Thethermopause is theatmospheric boundary of Earth's energy system, located at the top of thethermosphere.[1] The temperature of the thermopause could range from nearly absolute zero to 987.547 °C (1,810 °F).
Below this, the atmosphere is defined to be active[clarification needed] on theinsolation received, due to the increased presence of heavier gases such as monatomic oxygen. Thesolar constant is thus expressed at the thermopause. Beyond (above) this, theexosphere describes the thinnest remainder of atmospheric particles with large mean free path, mostly hydrogen and helium. As a lower boundary for the exosphere this boundary is also called theexobase.[1]
The exact altitude varies by the energy inputs of location, time of day, solar flux, season, etc. and can be between 500 and 1,000 kilometres (310 and 620 mi) high at a given place and time because of these. Aportion of themagnetosphere dips below this layer as well.
Although these are all named layers of the atmosphere, the pressure is so negligible that the chiefly-used definitions ofouter space are actually below this altitude. Orbiting satellites do not experience significant atmospheric heating, but their orbits do decay over time, depending on orbit altitude. Space missions such as theISS,Space Shuttle, andSoyuz operate under this boundary.
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