Editing Outer space (section)

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

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

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

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

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

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