"අභ්‍යවකාශය" හි සංශෝධන අතර වෙනස්කම්

rm English
සුළු (රොබෝ එකතු කරමින්: hi:अंतरिक्ष)
(rm English)
* [[විද්යුත් චුම්භක විකිරණ]], [[චුම්භක ක්ෂේත්‍ර]], සහ[[cosmic neutrino background|neutrinos]].
 
==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'']]. Based on this idea that a vacuum could not exist, it was widely held for many centuries that space could not be empty.<ref>{{cite book
|author=Porter, Roy; Park, Katharine; Daston, Lorraine
|title=The Cambridge History of Science: Early modern science
|page=27|work=Early Modern Science|volume=3
|publisher=Cambridge University Press|year=2006
|isbn=0521572444}}</ref> As late as the seventeenth century, the French philosopher [[René Descartes]] argued that the entirety of space must be filled.<ref>{{cite book|first=Michael|last=Eckert|year=2006|title=The dawn of fluid dynamics: a discipline between science and technology|page=5|publisher=Wiley-VCH|isbn=3527405135}}</ref>
It became known to [[Galileo Galilei]] that air had weight and so was subject to gravity. He also demonstrated that there was an established force that resisted the formation of a vacuum. However, it would remain for his pupil [[Evangelista Torricelli]] to create an apparatus that would produce a vacuum. At the time this experiment created a scientific sensation in Europe. The French mathematician [[Blaise Pascal]] reasoned that if the column of mercury was suspended by air then the column ought to be shorter at higher altitude. 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. These and other experiments were used to overthrow the principle of ''horror vacui''.<ref>{{cite book
| first=Florian | last=Cajori | year=1917
| title=A history of physics in its elementary branches: including the evolution of physical laboratories | pages=64–66
| location=New York | publisher=The Macmillan Company }}</ref>
 
Further work on the physics of the vacuum was performed by [[Otto von Guericke]]. 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.<ref>{{cite book
| first=Henning | last=Genz | year=2001 | pages=127–128
| title=Nothingness: the science of empty space
| publisher=Da Capo Press | isbn=0738206105 }}</ref>
 
Early speculations as to the infinite dimension of space was performed in the sixteenth century by the Italian philosopher [[Giordano Bruno]]. He extended the Copernican heliocentric cosmology to the concept of an infinite universe that is filled with a substance he called [[Aether (classical element)|aether]], which did not cause resistance to the motions of heavenly bodies.<ref>{{cite book
| first=Hilary | last=Gatti | year=2002 | pages=99–104
| title=Giordano Bruno and Renaissance science
| publisher=Cornell University Press | isbn=0801487854 }}</ref> 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.<ref>{{cite book
| first=Suzanne | last=Kelly
| title=The de muno of William Gilbert
| publisher=Menno Hertzberger & Co. | year=1965
| page=97&ndash;107 | location=Amsterdam
}}</ref> 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.<ref>{{cite book
| author=Olenick, Richard P.; Apostol, Tom M.; Goodstein, David L. | title=Beyond the mechanical universe: from electricity to modern physics
| page=356 | publisher=Cambridge University Press
| year=1986 | isbn=052130430X }}</ref>
 
The concept of a universe filled with a [[luminiferous aether]] remained in vogue among some scientists up until the twentieth century. This form of aether was viewed as the medium through which light could propagate. In 1887, the [[Michelson-Morley experiment]] was carried out as an attempt to detect the Earth's motion through this medium by looking for changes in the [[speed of light]] based on the direction of the planet's motion. However, the [[null result]] indicated something was wrong with the concept. Since then the idea of the luminiferous aether had essentially been abandoned, to be replaced by [[Albert Einstein]]'s theory of [[special relativity]]. The latter held that the speed of light is a constant in a vacuum, regardless of the observer's motion or [[frame of reference]].<ref>Olenick ''et al''. (1986:357&ndash;365).</ref><ref>{{cite book
| first=Paul | last=Thagard | year=1992
| pages=206&ndash;209 | title=Conceptual revolutions
| publisher=Princeton University Press | isbn=0691024901 }}</ref>
 
The first professional astronomer to support the concept of an infinite universe was the Englishman [[Thomas Digges]] in 1576.<ref>{{cite book
| first=Eli | last=Maor | year=1991
| title=To infinity and beyond: a cultural history of the infinite
| page=195 | publisher=Princeton paperbacks
| isbn=0691025118 }}</ref> However, the true scale of the universe remained unknown until the first successful measurement of the distance to a nearby star was performed in 1838 by the German astronomer [[Friedrich Bessel]]. He showed that the star [[61 Cygni]] had a [[parallax]] of just 0.31&nbsp;[[arcsecond]]s (compared to the modern value of 0.287&Prime;). This corresponded to a distance of over 10 [[light year]]s.<ref>{{cite book
| first=Stephen | last=Webb | year=1999
| title=Measuring the universe: the cosmological distance ladder
| pages=71–73 | publisher=Springer | isbn=1852331062,
}}</ref> The distance scale to the [[Andromeda galaxy]] was determined in 1923 by American astronomer [[Edwin Hubble]] when he measured the brightness of [[cepheid variable]]s within that galaxy. This established that the Andromeda galaxy, and by extension all galaxies, lay well outside the [[Milky Way]].<ref>{{cite book
| author=Tyson, Neil deGrasse; Goldsmith, Donald | year=2004
| title=Origins: fourteen billion years of cosmic evolution
| pages=114–115 | publisher=W. W. Norton & Company
| isbn=0393059928 }}</ref>
 
The modern concept of outer space is based upon the [[Big Bang]] [[cosmology]], which was first proposed in 1931 by the Belgian physicist [[Georges Lemaître]]. This theory holds that the observable universe originated from a very compact form that has since undergone continuous [[Hubble's law|expansion]]. 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.<ref>{{cite book
| first=Joseph | last=Silk | year=2000
| title=The Big Bang | edition=3rd
| publisher=Macmillan | isbn=080507256X }}</ref>
 
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 later popularized in 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 natural 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. However, even in the deep vacuum of [[Outer space#Intergalactic|intergalactic space]] there are still a few hydrogen atoms per cubic meter.<ref>{{cite journal
| last=Tadokoro | first=M. | title=A Study of the Local Group by Use of the Virial Theorem
| journal=Publications of the Astronomical Society of Japan
| volume=20 | page=230 | year=1968
| bibcode=1968PASJ...20..230T }} This source estimates a density of {{nowrap|7 × 10<sup>−29</sup> g/cm}} for the [[Local Group]]. An [[atomic mass unit]] is {{nowrap|1.66 × 10<sup>−24</sup> g}}, for roughly 40 atoms per cubic meter.</ref> By comparison, the air we breathe contains about 10<sup>25</sup> molecules per cubic meter.<ref>{{cite book
| author=Borowitz, Sidney; Beiser, Arthur | year=1971
| title=Essentials of physics: a text for students of science and engineering | series=Addison-Wesley series in physics
| edition=2nd | publisher=Addison-Wesley Publishing Company }} Note: this source gives a value of {{nowrap|2.7 × 10<sup>25</sup>}} molecules per cubic meter.</ref> The sparse density of matter in outer space means that [[electromagnetic radiation]] can travel great distances without being scattered; the [[mean free path]] for a [[photon]] in intergalactic space is about 10<sup>23</sup>&nbsp;km, or 10 billion [[light year]]s.<ref>{{cite book
| first=P. C. W. | last=Davies | year=1977
| title=The physics of time asymmetry | page=93
| publisher=University of California Press
| isbn=0520032470 }} Note: a light year is about 10<sup>13</sup>&nbsp;km.</ref> The deep vacuum of space could make it an attractive environment for certain industrial processes, for instance those that require ultraclean surfaces.<ref>{{cite conference
| first=Glenn | last=Chapmann
| editors=R. Blackledge, C. Radfield and S. Seida
| title=Space: the Ideal Place to Manufacture Microchips
| booktitle=Proceedings of the 10th International Space Development Conference | location=San Antonio, Texas
| date=May 22–27, 1991 | pages=25–33
| url=http://deneb.ensc.sfu.ca/papers/sdc-91w.pdf
| accessdate=2010-01-12 }}</ref>
 
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. This is known as the [[Kármán line]], 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 centimeter.
 
=== Temperature ===
All of the observable [[Universe]] is filled with [[photon]]s 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 this photon radiation is about {{convert|3|K|C F|0|lk=on}}. Some regions of outer space can contain highly energetic particles that have a much higher temperature than the CMB.
 
=== Effect on human bodies ===
{{See also|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. 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 arrived at the brain, death would quickly follow.
 
Humans and animals exposed to vacuum will lose consciousness after a few seconds and die of [[Hypoxia (medical)|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 |year=1973
|editor=edited by James F. Parker and Vita R. West
|title=Bioastronautics Data Book |edition=2nd
|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=August 7, 2007 |author=Landis, Geoffrey A. |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 | pages= 376–383 | pmid=4872696 | issue=4 }}</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 ineffectively; 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 spacesuits 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 sickness|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 lungs.<ref name="harding" /> Eardrums 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 (medical)|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 ==
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 designated scientific boundaries, 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&nbsp;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=9 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 |year=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" />
 
==ජාත්‍යන්තර නීතිය==
The [[Outer Space Treaty]] provides the basic framework for international space law. This treaty 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 signed by 98 states and ratified by an additional 27 states.<ref>{{cite web | date=January 1, 2008
| title=Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, including the Moon and Other Celestial Bodies
| publisher=United Nations Office for Outer Space Affairs
| url=http://www.unoosa.org/oosa/SpaceLaw/outerspt.html
| accessdate=2009-12-30 }}</ref>
 
Between 1958 and 2008, 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>{{cite web
| title=Index of Online General Assembly Resolutions Relating to Outer Space
| publisher=United Nations Office for Outer Space Affairs
| url=http://www.unoosa.org/oosa/en/SpaceLaw/gares
| accessdate=2009-12-30 }} {{Dead link|date=September 2010|bot=H3llBot}}</ref> 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]]. 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>{{cite web
| title=Columbus launch puts space law to the test
| date=November 5, 2007 | publisher=European Science Foundation | url=http://www.esf.org/research-areas/humanities/news/ext-news-singleview/article/columbus-launch-puts-space-law-to-the-test-358.html
| accessdate=2009-12-30 }}</ref>
 
== 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 1, 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 (32&nbsp;[[megajoule|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.
 
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 is 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'']], on [[STS-39]], May 1991 (orbital altitude: 260 km).]]
 
'''Geospace''' is the region of outer space near the Earth. Geospace includes the upper region of the 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]].
 
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 [[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 pole|magnetic poles]].
 
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 ===
'''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 [[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&ndash;10 [[proton]]s/cm<sup>3</sup> and is moving at a velocity of 350&ndash;400&nbsp;km/s.<ref name="papagiannis72" /> The distance and strength of the heliopause varies depending on the activity level of the solar wind.<ref>{{cite web
| first=Tony | last=Phillips | publisher=NASA
| title= Cosmic Rays Hit Space Age High | date=2009-09-29 | url=http://science.nasa.gov/headlines/y2009/29sep_cosmicrays.htm
| accessdate=2009-10-20 }}</ref> The discovery since 1995 of [[extrasolar planet]]s means that other stars must possess their own interplanetary media.<ref>{{cite conference
| author=Frisch, Priscilla C.; Müller, Hans R.; Zank, Gary P.; Lopate, C. | date=May 6–9, 2002
| title=Galactic environment of the Sun and stars: interstellar and interplanetary material
| pages=21–34 | booktitle=Astrophysics of life. Proceedings of the Space Telescope Science Institute Symposium
| location=Baltimore, MD, USA
| editors=Mario Livio, I. Neill Reid, William B. Sparks
| series=Space Telescope Science Institute symposium series, Volume 16 | publisher=Cambridge University Press
| ISBN=0-521-82490-7 | bibcode=2005asli.symp...21F }}</ref>
 
The volume of interplanetary space is an almost pure 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 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]].<ref>{{cite conference
| author=Flynn, G. J.; Keller, L. P.; Jacobsen, C.; Wirick, S.
| title=The Origin of Organic Matter in the Solar System: Evidence from the Interplanetary Dust Particles
| booktitle=Bioastronomy 2002: Life Among the Stars, Proceedings of IAU Symposium #213 | location=San Francisco
| editors=R. Norris and F. Stootman | year=2003
| publisher=Astronomical Society of the Pacific
| bibcode=2004IAUS..213..275F | accessdate=2009-10-20 }}</ref>
 
Interplanetary space contains the magnetic field generated by the Sun.<ref name="papagiannis72">{{cite book
| first=Michael D. | last=Papagiannis | year=1972
| title=Space Physics and Space Astronomy
| pages=12–149 | publisher=Taylor & Francis
| isbn=0677040008 }}</ref> There are also magnetospheres generated by planets such as Jupiter, Saturn 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 Belts]]. Planets without magnetic fields, such as Mars and Mercury, but excluding Venus, have their atmospheres gradually eroded by the solar wind.
 
===Interstellar===
'''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.
 
=== Intergalactic ===
'''Intergalactic space''' is the physical space between [[galaxy|galaxies]]. Generally free of dust and debris, intergalactic space is very close to a total vacuum. The space between galaxy clusters, called the [[void (astronomy)|voids]], is probably 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 planets, stars, 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> 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 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 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 10<sup>5</sup>&nbsp;[[kelvin|K]] to 10<sup>7</sup>&nbsp;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 10<sup>8</sup>&nbsp;K and above.
{{seealso|Intergalactic Stars}}
{{seealso|Intergalactic dust}}

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