Editing Outer space (section)

=== Boundary ===
{{For|the furthest reaches of space|observable universe}}
[[File:Earth's atmosphere.svg|thumb|upright=1.35|Illustration of Earth's atmosphere gradual transition into outer space]]

The transition between Earth's atmosphere and outer space lacks a well-defined physical boundary, with the air pressure steadily decreasing with altitude until it mixes with the [[solar wind]]. Various definitions for a practical boundary have been proposed, ranging from {{Convert|30|km|mi|abbr=on}} out to {{Convert|1600000|km|mi|abbr=on}}.<ref name="Betz"/> In 2009, measurements of the direction and speed of ions in the atmosphere were made from a [[sounding rocket]]. The altitude of {{Convert|118|km|mi|sigfig=3|abbr=on}} above Earth was the midpoint for charged particles transitioning from the gentle winds of the Earth's atmosphere to the more extreme flows of outer space. The latter can reach velocities well over {{Convert|268|m/s|ft/s|abbr=on}}.<ref name=thompton20090409/><ref name=jgr114/> 

High-altitude [[aircraft]], such as [[high-altitude balloon]]s have reached altitudes above Earth of up to 50 km.<ref name="Grush"/> Up until 2021, the United States designated people who travel above an altitude of {{convert|50|mi|km|abbr=on}} as astronauts.{{sfn|Wong|Fergusson|2010|p=16}} [[United States Astronaut Badge|Astronaut wings]] are now only awarded to spacecraft crew members that "demonstrated activities during flight that were essential to public safety, or contributed to human space flight safety".<ref name=FAA_2021/>

The region between airspace and outer space is termed "near space". There is no legal definition for this extent, but typically this is the altitude range from {{cvt|20|to|100|km|mi}}.<ref name=Hao_Fabio_2019/> For safety reasons, [[commercial aircraft]] are typically limited to altitudes of {{Cvt|12|km|mi}}, and air navigation services only extend to {{cvt|18|to|20|km|mi}}.<ref name=Hao_Fabio_2019>{{citation | postscript=.
 | title=Regulating Near-Space Activities: Using the Precedent of the Exclusive Economic Zone as a Model?
 | first1=Hao | last1=Liu | first2=Fabio | last2=Tronchetti
 | work=Ocean Development & International Law | year=2019
 | doi=10.1080/00908320.2018.1548452 }}</ref> The upper limit of the range is the [[Kármán line]], where [[astrodynamics]] must take over from [[aerodynamics]] in order to achieve flight.<ref name="j253"/> This range includes the [[stratosphere]], [[mesosphere]] and lower [[thermosphere]] layers of the Earth's atmosphere.<ref name="f457">{{citation | postscript=.
 | title=Spatial and Temporal Characterization of Near Space Temperature and Humidity and Their Driving Influences
 | last1=Luo | first1=Wenhui | last2=Ma | first2=Jinji
 | last3=Li | first3=Miao | last4=Xu | first4=Haifeng
 | last5=Wan | first5=Cheng | last6=Li | first6=Zhengqiang
 | display-authors=1 | journal=Remote Sensing
 | volume=16 | issue=22 | page=4307 | date=November 19, 2024
 | issn=2072-4292 | doi=10.3390/rs16224307 | doi-access=free| bibcode=2024RemS...16.4307L
 }}</ref>

Larger ranges for ''near space'' are used by some authors, such as {{cvt|18|to|160|km|mi}}.<ref name="k364"/> These extend to the altitudes where [[orbital flight]] in [[very low Earth orbit]]s becomes practical.<ref name="k364"/> Spacecraft have entered into a highly elliptical [[orbital flight|orbit]] with a perigee as low as {{Convert|80|to|90|km|mi|abbr=on}}, surviving for multiple orbits.<ref name=McDowell_2018/> At an altitude of {{Convert|120|km|mi|abbr=on}},<ref name=McDowell_2018/> descending spacecraft begin [[atmospheric entry]] as [[atmospheric drag]] becomes noticeable. For [[spaceplane]]s such as [[NASA]]'s [[Space Shuttle]], this begins the process of switching from steering with thrusters to maneuvering with [[Flight control surfaces|aerodynamic control surfaces]].<ref name=petty20030213/>

The Kármán line, established by the [[Fédération Aéronautique Internationale]], and used internationally by the [[United Nations]],<ref name="Betz"/> is set at an altitude of {{convert|100|km|mi|abbr=on}} as a working definition for the boundary between aeronautics and astronautics. This line is named after [[Theodore von Kármán]], who argued for an altitude where a vehicle would have to travel faster than [[Orbital speed|orbital velocity]] to derive sufficient [[aerodynamic lift]] from the atmosphere to support itself,{{sfn|O'Leary|2009|p=84}}<ref name=space_begin/> which he calculated to be at an altitude of about {{Convert|83.8|km|mi|abbr=on}}.<ref name="Grush"/> This distinguishes altitudes below as the region of [[aerodynamics]] and [[airspace]], and above as the space of [[astronautics]] and ''free space''.<ref name="Betz"/>

There is no internationally recognized legal altitude limit on national airspace, although the Kármán line is the most frequently used for this purpose. Objections have been made to setting this limit too high, as it could inhibit space activities due to concerns about airspace violations.<ref name=McDowell_2018/> It has been argued for setting no specified singular altitude in international law, instead applying different limits depending on the case, in particular based on the craft and its purpose. Increased commercial and military sub-orbital spaceflight has raised the issue of where to apply laws of airspace and outer space.<ref name="k364"/><ref name="j253"/> Spacecraft have flown over foreign countries as low as {{Convert|30|km|mi|abbr=on}}, as in the example of the Space Shuttle.<ref name="Grush"/>