Editing Outer space (section)

=== Earth orbit ===
{{main|Geocentric orbit|Orbital decay}}
[[File:Newton Cannon.svg|thumb|240px|[[Newton's cannonball]], an illustration of how objects can "fall" in a curve around the planet]]
When a rocket is launched to achieve orbit, its thrust must both counter gravity and accelerate it to [[orbital speed]]. After the rocket terminates its thrust, it follows an arc-like [[trajectory]] back toward the ground under the influence of the Earth's [[gravitational force]]. In a [[closed orbit]], this arc will turn into an [[ellipse|elliptical]] loop around the planet. That is, a spacecraft successfully enters Earth orbit when its [[centripetal acceleration|acceleration due to gravity]] pulls the craft down just enough to prevent its momentum from carrying it off into outer space.<ref name=NESDIS_2025/>

For a [[low Earth orbit]], orbital speed is about {{Convert|7.8|km/s |mph|-2|abbr=on}};<ref name=hill1999/> by contrast, the fastest piloted airplane speed ever achieved (excluding speeds achieved by deorbiting spacecraft) was {{Convert|2.2|km/s|mph|-2|abbr=on}} in 1967 by the [[North American X-15]].<ref name=shiner20071101/> The upper limit of orbital speed at {{Convert|11.2|km/s|mph|-2|abbr=on}} is the [[escape velocity|velocity required to pull free]] from Earth altogether and enter into a [[heliocentric orbit]].<ref name=williams2010/> The energy required to reach Earth orbital speed at an altitude of {{Convert|600|km|mi|abbr=on}} is about 36&nbsp;[[Megajoule|MJ]]/kg, which is six times the energy needed merely to climb to the corresponding altitude.<ref name=dimotakis1999/>

Very low Earth orbit (VLEO) has been defined as orbits that have a mean altitude below 450 km (280 mi), which can be better suited for Earth observation with small satellites.<ref name=Llop_et_al_2014/> Low Earth orbits in general range in altitude from {{cvt|180|to|2000|km|mi}} and are used for scientific satellites. [[Medium Earth orbit]]s extends from {{cvt|2000|to|35780|km|mi}}, which are favorable orbits for navigation and specialized satellites. Above {{cvt|35780|km|mi}} are the [[high Earth orbit]]s used for weather and some communication satellites.<ref name=Riebeck_2009/>

Spacecraft in orbit with a [[Apsis|perigee]] below about {{Convert|2000|km|mi|abbr=on}} (low Earth orbit) are subject to drag from the Earth's atmosphere,{{sfn|Ghosh|2000|pp=47–48}} which decreases the orbital altitude. The rate of orbital decay depends on the satellite's cross-sectional area and mass, as well as variations in the air density of the upper atmosphere, which is significantly effected by [[space weather]].<ref name="z356"/> At altitudes above {{cvt|800|km|mi|abbr=on}}, orbital lifetime is measured in centuries.<ref name=NASA_FAQ/> Below about {{cvt|300|km|mi|abbr=on}}, decay becomes more rapid with lifetimes measured in days. Once a satellite descends to {{cvt|180|km|mi|abbr=on}}, it has only hours before it vaporizes in the atmosphere.<ref name=slsa/> 

Radiation in orbit around Earth is concentrated in [[Van Allen radiation belt]]s, which trap [[cosmic radiation|solar and galactic radiation]]. Radiation is a threat to astronauts and space systems. It is difficult to shield against and space weather makes the radiation environment variable. The radiation belts are equatorial [[toroid]]al regions, which are bent towards Earth's poles, with the [[South Atlantic Anomaly]] being the region where charged particles approach Earth closest.<ref name=Baker_et_al_2018/><ref name="u460"/> The innermost radiation belt, the inner Van Allen belt, has its intensity peak at altitudes above the equator of half an Earth radius,<ref name="e494"/> centered at about 3000 km,<ref name="a298"/> increasing from the upper edge of low Earth orbit which it overlaps.<ref name=Irfan_et_al_2002/><ref name=Koteskey_2024/><ref name=Kovar_et_al_2020/>