Editing Outer space (section)

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

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

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

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

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