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എവിടെ നിന്നാണ് ബഹിരാകാശം തുടങ്ങുന്നത് എന്നതിനു പ്രത്യേകിച്ചു ഉത്തരം ശാസ്ത്രസമൂഹം നൽകിയിട്ടില്ല. പക്ഷേ സമുദ്രനിരപ്പിൽനിന്നും 100കി.മീ മുകളിൽ {{sfn|O'Leary|2009|p=84}} കർമാൻ ലൈനിൽ ബഹിരാകാശം തുടങ്ങുന്നതായാണ് സാധാരണ എല്ലാ ബഹിരാകാശകരാറുകളിലും പരാമർശിക്കാറുള്ളത്. 1967-ൽ അന്താരാഷ്ട്ര ബഹിരാകാശ നിയമത്തിനുവെണ്ടി ഐക്യരാഷ്ട്രസഭ ബഹിരാകാശക്കരാർ പാസാക്കി. ഈ കരാർ എല്ലാ രാജ്യങ്ങൾക്കും ബഹിരാകാശപര്യവേഷണങ്ങൾക്കു പൂർണ്ണ സ്വാതന്ത്ര്യം നൽകുന്നു. <!-- In 1979, the [[Moon Treaty]] made the surfaces of objects such as planets, as well as the orbital space around these bodies, the jurisdiction of the international community. --> ബഹിരാകാശം സമാധാനപരമായി ഉപയോഗിക്കാൻ യു.എൻ. വ്യവസ്ഥകളുണ്ടെങ്കിലും ഉപഗ്രഹവേധ ആയുധങ്ങൾ നിർമ്മിക്കുകയും ഭൂമിയുടെ ഭ്രമണപഥത്തിൽ പരീക്ഷിക്കുകയും ചെയ്തിട്ടുണ്ട്.
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Humans began the physical exploration of space during the 20th century with the advent of high-altitude [[Balloon (aircraft)|balloon flights]], followed by manned [[rocket launch]]es. [[Earth orbit]] was first achieved by [[Yuri Gagarin]] of the Soviet Union in 1961 and [[unmanned spacecraft]] have since reached all of the known [[planet]]s in the [[Solar System]]. Achieving [[low Earth orbit]] requires a minimum velocity of {{Convert|28100|kph|mph|abbr=on}}, much faster than any conventional aircraft. Outer space represents a challenging environment for human exploration because of the dual hazards of vacuum and [[radiation]]. [[Microgravity]] has a negative effect on human [[physiology]], causing [[muscle atrophy]] and [[Spaceflight osteopenia|bone loss]]. [[spaceflight|Space travel]] has been limited to low Earth orbit and the [[Moon]] for manned flight, and the [[Voyager program|vicinity of the Solar System]] for unmanned vehicles. In August 2012, [[Voyager 1]] became the first man-made craft to enter interstellar space.
== Discovery ==
In 350 BC, 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 [[Western culture|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 [[History of China#Ancient China|ancient China]], there were various schools of thought concerning the nature of the heavens, some of which bear a resemblance to the modern understanding. In 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 had mass and so was subject to gravity. In 1640, he demonstrated that an established force resisted the formation of a vacuum. However, it would remain for his pupil [[Evangelista Torricelli]] to create an apparatus that would produce a vacuum in 1643. This experiment resulted in the first mercury [[barometer]] and created a scientific sensation in Europe. The French mathematician [[Blaise Pascal]] reasoned that if the column of mercury was supported by air then the column ought to be shorter at higher altitude where the [[air pressure]] is lower.{{sfn|Holton|Brush|2001|pp=267–268}} In 1648, his brother-in-law, Florin Périer, repeated the experiment on the [[Puy-de-Dôme (mountain)|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 inflate, then deflate upon descent.{{sfn|Cajori|1917|pp=64–66}}
[[File:Magedurger Halbkugeln Luftpumpe Deutsches Museum.jpg|thumb|left|The original [[Magdeburg hemisphere]]s (lower 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}}
Back 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 cause resistance to the motions 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 Greece|ancient Greek]] philosophers, including Aristotle, who conceived of it as the medium through which the heavenly bodies moved.{{sfn|Olenick|Apostol|Goodstein|1986|p=356}}
The concept of a Universe filled with a [[luminiferous aether]] remained in vogue 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. However, 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 first successful measurement of the distance to a nearby star in 1838 by the German astronomer [[Friedrich Bessel]]. He showed that the star [[61 Cygni]] had a [[stellar parallax|parallax]] of just 0.31 [[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}} 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}}
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 K. British physicist [[Arthur Eddington]] made a similar calculation to derive a temperature of 3.18° in 1926. 1933 German physicist [[Erich Regener]] used the total measured energy of [[cosmic ray]]s to estimate an intergalactic temperature of 2.8 K.<ref name="Apeiron2_3_79"/>
The modern concept of outer space is based on the "Big Bang" [[cosmology]], first proposed in 1931 by the Belgian physicist [[Georges Lemaître]].<ref name=nature127_3210_706/> This theory holds that the [[observable Universe]] originated from a very compact form that has since undergone continuous [[Hubble's law|expansion]]. The background energy released during the initial expansion has steadily decreased in density, leading to a 1948 prediction by American physicts [[Ralph Alpher]] and [[Robert Herman]] of a temperature of 5 K for the temperature of space.<ref name="Apeiron2_3_79"/>
The term ''outer space'' was used as early as 1842 by the [[English people|English]] poet Lady [[Emmeline Stuart-Wortley]] in her poem "The Maiden of Moscow".{{sfn|Stuart Wortley|1841|p=410}} The expression ''outer space'' was used as an astronomical term by [[Alexander von Humboldt]] in 1845.{{sfn|Von Humboldt|1845|p=39}} It was later popularized in the writings of [[H. G. Wells]] in 1901.<ref name="entymonline"/> 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 name=harper2001/>
== Formation and state ==
{{Main|Big Bang}}
According to the Big Bang theory, the Universe originated in an extremely hot and dense state about 13.8 billion years ago and began [[Metric expansion of space|expanding]] rapidly. 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 space.<ref name="SciAm301_1_36"/> The matter that remained following the initial expansion has since undergone gravitational collapse to create [[star]]s, [[galaxy|galaxies]] and other [[astronomical object]]s, 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 also constrains the size of the directly observable Universe.<ref name="SciAm301_1_36"/> This leaves open the question as to whether the Universe is finite or infinite.
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 observable Universe is flat, meaning that photons on parallel paths at one point will 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 Universe at the equivalent of 5.9 protons per cubic meter, including dark energy, [[dark matter]], and [[Baryon#Baryonic matter|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, however, 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 voids that have much lower density, at least in terms of visible matter.<ref name=aj89_1461/> Unlike the matter and dark matter, the 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 orders of magnitude smaller than the influence of gravity from matter and dark matter within the Milky Way.<ref name=rvmphys_75_559 />
== Environment ==
[[File:Hubble Ultra Deep Field part d.jpg|right|thumb|Part of the [[Hubble Ultra-Deep Field]] image showing a typical section of space containing galaxies interspersed by deep vacuum. Given the finite [[speed of light]], this view covers the last {{nowrap|13 billion years}} of the history of outer space.|alt=A black background with luminous shapes of various sizes scattered randomly about. They typically have white, red or blue hues.]]
Outer space is the closest natural approximation to a [[perfect vacuum]]. It has effectively no [[friction]], allowing stars, [[planets]] and [[moons]] to move freely along their ideal [[orbit]]s. However, even the deep vacuum of [[Outer space#Intergalactic|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 we breathe contains about 10<sup>25</sup> molecules per cubic meter.{{sfn|Borowitz|Beiser|1971}} The sparse 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> km, or 10 billion [[light year]]s.{{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 boundary: the density of atmospheric gas gradually decreases with distance from the object until it becomes indistinguishable from the surrounding environment.{{sfn|Chamberlain|1978|p=2}} The Earth's atmospheric [[pressure]] drops to about {{nowrap|3.2 × 10<sup>−2</sup> }}[[Pascal (unit)|Pa]] at {{Convert|100|km|mi|abbr=off}} of altitude,<ref name=squire2000/> compared to 100 kPA for the [[International Union of Pure and Applied Chemistry]] (IUPAC) definition of [[Standard conditions for temperature and pressure|standard pressure]]. Beyond this altitude, 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/>
On the Earth, temperature is defined in terms of the [[Kinetic theory|kinetic]] activity of the surrounding atmosphere. However the temperature of the vacuum cannot be measured in this way. Instead, the temperature is determined by measurement of the radiation. 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]].) The current [[black body]] [[temperature]] of the background radiation is about {{convert|3|K|C F|0|lk=on}}.<ref name=apj707_2_916/> Some regions of outer space can contain highly energetic particles that have a much higher temperature than the CMB, such as the [[corona]] of the Sun where temperatures can range over 1.2–2.6 MK.<ref name=apj325_442/>
Outside of 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> [[Electronvolt|eV]] up to an extreme 10<sup>20</sup> 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> 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=11-217<!-- Note: this is not a page range -->}}
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"/>
=== Effect on human bodies ===
{{See also|Space exposure|Weightlessness}}
[[File:Astronaut-EVA.jpg|right|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.]]
Sudden exposure 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 of the chest.<ref name=ajeas2_4_573/> Even if the victim'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=ajeas2_4_573/>
As a consequence of rapid decompression, any [[oxygen]] dissolved in the blood will empty into the lungs to try to equalize the [[partial pressure]] gradient. Once the deoxygenated blood arrives at the brain, humans and animals will lose consciousness after a few seconds and die of [[Hypoxia (medical)|hypoxia]] within minutes.<ref name=bmj286/> Blood and other body fluids boil when the pressure drops below 6.3 kPa, 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 [[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 name=am39_376/> Space suits are needed at {{Convert|8|km|mi|abbr=on}} to provide enough oxygen for breathing and to prevent water loss, while above {{Convert|20|km|mi|abbr=on}} they are essential to prevent ebullism.{{sfn|Ellery|2000|p=68}} Most space suits use around 30–39 kPa of pure oxygen, about the same as on the Earth's surface. This pressure is high enough to prevent ebullism, but evaporation of blood could still cause [[decompression sickness]] and [[air embolism|gas embolisms]] if not managed.{{sfn|Davis|Johnson|Stepanek|2008|pp=270-271}}
Because humans are optimized for life in Earth [[Gravitation|gravity]], exposure to weightlessness has been shown to have deleterious effects on the [[health]] of the human body. Initially, more than 50% of astronauts experience [[space motion sickness]]. This can cause [[nausea]] and [[vomiting]], [[Vertigo (medical)|vertigo]], [[headache]]s, [[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.<ref name=spp22_15/> 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/>
For long duration space travel, radiation can pose an acute health hazard.
Exposure to radiation sources such as 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 increase in the 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, nearly the entire body would be traversed by high energy nuclei, each of which can cause ionization damage to cells. Fortunately, most such particles are significantly attenuated by the shielding provided by the aluminum walls of a spacecraft, and can be further diminished by water containers and other barriers. However, the impact of the cosmic rays upon the shielding produces additional radiation that can affect the crew. Further research will be needed to assess the radiation hazards and determine suitable countermeasures.<ref name=sas4_11_1013/>
== Boundary ==
[[File:Spaceship One in flight 1.jpg|right|thumb|[[SpaceShipOne]] completed the first [[Human spaceflight|manned]] [[private spaceflight]] in 2004, reaching an altitude of {{Convert|100.124|km|mi|abbr=on}}.|alt=A white rocketship with oddly-shaped wings against a blue sky.]]
There is no clear boundary between [[Earth's atmosphere]] and space, as the density of the atmosphere gradually decreases as the [[altitude]] increases. There are several standard boundary designations, namely:
* The [[Fédération Aéronautique Internationale]] has established the [[Kármán line]] at an altitude of {{convert|100|km|mi|abbr=on}} as a working definition for the boundary between aeronautics and astronautics. This is used because at an altitude of about {{Convert|100|km|mi|abbr=on}}, as [[Theodore von Kármán]] calculated, a vehicle would have to travel faster than [[Orbital speed|orbital velocity]] in order to derive sufficient [[aerodynamic lift]] from the atmosphere to support itself.{{sfn|O'Leary|2009|p=84}}
* The [[United States]] designates people who travel above an altitude of {{convert|50|mi|km}} as [[astronaut]]s.{{sfn|Wong|Fergusson|2010|p=16}}
* [[NASA]]'s mission control uses {{Convert|76|mi|km|abbr=on}} as their [[atmospheric reentry|re-entry]] altitude (termed the Entry Interface), 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.<ref name=petty20030213/>
In 2009, scientists at the [[University of Calgary]] reported detailed measurements with a Supra-Thermal Ion Imager (an instrument that measures the direction and speed of ions), which allowed them to establish a boundary at {{Convert|118|km|mi|abbr=on}} 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|268|m/s|mph|abbr=on}}.<ref name=thompton20090409/><ref name=jgr114/>
The altitude where the atmospheric pressure matches the [[vapor pressure of water]] at the [[Human body temperature|temperature of the human body]] is called the [[Armstrong line]],
named after American physician [[Harry G. Armstrong]]. Located at an altitude of around {{convert|19.14|km|mi|abbr=on}}, this is the height at which water in the blood stream changes phase from liquid to gas; in other words, the blood begins to boil. Hence, at this altitude the human body requires a pressure suit, or a pressurized capsule, to survive.{{sfn|Piantadosi|2003|pp=188-189}} The region between the Armstrong line and the Karman line is sometimes termed [[near space]].
== Legal status ==
{{Main|Space law}}
[[File:SM-3 launch to destroy the NRO-L 21 satellite.jpg|thumb|right|2008 launch of the [[RIM-161 Standard Missile 3|SM-3 missile]] used to destroy American [[spy satellite]] [[USA-193]]|alt=At top, a dark rocket is emitting a bright plume of flame against a blue sky. Underneath, a column of smoke is partly concealing a navy ship.]]
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]]. It also 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 USSR, the United States of America and the United Kingdom. As of January 1, 2008 the treaty has been ratified by 98 states and signed by an additional 27 states.<ref name=unoosa/>
Beginning in 1958, outer space has been the subject of multiple resolutions by the United Nations General Assembly. 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 US, USSR and China.{{sfn|Wong|Fergusson|2010|p=4}} The 1979 [[Moon Treaty]] turned the jurisdiction of all heavenly bodies (including the orbits around such bodies) over to the international community. However, this treaty has not been ratified by any nation that currently practices manned spaceflight.<ref name=esf20071105/>
In 1976 eight equatorial states ([[Ecuador]], [[Colombia]], [[Brazil]], [[Republic of the Congo|Congo]], [[Democratic Republic of the Congo|Zaire]], [[Uganda]], [[Kenya]], and [[Indonesia]]) met in [[Bogotá]], Colombia. They made the "Declaration of the First Meeting of Equatorial Countries," also known as "the Bogotá Declaration", where they made a claim to control the segment of the geosynchronous orbital path corresponding to each country.<ref name=bogota1976/> These claims are not internationally accepted.<ref name=aasl31_2006/>
== Earth orbit ==
A spacecraft enters orbit when it has enough horizontal velocity for its [[Centripetal force|centripetal]] [[acceleration]] due to [[Gravitation|gravity]] to be less than or equal to the [[centrifugal force|centrifugal]] acceleration due to the horizontal component of its velocity. For a [[low Earth orbit]], this velocity is about {{Convert|7800|m/s|km/h mph|-2|abbr=on}};<ref name=hill1999/> by contrast, the fastest manned 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 name=shiner20071101/>
To achieve an orbit, a [[spacecraft]] must travel faster than a [[sub-orbital spaceflight]]. The energy required to reach Earth orbital velocity at an altitude of {{Convert|600|km|mi|abbr=on}} is about 36 [[Joule#Megajoule|MJ]]/kg, which is six times the energy needed merely to climb to the corresponding altitude.<ref name=dimotakis1999/> Spacecraft with a [[Apsis|perigee]] below about {{Convert|2,000|km|mi|abbr=on}} are subject to drag from the Earth's atmosphere, which will cause the orbital altitude to decrease. 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. Below about {{Convert|300|km|mi|abbr=on}}, decay becomes more rapid with lifetimes measured in days. Once a satellite descends to {{Convert|180|km|mi|abbr=on}}, it will start to burn up in the atmosphere.<ref name=slsa/> The [[escape velocity]] required to pull free of Earth's gravitational field altogether and move into interplanetary space is about {{Convert|11200|m/s|km/h mph|-2|abbr=on}}.<ref name=williams2010/>
Earth's gravity reaches out far past the [[Van Allen radiation belt]] and keeps the Moon in orbit at an average distance of {{Convert|384403|km|mi|abbr=on}}. The region of space where the gravity of a planet tends to dominate the motion of objects in the presence of other perturbing bodies (such as another planet) is known as the [[Hill sphere]]. For Earth, this sphere has a radius of about {{Convert|1500000|km|mi|abbr=on}}.<ref name=yoder1995/>
== Regions ==
Space is a partial 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 [[Heliosphere#Heliopause|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 [[Void (astronomy)|intergalactic void]].
=== Geospace ===
[[File:Aurora-SpaceShuttle-EO.jpg|thumb|300px|right|[[Aurora (astronomy)|Aurora australis]] observed from the [[Space Shuttle]] ''[[Space Shuttle Discovery|Discovery]]'', on [[STS-39]], May 1991 (orbital altitude: 260 km)|alt=The lower half is the blue-white planet in low illumination. Nebulous red streamers climb upward from the limb of the disk toward the black sky. The Space Shuttle is visible along the left edge.]]
Geospace is the region of outer space near the Earth. Geospace includes the upper region of the atmosphere and the [[magnetosphere]].{{sfn|Schrijver|Siscoe|2010|p=363}} The Van Allen radiation belt lies within the geospace. The outer boundary of geospace is the [[magnetopause]], which forms an interface between the planet's magnetosphere and the solar wind. The inner boundary is the [[ionosphere]].<ref name=geospace/> As the physical properties and behavior of near Earth space is affected by the behavior of the Sun and [[space weather]], the field of [[wiktionary:geospace|geospace]] is interlinked with [[heliophysics]]; the study of the Sun and its impact on the Solar System planets.{{sfn|Fichtner|Liu|2011|pp=341–345}}
The volume of geospace defined by the magnetopause is compacted in the direction of the Sun by the pressure of the solar wind, giving it a typical subsolar distance of 10 Earth radii from the
center of the planet. However, the tail can extend outward to more than 100–200 Earth radii.{{sfn|Koskinen|2010|pp=32, 42}} The Moon passes through the geospace tail during roughly four days each month, during which time the surface is shielded from the solar wind.{{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. During [[geomagnetic storm]]s two regions of geospace, the radiation belts and the ionosphere, can become strongly disturbed. These storms increase fluxes of energetic electrons that can permanently damage satellite electronics, disrupting telecommunications and [[Global Positioning System|GPS]] technologies, and can also be a hazard to astronauts, even in low Earth orbit. They also create [[Aurora (astronomy)|aurorae]] seen near the [[Earth's magnetic field#Magnetic poles|magnetic poles]].<ref name=oecd/>
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.<ref name=slsa/> This region 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.<ref name=portree_loftus1999/>
==== Cislunar space ====
The region outside Earth's atmosphere and extending out to just beyond the Moon’s orbit, including the [[Lagrangian point]]s, is sometimes referred to as ''cis-lunar space''.<ref>{{cite web|title=The cislunar gateway with no gate|url=http://www.thespacereview.com/article/2165/1|publisher=The Space Review}}</ref>
=== Interplanetary ===
[[File:Comet Hale Bopp NASA.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, the space around the Sun and planets of the [[Solar System]], is the region dominated by the [[interplanetary medium]], which extends out to the [[Heliosphere#Heliopause|heliopause]] where the influence of the galactic environment starts to dominate over the magnetic field and particle flux from the Sun. 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. 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 distance and strength of the heliopause varies depending on the activity level of the solar wind.<ref name=phillips2009/> The discovery since 1995 of [[extrasolar planet]]s means that other stars must possess their own interplanetary media.{{sfn|Frisch|Müller|Zank|Lopate|2002|pp=21–34}}
The volume 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. However, this space is not completely empty, and is sparsely filled with cosmic rays, which include [[ion]]ized [[atomic nucleus|atomic nuclei]] and various [[subatomic particle]]s. There is also gas, [[Plasma (physics)|plasma]] and dust, small [[Meteoroid#Meteor|meteors]], and several dozen types of [[organic chemistry|organic]] [[molecule]]s discovered to date by [[rotational spectroscopy|microwave spectroscopy]].<ref name=asp2003/>
Interplanetary space contains the magnetic field generated by the Sun.{{sfn|Papagiannis|1972|pp=12–149}} There are also magnetospheres generated by planets such as Jupiter, Saturn, 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 magnetic particles such as the Van Allen radiation belt. Planets without magnetic fields, such as Mars, have their atmospheres gradually eroded by the solar wind.<ref name=ssr69_3_215/>
=== Interstellar ===
{{main|Interstellar medium}}
{{redirect|Interstellar|the 2014 film|Interstellar (film)}}
[[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 within a galaxy not occupied by stars or their planetary systems. The [[interstellar medium]] resides—by definition—in interstellar space. The average density of matter in this region is about 10<sup>6</sup> particles per m<sup>3</sup>, but this varies from a low of about 10<sup>4</sup> – 10<sup>5</sup> in regions of sparse matter up to about 10<sup>8</sup> – 10<sup>10</sup> in [[dark nebula]]. Regions of [[star formation]] may reach 10<sup>12</sup> – 10<sup>14</sup> particles per m<sup>3</sup> (as a comparison, Earth's atmospheric density at sea level is on the order of 10<sup>25</sup> particles per m<sup>3</sup><ref>{{cite web|last=Tyson|first=Patrick|title=The Kinetic Atmosphere: Molecular Numbers|url=http://www.climates.com/KA/BASIC%20PARAMETERS/molecularnumbers.pdf|accessdate=13 September 2013|month=January|year=2012}}</ref>). Nearly 70% of the mass of the interstellar medium consists of lone hydrogen atoms. This is enriched with helium atoms as well as trace amounts of heavier atoms formed through [[stellar nucleosynthesis]]. These atoms can be ejected into the interstellar medium by [[stellar wind]]s, or when evolved stars begin to shed their outer envelopes such as during the formation of a [[planetary nebula]]. The cataclysmic explosion of a [[supernova]] will generate an expanding [[shock wave]] consisting of ejected materials.
A [[List of interstellar and circumstellar molecules|number of molecules]] exist in interstellar space, as can tiny, 0.1 [[Micrometre|μm]] dust particles.{{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 cloud]]s 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 [[ion]]ize 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 [[parsec]]s (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 cloud]]s 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 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|astrosphere]]s 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 K and radii of 0.5–5 pc.<ref name=redfield2006/>
When stars are moving at a sufficiently high [[peculiar velocity]], their astrosphere can generate a [[bow shock]] as it collides 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 [[Voyager program|Voyagers]] 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 the third boundary of an astrosphere after the [[Heliosphere#Termination shock|termination shock]] and the astropause (called the [[Heliosphere#Heliopause|heliopause]] in the Solar System).<ref name=bow/>
=== Intergalactic ===
[[File:LH 95.jpg|thumb|right|[[LH 95|A]] [[star]] forming region in the [[Large Magellanic Cloud]], perhaps the closest Galaxy to Earth's [[Milky Way]]]]
Intergalactic space is the physical space between galaxies. The huge spaces between [[Galaxy groups and clusters|galaxy clusters]] are called the [[void (astronomy)|voids]]. Surrounding and stretching between galaxies, there is a [[rarefaction|rarefied]] plasma<ref name=jafelice_opher1992/> that is organized in a [[galaxy filament|cosmic filamentary structure]].<ref name=wadsley2002/> This material is called the intergalactic medium (IGM). The density of the IGM is 5-200 times the average density of the Universe.<ref name="apj_714_1715"/> It consists mostly of [[ionization|ionized]] hydrogen; i.e. a plasma consisting of equal numbers of [[electron]]s and [[proton]]s. As gas falls into the intergalactic medium from the voids, it heats up to temperatures of 10<sup>5</sup> K to 10<sup>7</sup> K,<ref name=baas41_908/> which is high enough so that collisions between atoms have enough energy to cause the bound electrons to escape from the hydrogen nuclei; this is why the IGM is ionized. At these temperatures, it is called the [[warm–hot intergalactic medium]] (WHIM). (Although the gas 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> K and above in the so-called [[intracluster medium]].<ref name="apj546_100"/>
== Exploration and applications ==
{{Main|Space exploration|Space colonization|Space manufacturing}}
[[File:As08-16-2593.jpg|right|thumb|The first image taken of the entire Earth by astronauts was shot during the [[Apollo 8]] mission|alt=An blue-white disk against a black background. Brown areas of ground are visible in some areas through openings in the swirling white clouds. The lower left of the disk is in partial shadow.]]
For the majority of human history, space was explored by remote observation; initially with the unaided eye and then with the telescope. Prior to the advent of reliable rocket technology, the closest that humans had come to reaching outer space was through the use of balloon flights. In 1935, the U.S. ''[[Explorer II]]'' manned balloon flight had 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 unmanned 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 Earth orbit were [[Frank Borman]], [[Jim Lovell]] and [[William Anders]] in 1968 on board the U.S. [[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 the planet [[Venus]], although contact was lost before reaching Venus. The first successful planetary mission was the [[Mariner 2]] fly-by of Venus in 1962.{{sfn|Collins|2007|p=86}} The first spacecraft to perform a fly-by of Mars was [[Mariner 4]], which reached the planet in 1964. Since that time, unmanned 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 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>{{cite web|last=Wall|first=Mike|title=Voyager 1 Has Left Solar System|url=http://m.space.com/22729-voyager-1-spacecraft-interstellar-space.html|work=Web|publisher=Space.com|accessdate=13 September 2013}}</ref>
The absence of air makes outer space (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 13.8 billion years ago—almost to the time of the Big Bang—to be observed. However, 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 planet]]s. Moving an [[infrared telescope]] out past the dust will increase the effectiveness of the instrument.<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/>
Unmanned spacecraft in Earth orbit have become an essential technology of modern civilization. They allow direct monitoring of [[Weather satellite|weather conditions]], relay [[Communications satellite|long-range communications]] including telephone calls and television signals, provide a means of [[Satellite navigation|precise navigation]], and allow [[remote sensing]] of the Earth. The latter role serves a wide 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}}
The deep vacuum of space could make it an attractive environment for certain industrial processes, such as those that require ultraclean surfaces.<ref name=chapman1991/> However, like [[asteroid mining]], [[space manufacturing]] requires a significant investment with little prospect of an immediate return.<ref name="IJA10_307"/>
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== ഇതും കാണുക ==
== അവലംബം ==
{{Reflist|2|refs=
<ref name="CBE2008">{{Citation | first1 = David T. | last1 = Chuss | title = Cosmic Background Explorer | publisher = NASA Goddard Space Flight Center | date = June 26, 2008 | url = http://lambda.gsfc.nasa.gov/product/cobe/ | accessdate= 2013-04-27 | postscript= . }}</ref>
<!-- ഉപയോഗിച്ചിട്ടില്ല
<ref name=bogota1976>{{citation | author=Representatives of the States traversed by the Equator | title=Declaration of the first meeting of equatorial countries | date=December 3, 1976 | location=Bogota, Republic of Colombia | work=Space Law | publisher=JAXA | url=http://www.jaxa.jp/library/space_law/chapter_2/2-2-1-2_e.html | accessdate=2011-10-14 | postscript=. }}</ref>
<ref name=nature127_3210_706>{{citation | last1=Lemaître | first1=G. | authorlink=Georges Lemaître | title=The Beginning of the World from the Point of View of Quantum Theory | month=May | year=1931 | journal=Nature | volume=127 | issue=3210 | page=706 | doi=10.1038/127706b0 | bibcode=1931Natur.127..706L | postscript=. }}</ref>
<!-- Unused citation
<ref name=nasa19970603>{{citation | url= http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/970603.html | title=Human Body in a Vacuum | date=June 3, 1997 | publisher=NASA | accessdate=2009-06-19 | postscript=. }}</ref> -->
<ref name=landis20070807>{{citation | url=http://www.geoffreylandis.com/vacuum.html | title=Human Exposure to Vacuum | date=August 7, 2007 | last1=Landis | first1=Geoffrey A. | publisher=www.geoffreylandis.com | accessdate=2009-06-19 | postscript=. }}</ref>
<ref name="IJA10_307">{{Citation | last1 = Forgan | first1 = Duncan H. | last2 = Elvis | first2 = Martin | title = Extrasolar asteroid mining as forensic evidence for extraterrestrial intelligence | work = International Journal of Astrobiology | volume = 10 | pages = 307–313 | month = October | year = 2011 | doi = 10.1017/S1473550411000127 | bibcode = 2011IJAsB..10..307F | arxiv = 1103.5369 | postscript= . }}</ref>
<ref name="CBE2008">{{Citation | first1 = David T. | last1 = Chuss | title = Cosmic Background Explorer | publisher = NASA Goddard Space Flight Center | date = June 26, 2008 | url = http://lambda.gsfc.nasa.gov/product/cobe/ | accessdate= 2013-04-27 | postscript= . }}</ref>
<ref name="PNAS103_33_12232">{{Citation | first1 = William | last1 = Klemperer | title = Interstellar chemistry | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 103 | issue = 33 | pages = 12232–12234 | date = August 15, 2006 | doi = 10.1073/pnas.0605352103 | pmc = 1567863 | postscript= . |bibcode = 2006PNAS..10312232K }}</ref>
<ref name="WMAP">{{cite web | url=http://map.gsfc.nasa.gov/universe/uni_shape.html | title=WMAP — Shape of the universe | publisher=NASA | accessdate=June 4, 2013 | date=December 21, 2012}}</ref>
-->
}}
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