Low Earth orbit

A low Earth orbit (LEO) is an Earth-centred orbit with an altitude of 2,000 km (1,200 mi) or less (approximately one-third of the radius of Earth),[1] or with at least 11.25 periods per day (an orbital period of 128 minutes or less) and an eccentricity less than 0.25.[2] Most of the artificial objects in outer space are in LEO.[3]

Orbit size comparison of GPS, GLONASS, Galileo, BeiDou-2, and Iridium constellations, the International Space Station, the Hubble Space Telescope, and geostationary orbit (and its graveyard orbit), with the Van Allen radiation belts 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.)

There is a large variety of other sources[4][5][6] 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 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 the region in space that LEO orbits occupy.[1][7][8][9] 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 2,000 km (1,242.7 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.

All crewed space stations to date, as well as the majority of satellites, have been in LEO. From 1968 to 1972 the Apollo program's lunar missions sent humans beyond LEO. Since the end of the Apollo program there have been no human spaceflights beyond LEO.

Orbital characteristics

The mean orbital velocity needed to maintain a stable low Earth orbit is about 7.8 km/s (28,000 km/h; 17,000 mph), but reduces with increased orbital altitude. Calculated for circular orbit of 200 km (120 mi) it is 7.79 km/s (28,000 km/h; 17,400 mph), and for 1,500 km (930 mi) it is 7.12 km/s (25,600 km/h; 15,900 mph).[10] The delta-v needed to achieve low Earth orbit starts around 9.4 km/s. Atmospheric and gravity drag associated with launch typically adds 1.3–1.8 km/s (4,700–6,500 km/h; 2,900–4,000 mph) to the launch vehicle delta-v required to reach normal LEO orbital velocity of around 7.8 km/s (28,080 km/h; 17,448 mph).[11]

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 500 km or 311 mi and up), depending on orbit height. Due to atmospheric drag, satellites do not usually orbit below 300 km (190 mi). 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 of low-latitude places on Earth and have the lowest delta-v requirement (i.e., fuel spent) of any orbit, provided they have the direct (not retrograde) orientation with respect to the Earth's rotation. 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.

In 2017, "very low Earth" orbits began to be seen in regulatory filings. These orbits, referred to as "VLEO", require the use of novel technologies for orbit raising because they operate in orbits that would ordinarily decay too soon to be economically useful.[12]

Use of LEO

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
  • The International Space Station is in a LEO about 400 km (250 mi) to 420 km (260 mi) above Earth's surface,[13] and needs reboosting a few times a year due to orbital decay.
  • Iridium 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. They are also able to traverse the surface of the Earth. A majority of artificial satellites are placed in LEO,[14] making one complete revolution around the Earth in about 90 minutes. Satellites can also take advantage of Sun-synchronous LEO orbits at an altitude of about 800 km (500 mi) 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 770 km (480 mi)).
    • Gravimetry missions such as GOCE orbited at about 255 km (158 mi) to measure Earth's gravity field at highest sensitivity (the mission lifetime was limited because of atmospheric drag); GRACE were, and GRACE-FO are, orbiting at about 500 km (310 mi)
  • The Hubble Space Telescope orbits at about 540 km (340 mi) above Earth.
  • The Chinese Tiangong-1 station was in orbit at about 355 kilometres (221 mi).,[15] 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.

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 Combined 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.[16] However, a limited Arecibo Observatory study suggested there could be approximately one million objects larger than 2 millimeters,[17] which are too small to be visible from Earth-based observatories.[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, approximately 6.673×10−11 Nm2/kg2; M, mass of Earth, approximately 5.98×1024 kg.
  2. Approximately 8.6 times (in radius and length) when the moon is nearest (363104 km ÷ 42164 km) to 9.6 times when the moon is farthest (405696 km ÷ 42164 km).

References
  1. "IADC Space Debris Mitigation Guidelines" (PDF). INTER-AGENCY SPACE DEBRIS COORDINATION COMMITTEE: Issued by Steering Group and Working Group 4. September 2007. 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
  2. "Current Catalog Files". Retrieved July 13, 2018. LEO: Mean Motion > 11.25 & Eccentricity < 0.25
  3. 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.
  4. "Definition of LOW EARTH ORBIT". Merriam-Webster Dictionary. Retrieved 2018-07-08.
  5. "Frequently Asked Questions". FAA. Retrieved 2020-02-14. 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". NASA. Retrieved 2018-07-12. 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?". 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. 2015-06-01. Retrieved 2018-07-08. LEO is the first 100 to 200 miles (161 to 322 km) of space.CS1 maint: others (link)
  8. Sen, Abhijit; Tiwari, Sanat Kumar (2014). "Charging of space debris in the LEO and GEO regions". 40th COSPAR Scientific Assembly. 40: PEDAS.1–41–14. Bibcode:2014cosp...40E2964S. LEO region (100 kms [sic] to 1000 kms)
  9. Steele, Dylan (2016-05-03). "A Researcher's Guide to: Space Environmental Effects". NASA. p. 7. Retrieved 2018-07-12. the low-Earth orbit (LEO) environment, defined as 200–1,000 km above Earth's surface
  10. "LEO parameters". www.spaceacademy.net.au. Retrieved 2015-06-12.
  11. Swinerd, Graham (2008). How Spacecraft Fly. Praxis Publishing. pp. 103–104. ISBN 978-0387765723.
  12. Messier, Doug (2017-03-03). "SpaceX Wants to Launch 12,000 Satellites". Parabolic Arc. Retrieved 2018-01-22.
  13. "Higher Altitude Improves Station's Fuel Economy". NASA. Retrieved 2013-02-12.
  14. Holli, Riebeek (2009-09-04). "NASA Earth Observatory". earthobservatory.nasa.gov. Retrieved 2015-11-28.
  15. "天宫一号成功完成二次变轨"
  16. Fact Sheet: Joint Space Operations Center Archived 2010-02-03 at the Wayback Machine
  17. archive of astronomy: space junk
  18. ISS laser broom, project Orion Archived 2011-07-28 at the Wayback Machine

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