فضائے بسیط]] [[yi:דערויסענדער ספעיס]] [[zh-yue:太空]] [[zh:外层空间]]'