Editing Vacuum (section)

== Outer space ==
{{Main|Outer space}}

[[File:Structure_of_the_magnetosphere_LanguageSwitch.svg|lang=en|left|thumb|upright=1.75|Structure of the [[magnetosphere]] - is not a perfect vacuum, but a tenuous [[Plasma (physics)|plasma]] awash with charged particles, free elements such as [[hydrogen]], [[helium]] and [[oxygen]], [[electromagnetic field]]s.]]

[[Outer space]] has very low density and pressure, and is the closest physical approximation of a perfect vacuum. But no vacuum is truly perfect, not even in interstellar space, where there are still a few hydrogen atoms per cubic meter.<ref name=tadokoro />

Stars, planets, 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 {{convert|32|mPa}} at {{convert|100|km|mi}} of altitude,<ref name=squire2000>{{cite journal | first=Tom | last=Squire | date=September 27, 2000 | title=U.S. Standard Atmosphere, 1976 | journal=Thermal Protection Systems Expert and Material Properties Database | url=http://tpsx.arc.nasa.gov/cgi-perl/alt.pl | access-date=2011-10-23  | url-status=dead | archive-url=https://web.archive.org/web/20111015062917/http://tpsx.arc.nasa.gov/cgi-perl/alt.pl | archive-date=October 15, 2011 }}</ref> the [[Kármán line]], which is a common definition of the boundary with outer space. Beyond this line, isotropic gas pressure rapidly becomes insignificant when compared to [[radiation pressure]] from the [[Sun]] and the [[dynamic pressure]] of the [[solar wind]]s, so the definition of pressure becomes difficult to interpret. The [[thermosphere]] in this range has large gradients of pressure, temperature and composition, and varies greatly due to [[space weather]]. Astrophysicists prefer to use [[number density]] to describe these environments, in units of particles per cubic centimetre.

But 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 couple of weeks or a few times a year (depending on solar activity).<ref>{{Cite web|url=https://earthobservatory.nasa.gov/features/OrbitsCatalog/page3.php|title=Catalog of Earth Satellite Orbits|date=2009-09-04|website=earthobservatory.nasa.gov|language=en|access-date=2019-01-28}}</ref> 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]].<ref>{{cite journal|last1=Andrews|first1=Dana G.|last2=Zubrin|first2=Robert M. |url=http://pdfs.semanticscholar.org/9f10/d81e06d0f8515411bff54728029f0b5551dc.pdf|archive-url=https://web.archive.org/web/20190302064738/http://pdfs.semanticscholar.org/9f10/d81e06d0f8515411bff54728029f0b5551dc.pdf|url-status=dead|archive-date=2019-03-02|title=Magnetic Sails & Interstellar Travel |date=1990|journal=Journal of the British Interplanetary Society|doi=10.2514/3.26230|volume=43|pages=265–272|s2cid=55324095|access-date=2019-07-21}}</ref> 

All of the [[observable universe]] is filled with large numbers of [[photon]]s, the so-called [[cosmic background radiation]], and quite likely a correspondingly large number of [[neutrino]]s. The current [[temperature]] of this radiation is about {{convert|3|K|C F|lk=on}}.