Astronomy:Low Earth orbit

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Short description: Orbit around Earth between 160 and 2000 km
Comparison of geostationary, GPS, GLONASS, Galileo, Compass (MEO), International Space Station, Hubble Space Telescope, Iridium constellation and graveyard orbits, with the Radiation|Radiation

belt]]s and the Earth to scale.[lower-alpha 1] The Moon's orbit is around 9 times as large as geostationary orbit.[lower-alpha 2] (In the SVG file, hover over an orbit or its label to highlight it; click to load its article.)]]

A low Earth orbit (LEO) is an orbit around Earth with a period of 128 minutes or less (making at least 11.25 orbits per day) and an eccentricity less than 0.25.[1] Most of the artificial objects in outer space are in LEO, with an altitude never more than about one-third of the radius of Earth (or about 2000 kilometers).[2]

The term LEO region is also used for the area of space below an altitude of 2,000 km (1,200 mi) (about one-third of Earth's radius).[3] Objects in orbits that pass through this zone, even if they have an apogee further out or are sub-orbital, are carefully tracked since they present a collision risk to the many LEO satellites.

No human spaceflights other than the lunar missions of the Apollo program have taken place beyond LEO. All crewed space stations to date have operated within LEO.

Defining characteristics

A wide variety of sources[4][5][6] define LEO in terms of altitude. The altitude of an object in an elliptic orbit can vary significantly along the orbit. Even for circular orbits, the altitude above ground can vary by as much as 30 km (19 mi) (especially for polar orbits) due to the oblateness of Earth's spheroid figure and local topography. 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 to Kepler's third law, this corresponds to a semi-major axis of 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.[3][7][8] Some highly elliptical orbits may pass through the LEO region near their lowest altitude (or perigee) but are not in a LEO orbit because their highest altitude (or apogee) exceeds 2,000 km (1,243 mi). Sub-orbital objects can also reach the LEO region but are not in a LEO orbit because they re-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.

Orbitalaltitudes.svg

Orbital characteristics

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).[9] The launch vehicle's delta-v needed to achieve low Earth orbit starts around 9.4 km/s (5.8 mi/s).

The pull of gravity in 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 permanent free fall around Earth, because in orbit the gravitational force and the centrifugal force balance each other out.[lower-alpha 3] As a result, spacecraft in orbit continue to stay in orbit, and people inside or outside such craft continuously experience weightlessness.

Objects in LEO encounter atmospheric drag from gases in the thermosphere (approximately 80–600 km above the surface) or exosphere (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 inner Van 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 lower delta-v launch requirements because they take advantage of the Earth's rotation. Other useful LEO orbits including polar orbits and Sun-synchronous orbits have a higher inclinations to the equator and provide coverage for higher latitudes on Earth. Some of the first generation of Starlink satellites 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 include medium 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 intense radiation and charge accumulation.

In 2017, "very low Earth orbits" (VLEO) began to be seen in regulatory filings. These orbits, below about 450 km (280 mi), require the use of novel technologies for orbit raising because they operate in orbits that would ordinarily decay too soon to be economically useful.[10][11]

Use

File:Sunrise To Sunset Aboard The ISS.OGG

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

Since it requires less energy to 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 the Iridium phone system. Some communication satellites use much higher geostationary orbits and move at the same angular velocity as the Earth as to appear stationary above one location on the planet.

Disadvantages

Unlike geosynchronous satellites, satellites in LEO have a small field of view and can only observe and communicate with a fraction of the Earth at a given time. This means that a network (or constellation) of satellites is required to provide continuous coverage. Satellites in lower regions of LEO also suffer from rapid orbital decay, requiring either periodic re-boosting to maintain stable orbits or the launching of replacements for those that re-enter the atmosphere.

Examples

  • The International Space Station is in a LEO about 400 km (250 mi) to 420 km (260 mi) above Earth's surface,[12] and needs re-boosting a few times a year due to orbital decay.
  • The Iridium telecom satellites orbit at about 780 km (480 mi).
  • Earth observation satellites, also known as remote sensing satellites, including spy satellites and other Earth imaging satellites, use LEO as they are able to see the surface of the Earth more clearly by being closer to it. A majority of artificial satellites are placed in LEO.[13] Satellites can also take advantage of consistent lighting of the surface below via Sun-synchronous LEO orbits at an altitude of about 800 km (500 mi) and near polar inclination. Envisat (2002–2012) is one example.
  • The Hubble Space Telescope orbits at about 540 km (340 mi) above Earth.
  • The Chinese Tiangong space station was launched in April 2021, and currently orbits between about 340 kilometres (210 mi) and 450 kilometres (280 mi).
  • The gravimetry mission GRACE-FO orbits at about 500 km (310 mi) as did its predecessor, GRACE.

Former

  • The Chinese Tiangong-1 station was in orbit at about 355 kilometres (221 mi),[14] until its de-orbiting in 2018.
  • The Chinese Tiangong-2 station 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).

In fiction

  • In the film 2001: A Space Odyssey, Earth's transit station ("Space Station V") "orbited 300 km above Earth".[15]

Space debris

The LEO environment is becoming congested with space debris because of the frequency of object launches.[16] 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 a domino effect known as Kessler syndrome. NASA's Orbital 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.[17] 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.[18]

See also

Notes

  1. ↑ Orbital periods and speeds are calculated using the relations 4π2R3 = T2GM and V2R = GM, where R = radius of orbit in metres, T = orbital period in seconds, V = orbital speed in m/s, G = gravitational constant ≈ 6.673×10−11 Nm2/kg2, M = mass of Earth ≈ 5.98×1024 kg.
  2. ↑ Approximately 8.6 times (in radius and length) when the moon is nearest (363 104 km ÷ 42 164 km) to 9.6 times when the moon is farthest (405 696 km ÷ 42 164 km).
  3. ↑ It is important to note here that “free fall” by definition requires that gravity is the only force acting on the object. That definition is still fulfilled when falling around Earth, as the other force, the centrifugal force is a fictitious force.

References

  1. ↑ "Current Catalog Files". https://www.space-track.org/#/recent. "LEO: Mean Motion > 11.25 & Eccentricity < 0.25" 
  2. ↑ Sampaio, Jarbas; Wnuk, Edwin; Vilhena de Moraes, Rodolpho; Fernandes, Sandro (2014-01-01). "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. https://www.researchgate.net/publication/273597440. Retrieved 2018-07-13. 
  3. ↑ 3.0 3.1 "IADC Space Debris Mitigation Guidelines". INTER-AGENCY SPACE DEBRIS COORDINATION COMMITTEE: Issued by Steering Group and Working Group 4. September 2007. http://www.unoosa.org/documents/pdf/spacelaw/sd/IADC-2002-01-IADC-Space_Debris-Guidelines-Revision1.pdf. "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" 
  4. ↑ "Definition of LOW EARTH ORBIT" (in en). https://www.merriam-webster.com/dictionary/low%20earth%20orbit. 
  5. ↑ "Frequently Asked Questions" (in en-us). FAA. https://www.faa.gov/space/additional_information/faq/#s1. "LEO refers to orbits that are typically less than 2,400 km (1,491 mi) in altitude." 
  6. ↑ Campbell, Ashley (2015-07-10). "SCaN Glossary" (in en). NASA. https://www.nasa.gov/directorates/heo/scan/definitions/glossary/index.html#L. "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." 
  7. ↑ "What Is an Orbit?" (in en). NASA. 2015-06-01. https://www.nasa.gov/audience/forstudents/5-8/features/nasa-knows/what-is-orbit-58.html. "LEO is the first 100 to 200 miles (161 to 322 km) of space." 
  8. ↑ Steele, Dylan (2016-05-03). "A Researcher's Guide to: Space Environmental Effects" (in en). NASA: p. 7. https://www.nasa.gov/connect/ebooks/researchers_guide_space_environment_detail.html. "the low-Earth orbit (LEO) environment, defined as 200–1,000 km above Earth's surface" 
  9. ↑ "LEO parameters". http://www.spaceacademy.net.au/watch/track/leopars.htm. 
  10. ↑ Crisp, N. H.; Roberts, P. C. E.; Livadiotti, S.; Oiko, V. T. A.; Edmondson, S.; Haigh, S. J.; Huyton, C.; Sinpetru, L. et al. (August 2020). "The Benefits of Very Low Earth Orbit for Earth Observation Missions". Progress in Aerospace Sciences 117: 100619. doi:10.1016/j.paerosci.2020.100619. Bibcode: 2020PrAeS.11700619C. 
  11. ↑ Messier, Doug (2017-03-03). "SpaceX Wants to Launch 12,000 Satellites". Parabolic Arc. http://www.parabolicarc.com/2017/03/03/spacex-launch-12000-satellites/. 
  12. ↑ "Higher Altitude Improves Station's Fuel Economy". NASA. http://www.nasa.gov/mission_pages/station/expeditions/expedition26/iss_altitude.html. 
  13. ↑ Holli, Riebeek (2009-09-04). "NASA Earth Observatory" (in en). http://earthobservatory.nasa.gov/Features/OrbitsCatalog/. 
  14. ↑ ""天宫一号成功完成二次变轨"". http://www.people.com.cn/h/2011/1001/c25408-1418236438.html. 
  15. ↑ "Space station from 2001: A Space Odyssey". https://www.esa.int/ESA_Multimedia/Images/2012/01/Space_station_from_2001_A_Space_Odyssey. 
  16. ↑ United Nations Office for Outer Space Affairs (2010). "Space Debris Mitigation Guidelines of the Committee on the Peaceful Uses of Outer Space". https://www.iadc-home.org/documents_public/view/search_field/eNortjI2tlJy1CsuTcpKTS7RdtTLS8xNBVIlielAMjk_ryQ1r6RYyRpcMAypDUU~/page/1/id/126#u. 
  17. ↑ "ARES | Orbital Debris Program Office | Frequently Asked Questions". https://www.orbitaldebris.jsc.nasa.gov/faq/. 
  18. ↑ Garcia, Mark (2015-04-13). "Space Debris and Human Spacecraft". http://www.nasa.gov/mission_pages/station/news/orbital_debris.html. 

 This article incorporates public domain material from websites or documents of the National Aeronautics and Space Administration.



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