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A '''low Earth orbit''' ('''LEO''') is a circular [[Orbit|orbit]] around [[Earth]] between the atmosphere and the [[Van Allen radiation belt]]. These boundaries are not firmly defined, but are typically around 350 - 800 km above the [[Earth|Earth's]] surface, with inclination angles less than 60 degrees from the equator. This is generally below [[intermediate circular orbit]] (ICO), Sun-synchronous orbit and far below [[geostationary orbit]]. Orbits lower than this are not stable, and will decay rapidly because of atmospheric drag. Orbits higher than this are subject to early electronic failure because of intense radiation and charge accumulation. Orbits with a higher inclination angle (>~ 70 degrees) are usually called [[polar orbit]]s.
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Objects in low Earth orbit encounter atmospheric gases in the [[thermosphere]] (approximately 80-500 km up) or [[exosphere]] (approximately 500 km and up), depending on orbit height.
 
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Most [[manned spaceflight]]s have been in '''LEO''', including all [[Space Shuttle]] and various [[space station]] missions; the only exceptions have been suborbital test flights such as the early [[Project Mercury]] missions (which did not reach '''LEO'''), and the [[Project Apollo]] missions to the Moon (which went beyond '''LEO''').
 
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Most early artificial [[satellite]]s were placed in '''LEO'''. Here they travel at about 27,400 km/h (8 km/s), making one revolution in about 90 minutes.  The primary exceptions are [[communication satellites]], now common, that now mostly use geostationary orbit to obviate the requirement for dishes to track the satellite's movement and [[meteorological satellites]]. It requires less energy to place a satellite into '''LEO''' and the satellite needs less powerful transmitters for data transfer, so '''LEO''' is still used for occasional communication applications.  Because these orbits are not geostationary, a network of satellites is required to provide continuous coverage.  Lower orbits also aid [[Remote Sensing|remote sensing]] because of the added detail that can be gained.
 
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The '''LEO''' environment is becoming congested, not least with [[Space debris|space debris]]. The [[United States Space Command]] tracks more than 8,000 objects larger than 10cm in '''LEO'''.
 
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Although [[Gravity|gravity]] in '''LEO''' is not much less than on the surface of the [[Earth]] (it reduces 1% every 30 km), people and objects in any orbit experience [[Weightlessness|weightlessness]]. This is an effect of freefall and has nothing to do with the strength of the gravitational field.
 
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[[Atmosphere|Atmospheric]] and gravitational losses associated with launch typically add 1,500-2,000 m/s to the [[Delta-V|delta-v]] required to reach normal '''LEO''' orbital [[Velocity|velocity]] of ~7,800 m/s.
 
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==See also==
 
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*[[Geostationary earth orbit]]
 
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*[[Powered flight losses]]
 
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*[[impulse]]
 
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[[Category: Articles]]
 
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[[Category:Glossary]]
 
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[[Category:Spaceflight]]
 

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