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low Earth orbit
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{{short description|Orbit around Earth with an altitude between 160 kilometers and 2,000 kilometers}}{{Comparison satellite navigation orbits}}A low Earth orbit (LEO) is an Earth-centered orbit with an altitude of {{cvt|2000|km|mi}} or less (approximately one-third of the radius of Earth), or with at least 11.25 periods per day (an orbital period of 128 minutes or less) and an eccentricity less than 0.25.WEB,weblink Current Catalog Files, July 13, 2018, LEO: Mean Motion > 11.25 & Eccentricity < 0.25, Most of the manmade objects in outer space are in LEO.JOURNAL, Sampaio, Jarbas, Wnuk, Edwin, Vilhena de Moraes, Rodolpho, Fernandes, Sandro, 2014-01-01, Resonant Orbital Dynamics in LEO Region: Space Debris in Focus,weblink Mathematical Problems in Engineering, 2014, Figure 1: Histogram of the mean motion of the cataloged objects, 10.1155/2014/929810, There is a large variety of other sourcesWEB,weblink Definition of LOW EARTH ORBIT, www.merriam-webster.com, en, 2018-07-08, WEB,weblink Frequently Asked Questions, www.faa.gov, en-us, 2018-07-08, LEO refers to orbits that are typically less than 2,400 km (1,491 mi) in altitude., NEWS,weblink SCaN Glossary, Campbell, Ashley, 2015-07-10, NASA, 2018-07-12, en, 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., that 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 {{cvt|30|km|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 {{cvt|8413|km|mi}}. For circular orbits, this in turn corresponds to an altitude of {{cvt|2042|km|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 the region in space that LEO orbits occupy.WEB,weblink IADC Space Debris Mitigation Guidelines, September 2007, INTER-AGENCY SPACE DEBRIS COORDINATION COMMITTEE: Issued by Steering Group and Working Group 4, 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, NEWS,weblink What Is an Orbit?, 2015-06-01, NASA, 2018-07-08, 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;, en, LEO is the first 100 to 200 miles (161 to 322 km) of space., JOURNAL, LEO region (100 kms [sic] to 1000 kms), 2014cosp...40E2964S, Charging of space debris in the LEO and GEO regions, 40th COSPAR Scientific Assembly, 40, PEDAS.1–41–14, Sen, Abhijit, Tiwari, Sanat Kumar, 2014, NEWS,weblink A Researcher's Guide to: Space Environmental Effects, Steele, Dylan, 2016-05-03, NASA, 2018-07-12, 7, en, the low-Earth orbit (LEO) environment, defined as 200–1,000 km above Earth's surface, Some highly elliptical orbits may pass through the LEO region near their lowest altitude (or perigee) but are not in an LEO Orbit because their highest altitude (or apogee) exceeds {{cvt|2000|km|mi}}. Sub-orbital objects can also reach the LEO region but are not in an 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. The International Space Station conducts operations in LEO. All crewed space stations to date, as well as the majority of satellites, have been in LEO. The altitude record for human spaceflights in LEO was Gemini 11 with an apogee of {{cvt|1374.1|km|mi}}. Apollo 8 was the first mission to carry humans beyond LEO on December 21–27, 1968. The Apollo program continued during the four-year period spanning 1968 through 1972 with 24 astronauts who flew lunar flights but since then there have been no human spaceflights beyond LEO.

Orbital characteristics

The mean orbital velocity needed to maintain a stable low Earth orbit is about {{Cvt|7.8|km/s|kph mph}}, but reduces with increased orbital altitude. Calculated for circular orbit of {{cvt|200|km}} it is {{cvt|7.79|km/s|kph mph}}, and for {{cvt|1500|km}} it is {{cvt|7.12|km/s|kph mph}}.WEB,weblink LEO parameters, www.spaceacademy.net.au, 2015-06-12, The delta-v needed to achieve low Earth orbit starts around 9.4 km/s. Atmospheric and gravity drag associated with launch typically adds {{cvt|1.3|–|1.8|km/s|kph mph}} to the launch vehicle delta-v required to reach normal LEO orbital velocity of around {{cvt|7.8|km/s|km/h mph|0}}.BOOK, How Spacecraft Fly, Swinerd, Graham, Praxis Publishing, 2008, 978-0387765723, 103–104, (File:Orbitalaltitudes.jpg|center|700px)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 far less than the Earth's radius. However, an object in orbit is, by definition, in free fall, since there is no force holding it up. As a result objects in orbit, including people, experience a sense of weightlessness, even though they are not actually without weight.Objects in LEO encounter atmospheric drag from gases in the thermosphere (approximately 80–500 km above the surface) or exosphere (approximately {{cvt|500|km|0|disp=or}} and up), depending on orbit height. Due to atmospheric drag, satellites do not usually orbit below {{cvt|300|km}}. 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 and have the lowest delta-v requirement (i.e., fuel spent) of any orbit. Orbits with a high inclination angle to the equator are usually called polar orbits.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.{{redirect|VLEO}}In 2017, a very low-Earth orbit (or very-low LEO orbit) began to be seen in regulatory filings. This orbit, referred to as "VLEO", requires the use of novel technologies for orbit raising because they operate in orbits that would ordinarily decay too soon to be economically useful.NEWS, Messier, Doug,weblink SpaceX Wants to Launch 12,000 Satellites, Parabolic Arc, 2017-03-03, 2018-01-22,

Use of LEO

File:Sunrise To Sunset Aboard The ISS.OGG|thumb|Roughly half an orbit of the ISS.]]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

Satellites in LEO have a small momentary field of view, only able to observe and communicate with a fraction of the Earth at a time, meaning a network (or "constellation") of satellites is required to in order to provide continuous coverage. Satellites in lower regions of LEO also suffer from fast orbital decay, requiring either periodic reboosting to maintain a stable orbit, or launching replacement satellites when old ones re-enter.

Examples

  • Earth observation satellites and spy satellites use LEO as they are able to see the surface of the Earth clearly by being close to it. They are also able to traverse the surface of the Earth. A majority of artificial satellites are placed in LEO,WEB, NASA Earth Observatory,weblink earthobservatory.nasa.gov, 2009-09-04, 2015-11-28, en, Riebeek, Holli, making one complete revolution around the Earth in about 90 minutes.
  • The International Space Station is in a LEO about {{convert|330|km|mi|-1|abbr=on}} to {{convert|420|km|mi|-1|abbr=on}} above Earth's surface,WEB,weblink Higher Altitude Improves Station's Fuel Economy, NASA, 2013-02-12, and needs reboosting a few times a year due to orbital decay.
  • Iridium satellites orbit at about {{convert|780|km|mi|abbr=on}}.
  • Lower orbits also aid remote sensing satellites because of the added detail that can be gained. Remote sensing satellites can also take advantage of sun-synchronous LEO orbits at an altitude of about {{convert|800|km|mi|-1|abbr=on}} and near polar inclination. Envisat (2002–2012) is one example of an Earth observation satellite that makes use of this particular type of LEO (at {{convert|770|km|mi|abbr=on}}).
  • GOCE orbited at about {{convert|255|km|mi|abbr=on}} to measure Earth's gravity field.
  • GRACE were, and GRACE-FO are, orbiting at about {{convert|500|km|mi|abbr=on}}
  • The Hubble Space Telescope orbits at about {{convert|540|km|mi|abbr=on}} above Earth.
  • The Chinese Tiangong-2 station orbits at about {{convert|370|km|mi|abbr=on}}.

Space debris

The LEO environment is becoming congested with space debris because of the frequency of object launches. This has caused growing concern in recent years, since collisions at orbital velocities can easily be dangerous, and even deadly. Collisions can produce even more space debris in the process, creating a domino effect, something known as Kessler Syndrome. The Joint Space Operations Center, part of United States Strategic Command (formerly the United States Space Command), currently tracks more than 8,500 objects larger than 10 cm in LEO.Fact Sheet: Joint Space Operations Center {{webarchive|url=https://web.archive.org/web/20100203082814weblink |date=2010-02-03 }} However, a limited Arecibo Observatory study suggested there could be approximately one million objects larger than 2 millimeters,archive of astronomy: space junk which are too small to be visible from Earth-based observatories.ISS laser broom, project Orion {{webarchive|url=https://web.archive.org/web/20110728181940weblink |date=2011-07-28 }}

See also

{{colbegin}} {{colend}}{{clear}}

Notes

{{reflist|group=lower-alpha}}

References

{{reflist|30em}}{{Include-NASA}}{{orbits}}{{People currently in space}}{{Authority control}}

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