Jump to content

Low Earth orbit

From Wikipedia, the free encyclopedia
(Redirected fromLow Earth Orbit)
A view from aspace stationin a low Earth orbit (LEO) at about 400 km (250 mi), with yellow-greenairglowvisible at Earth'shorizon,where roughly at an altitude of 100 km (62 mi) theboundary between Earth and outer spacelies and flying speeds reachorbital velocities.

Alow Earth orbit(LEO) is anorbit around Earthwith aperiodof 128 minutes or less (making at least 11.25 orbits per day) and aneccentricityless than 0.25.[1]Most of the artificial objects inouter spaceare in LEO, peaking in number at an altitude around 800 km (500 mi),[2]while the farthest in LEO, beforemedium Earth orbit(MEO), have an altitude more than about one-third of theradius of Earth(or about 2000 kilometers),[3]roughly at the beginning of theinner Van Allen radiation belt.

The termLEO regionis also used for the area of space below analtitudeof 2,000 km (1,200 mi) (about one-third of Earth's radius).[4]Objects in orbits that pass through this zone, even if they have anapogeefurther out or aresub-orbital,are carefully tracked since they present a collision risk to the many LEO satellites.

Nohuman spaceflightsother than the lunar missions of theApollo programhave taken place beyond LEO. Allspace stationsto date have operatedgeocentricwithin LEO.

Defining characteristics[edit]

A wide variety of sources[5][6][7]define LEO in terms ofaltitude.The altitude of an object in anelliptic orbitcan vary significantly along the orbit. Even forcircular orbits,the altitude above ground can vary by as much as 30 km (19 mi) (especially forpolar orbits) due to theoblatenessofEarth's spheroid figureand localtopography.While definitions based on altitude are inherently ambiguous, most of them fall within the range specified by an orbit period of 128 minutes because, according toKepler's third law,this corresponds to asemi-major axisof 8,413 km (5,228 mi). For circular orbits, this in turn corresponds to an altitude of 2,042 km (1,269 mi) above the mean radius of Earth, which is consistent with some of the upper altitude limits in some LEO definitions.

The LEO region is defined by some sources as a region in space that LEO orbits occupy.[4][8][9]Somehighly elliptical orbitsmay pass through the LEO region near their lowest altitude (orperigee) but are not in a LEO orbit because their highest altitude (orapogee) exceeds 2,000 km (1,243 mi).Sub-orbitalobjects can also reach the LEO region but are not in a LEO orbit because theyre-enter the atmosphere.The distinction between LEO orbits and the LEO region is especially important for analysis of possible collisions between objects which may not themselves be in LEO but could collide with satellites or debris in LEO orbits.

Orbital characteristics[edit]

The mean orbital velocity needed to maintain a stable low Earth orbit is about 7.8 km/s (4.8 mi/s), which translates to 28,000 km/h (17,000 mph). However, this depends on the exact altitude of the orbit. Calculated for a circular orbit of 200 km (120 mi) the orbital velocity is 7.79 km/s (4.84 mi/s), but for a higher 1,500 km (930 mi) orbit the velocity is reduced to 7.12 km/s (4.42 mi/s).[10]The launch vehicle'sdelta-vneeded to achieve low Earth orbit starts around 9.4 km/s (5.8 mi/s).

The pull ofgravityin LEO is only slightly less than on the Earth's surface. This is because the distance to LEO from the Earth's surface is much less than the Earth's radius. However, an object in orbit is in a permanentfree fallaround Earth, because in orbit thegravitational forceand thecentrifugal forcebalance each other out.[a]As a result, spacecraft in orbit continue to stay in orbit, and people inside or outside such craft continuously experienceweightlessness.

Objects in LEO encounter atmospheric drag fromgasesin thethermosphere(approximately 80–600 km above the surface) orexosphere(approximately 600 km or 400 mi and higher), depending on orbit height. Orbits of satellites that reach altitudes below 300 km (190 mi) decay fast due to atmospheric drag. Objects in LEO orbit Earth between the denser part of the atmosphere and below the innerVan Allen radiation belt.

Equatorial low Earth orbits (ELEO) are a subset of LEO. These orbits, with low inclination to the Equator, allow rapid revisit times over low-latitude locations on Earth. Prograde equatorial LEOs also have lowerdelta-vlaunch requirements because they take advantage of the Earth's rotation. Other useful LEO orbits includingpolar orbitsandSun-synchronous orbitshave a higher inclinations to the equator and provide coverage for higher latitudes on Earth. Some of the first generation ofStarlinksatellites used polar orbits which provide coverage everywhere on Earth. Later Starlink constellations orbit at a lower inclination and provide more coverage for populated areas.

Higher orbits includemedium Earth orbit(MEO), sometimes called intermediate circular orbit (ICO), and further above,geostationary orbit(GEO). Orbits higher than low orbit can lead to early failure of electronic components due to intenseradiationand charge accumulation.

In 2017, "very low Earth orbits"(VLEO) began to be seen inregulatoryfilings. These orbits, below about 450 km (280 mi), require the use of novel technologies fororbit raisingbecause they operate in orbits that would ordinarily decay too soon to be economically useful.[11][12]

Use[edit]

Roughly half an orbit of theInternational Space Station

A low Earth orbit requires the lowest amount of energy for satellite placement. It provides high bandwidth and low communicationlatency.Satellites and space stations in LEO are more accessible for crew and servicing.

Since it requires lessenergyto place a satellite into a LEO, and a satellite there needs less powerful amplifiers for successful transmission, LEO is used for many communication applications, such as theIridium phone system.Somecommunication satellitesuse much highergeostationary orbitsand move at the same angular velocity as the Earth as to appear stationary above one location on the planet.

Disadvantages[edit]

Unlikegeosynchronous satellites,satellites in low orbit have a smallfield of viewand can only observe and communicate with a fraction of the Earth at a given time. This means that a large network (orconstellation) of satellites is required to provide continuous coverage.

Satellites at lower altitudes of orbit are in the atmosphere and suffer from rapidorbital decay,requiring either periodic re-boosting to maintain stable orbits, or the launching of replacements for those that re-enter the atmosphere. The effects of adding such quantities of vaporized metals to Earth'sstratosphereare potentially of concern but currently unknown.[13]

Examples[edit]

Former[edit]

  • The ChineseTiangong-1station was in orbit at about 355 kilometres (221 mi),[16]until its de-orbiting in 2018.
  • The ChineseTiangong-2station was in orbit at about 370 km (230 mi), until its de-orbiting in 2019.
  • GOCE,another gravimetry mission, orbited at about 255 km (158 mi).
  • Super Low Altitude Test Satellite,nicknamedTsubame,orbited at 167.4 km (104.0 mi), the lowest altitude ever among Earth observation satellites.[17]

In fiction[edit]

Space debris[edit]

The LEO environment is becoming congested withspace debrisbecause of the frequency of object launches.[19]This has caused growing concern in recent years, since collisions at orbital velocities can be dangerous or deadly. Collisions can produce additional space debris, creating adomino effectknown asKessler syndrome.NASA'sOrbital Debris Program tracks over 25,000 objects larger than 10 cm diameter in LEO, while the estimated number between 1 and 10 cm is 500,000, and the number of particles bigger than 1 mm exceeds 100 million.[20]The particles travel at speeds up to 7.8 km/s (28,000 km/h; 17,500 mph), so even a small impact can severely damage a spacecraft.[21]

See also[edit]

Notes[edit]

  1. ^It is important to note here that “free fall” by definition requires thatgravityis the only force acting on the object. That definition is still fulfilled when falling around Earth, as the other force, thecentrifugal forceis afictitious force.

References[edit]

  1. ^"Current Catalog Files".Archivedfrom the original on 26 June 2018.Retrieved13 July2018.LEO: Mean Motion > 11.25 & Eccentricity < 0.25
  2. ^Muciaccia, Andrea (2021).Fragmentations in low Earth orbit: event detection and parent body identification(Thesis).doi:10.13140/RG.2.2.27621.52966.
  3. ^Sampaio, Jarbas; Wnuk, Edwin; Vilhena de Moraes, Rodolpho; Fernandes, Sandro (1 January 2014)."Resonant Orbital Dynamics in LEO Region: Space Debris in Focus".Mathematical Problems in Engineering.2014:Figure 1: Histogram of the mean motion of the cataloged objects.doi:10.1155/2014/929810.Archivedfrom the original on 1 October 2021.Retrieved13 July2018.
  4. ^ab"IADC Space Debris Mitigation Guidelines"(PDF).INTER-AGENCY SPACE DEBRIS COORDINATION COMMITTEE: Issued by Steering Group and Working Group 4. September 2007.Archived(PDF)from the original on 17 July 2018.Retrieved17 July2018.Region A, Low Earth Orbit (or LEO) Region – spherical region that extends from the Earth's surface up to an altitude (Z) of 2,000 km
  5. ^"Definition of LOW EARTH ORBIT".Merriam-Webster Dictionary.Archivedfrom the original on 8 July 2018.Retrieved8 July2018.
  6. ^"Frequently Asked Questions".FAA.Archivedfrom the original on 2 June 2020.Retrieved14 February2020.LEO refers to orbits that are typically less than 2,400 km (1,491 mi) in altitude.
  7. ^Campbell, Ashley (10 July 2015)."SCaN Glossary".NASA.Archivedfrom the original on 3 August 2020.Retrieved12 July2018.Low Earth Orbit (LEO): A geocentric orbit with an altitude much less than the Earth's radius. Satellites in this orbit are between 80 and 2000 kilometers above the Earth's surface.
  8. ^"What Is an Orbit?".NASA.David Hitt: NASA Educational Technology Services, Alice Wesson: JPL, J.D. Harrington: HQ;, Larry Cooper: HQ;, Flint Wild: MSFC;, Ann Marie Trotta: HQ;, Diedra Williams: MSFC. 1 June 2015.Archivedfrom the original on 27 March 2018.Retrieved8 July2018.LEO is the first 100 to 200 miles (161 to 322 km) of space.{{cite news}}:CS1 maint: others (link)
  9. ^Steele, Dylan (3 May 2016)."A Researcher's Guide to: Space Environmental Effects".NASA.p. 7.Archivedfrom the original on 17 November 2016.Retrieved12 July2018.the low-Earth orbit (LEO) environment, defined as 200–1,000 km above Earth's surface
  10. ^"LEO parameters".spaceacademy.net.au.Archivedfrom the original on 11 February 2016.Retrieved12 June2015.
  11. ^Crisp, N. H.; Roberts, P. C. E.; Livadiotti, S.; Oiko, V. T. A.; Edmondson, S.; Haigh, S. J.; Huyton, C.; Sinpetru, L.; Smith, K. L.; Worrall, S. D.; Becedas, J. (August 2020). "The Benefits of Very Low Earth Orbit for Earth Observation Missions".Progress in Aerospace Sciences.117:100619.arXiv:2007.07699.Bibcode:2020PrAeS.11700619C.doi:10.1016/j.paerosci.2020.100619.S2CID220525689.
  12. ^Messier, Doug (3 March 2017)."SpaceX Wants to Launch 12,000 Satellites".Parabolic Arc.Archivedfrom the original on 22 January 2020.Retrieved22 January2018.
  13. ^https:// scientificamerican /article/space-junk-is-polluting-earths-stratosphere-with-vaporized-metal/
  14. ^"Higher Altitude Improves Station's Fuel Economy".NASA.Archivedfrom the original on 15 May 2015.Retrieved12 February2013.
  15. ^Holli, Riebeek (4 September 2009)."NASA Earth Observatory".earthobservatory.nasa.gov.Archivedfrom the original on 27 May 2018.Retrieved28 November2015.
  16. ^""Thiên cung nhất hào thành công hoàn thành lần thứ hai biến quỹ"".Archived fromthe originalon 13 November 2011.Retrieved13 October2020.
  17. ^"Japan's low altitude satellite Tsubame registered in Guinness World Records".The Japan Times.30 December 2019.Retrieved25 June2024.
  18. ^"Space station from 2001: A Space Odyssey".
  19. ^United Nations Office for Outer Space Affairs (2010)."Space Debris Mitigation Guidelines of the Committee on the Peaceful Uses of Outer Space".Inter-Agency Space Debris Coordination Committee (IADC).Retrieved19 October2021.
  20. ^"ARES | Orbital Debris Program Office | Frequently Asked Questions".NASA.gov.Archived fromthe originalon 2 September 2022.Retrieved2 September2022.
  21. ^Garcia, Mark (13 April 2015)."Space Debris and Human Spacecraft".NASA.gov.Archived fromthe originalon 8 September 2022.Retrieved2 September2022.

Public DomainThis article incorporatespublic domain materialfrom websites or documents of theNational Aeronautics and Space Administration.

Retrieved from "https://en.wikipedia.org/w/index.php?title=Low_Earth_orbit&oldid=1231108006"