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 Eccentricity or eccentric may refer to: * Eccentricity (behavior), odd behavior on the part of a person, as opposed to being "normal" Mathematics, science and technology Mathematics * Off-Centre (geometry), center, in geometry * Eccentricity (g ...

less than 0.25. Most of the artificial objects in outer space are in LEO, peaking in number at an altitude around , while the farthest in LEO, before medium Earth orbit (MEO), have an altitude of 2,000 km, about one-third of the radius of Earth and near the beginning of the inner Van Allen radiation belt. The term ''LEO region'' is used for the area of space below an

altitude Altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum (geodesy), datum and a point or object. The exact definition and reference datum varies according to the context (e.g., aviation, geometr ...

of (about one-third of Earth's radius). 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 spaceflight Human spaceflight (also referred to as manned spaceflight or crewed spaceflight) is spaceflight with a crew or passengers aboard a spacecraft, often with the spacecraft being operated directly by the onboard human crew. Spacecraft can also be ...

s other than the lunar missions of the

Apollo program The Apollo program, also known as Project Apollo, was the United States human spaceflight program led by NASA, which Moon landing, landed the first humans on the Moon in 1969. Apollo followed Project Mercury that put the first Americans in sp ...

(1968-1972) have gone beyond LEO. The

Artemis program The Artemis program is a Exploration of the Moon, Moon exploration program led by the United States' National Aeronautics and Space Administration (NASA), formally established in 2017 via Space Policy Directive 1. The program's stated long-ter ...

of lunar missions is scheduled to go beyond LEO. All

space station A space station (or orbital station) is a spacecraft which remains orbital spaceflight, in orbit and human spaceflight, hosts humans for extended periods of time. It therefore is an artificial satellite featuring space habitat (facility), habitat ...

s to date have operated geocentric within LEO.

Defining characteristics

A wide variety of sources define LEO in terms of

. The altitude of an object in an elliptic orbit can vary significantly along the orbit. Even for

circular orbit A circular orbit is an orbit with a fixed distance around the barycenter; that is, in the shape of a circle. In this case, not only the distance, but also the speed, angular speed, Potential energy, potential and kinetic energy are constant. T ...

s, the altitude above ground can vary by as much as (especially for polar orbits) due to the oblateness of Earth's spheroid figure and local

topography Topography is the study of the forms and features of land surfaces. The topography of an area may refer to the landforms and features themselves, or a description or depiction in maps. Topography is a field of geoscience and planetary sci ...

. 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 . For circular orbits, this in turn corresponds to an altitude of 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. 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 . 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

Orbital characteristics

The mean orbital velocity needed to maintain a stable low Earth orbit is about , which translates to . However, this depends on the exact altitude of the orbit. Calculated for a circular orbit of the orbital velocity is , but for a higher orbit the velocity is reduced to . The launch vehicle's delta-v needed to achieve low Earth orbit starts around . The pull of

gravity In physics, gravity (), also known as gravitation or a gravitational interaction, is a fundamental interaction, a mutual attraction between all massive particles. On Earth, gravity takes a slightly different meaning: the observed force b ...

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 In classical mechanics, free fall is any motion of a physical object, body where gravity is the only force acting upon it. A freely falling object may not necessarily be falling down in the vertical direction. If the common definition of the word ...

around Earth, because in orbit the gravitational force and the centrifugal force balance each other out. As a result, spacecraft in orbit continue to stay in orbit, and people inside or outside such craft continuously experience weightlessness. Objects in LEO orbit Earth between the denser part of the atmosphere and below the inner Van Allen radiation belt. They encounter atmospheric drag from gases in the thermosphere (approximately 80–600 km above the surface) or

exosphere The exosphere is a thin, atmosphere-like volume surrounding a planet or natural satellite where molecules are gravitationally bound to that body, but where the density is so low that the molecules are essentially collision-less. In the case of ...

(approximately and higher), depending on orbit height. Satellites in orbits that reach altitudes below decay quickly due to atmospheric drag. Equatorial low Earth orbits (ELEO) are a subset of LEO. These orbits, with low orbital inclination, 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 orbit A Sun-synchronous orbit (SSO), also called a heliosynchronous orbit, is a nearly polar orbit around a planet, in which the satellite passes over any given point of the planet's surface at the same local mean solar time. More technically, it is ...

s 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 A geostationary orbit, also referred to as a geosynchronous equatorial orbit''Geostationary orbit'' and ''Geosynchronous (equatorial) orbit'' are used somewhat interchangeably in sources. (GEO), is a circular orbit, circular geosynchronous or ...

(GEO). Orbits higher than low orbit can lead to early failure of electronic components due to intense

radiation In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or a material medium. This includes: * ''electromagnetic radiation'' consisting of photons, such as radio waves, microwaves, infr ...

and charge accumulation. In 2017, " very low Earth orbits" (VLEO) began to be seen in regulatory filings. These orbits, below about , require the use of novel technologies for orbit raising because they operate in orbits that would ordinarily decay too soon to be economically useful.

Use

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 Energy () is the physical quantity, quantitative physical property, property that is transferred to a physical body, body or to a physical system, recognizable in the performance of Work (thermodynamics), work and in the form of heat and l ...

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

s 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 low orbit have a small

field of view The field of view (FOV) is the angle, angular extent of the observable world that is visual perception, seen at any given moment. In the case of optical instruments or sensors, it is a solid angle through which a detector is sensitive to elec ...

and can only observe and communicate with a fraction of the Earth at a given time. This means that a large network (or

constellation A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The first constellati ...

) of satellites is required to provide continuous coverage. Satellites at lower altitudes of orbit are in the atmosphere and 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. The effects of adding such quantities of vaporized metals to Earth's stratosphere are potentially of concern but currently unknown.

Examples

* The

International Space Station The International Space Station (ISS) is a large space station that was Assembly of the International Space Station, assembled and is maintained in low Earth orbit by a collaboration of five space agencies and their contractors: NASA (United ...

is in LEO about above the Earth's surface. The station’s orbit decays by about and consequently needs re-boosting a few times a year. * The Iridium telecom satellites orbit at about . * Earth observation satellites, also known as

remote sensing Remote sensing is the acquisition of information about an physical object, object or phenomenon without making physical contact with the object, in contrast to in situ or on-site observation. The term is applied especially to acquiring inform ...

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

satellite A satellite or an artificial satellite is an object, typically a spacecraft, placed into orbit around a celestial body. They have a variety of uses, including communication relay, weather forecasting, navigation ( GPS), broadcasting, scient ...

s are placed in LEO. Satellites can also take advantage of consistent lighting of the surface below via Sun-synchronous LEO orbits at an altitude of about and near polar inclination. Envisat (2002–2012) is one example. * The

Hubble Space Telescope The Hubble Space Telescope (HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the Orbiting Solar Observatory, first space telescope, but it is one of the largest and most ...

orbits at about above Earth. * Satellite internet constellations such as Starlink. * The Chinese Tiangong space station was launched in April 2021 and currently orbits between above the Earth's surface. * The gravimetry mission GRACE-FO orbits at about as did its predecessor, GRACE.

Former

* GOCE (2009-2013), an ESA gravimetry mission, orbited at about 255 km (158 mi). * Super Low Altitude Test Satellite (2017-2019), nicknamed ''Tsubame'', orbited at , the lowest altitude ever among Earth observation satellites.

In fiction

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

Space debris

The LEO environment is becoming congested with

space debris Space debris (also known as space junk, space pollution, space waste, space trash, space garbage, or cosmic debris) are defunct human-made objects in spaceprincipally in Earth orbitwhich no longer serve a useful function. These include dere ...

because of the frequency of object launches. 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 A domino effect is the cumulative effect produced when one event sets off a series of similar or related events, a form of chain reaction. The term is an analogy to a falling row of dominoes. It typically refers to a linked sequence of events ...

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. The particles travel at speeds up to , so even a small impact can severely damage a spacecraft.

Notes

References

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