A low Earth orbit (LEO) is an orbit around Earth with a

period Period may refer to: Common uses * Period (punctuation) * Era, a length or span of time *Menstruation, commonly referred to as a "period" Arts, entertainment, and media * Period (music), a concept in musical composition * Periodic sentence (o ...

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 A medium Earth orbit (MEO) is an geocentric orbit, Earth-centered orbit with an altitude above a low Earth orbit (LEO) and below a high Earth orbit (HEO) – between above sea level.

(MEO), have an altitude of 2,000 km, about one-third of the

radius of Earth Earth radius (denoted as ''R''🜨 or ''R''E) is the distance from the center of Earth to a point on or near its surface. Approximating the figure of Earth by an Earth spheroid (an oblate ellipsoid), the radius ranges from a maximum (equatoria ...

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 An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. The line of apsides (also called apse line, or major axis of the orbit) is the line connecting the two extreme values. Apsides perta ...

further out or are

sub-orbital A sub-orbital spaceflight is a spaceflight in which the spacecraft reaches outer space, but its trajectory intersects the surface of the gravitating body from which it was launched. Hence, it will not complete one orbital revolution, will no ...

, 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 In astronomy, the geocentric model (also known as geocentrism, often exemplified specifically by the Ptolemaic system) is a superseded description of the Universe with Earth at the center. Under most geocentric models, the Sun, Moon, stars, an ...

within LEO.

Defining characteristics

A wide variety of sources define LEO in terms of

. The altitude of an object in an

elliptic orbit In astrodynamics or celestial mechanics, an elliptical orbit or eccentric orbit is an orbit with an eccentricity of less than 1; this includes the special case of a circular orbit, with eccentricity equal to 0. Some orbits have been referre ...

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 orbit A polar orbit is one in which a satellite passes above or nearly above both poles of the body being orbited (usually a planet such as the Earth, but possibly another body such as the Moon or Sun) on each revolution. It has an inclination of abo ...

s) due to the

oblateness Flattening is a measure of the compression of a circle or sphere along a diameter to form an ellipse or an ellipsoid of revolution (spheroid) respectively. Other terms used are ellipticity, or oblateness. The usual notation for flattening is f ...

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 In astronomy, Kepler's laws of planetary motion, published by Johannes Kepler in 1609 (except the third law, which was fully published in 1619), describe the orbits of planets around the Sun. These laws replaced circular orbits and epicycles in ...

, this corresponds to a

semi-major axis In geometry, the major axis of an ellipse is its longest diameter: a line segment that runs through the center and both foci, with ends at the two most widely separated points of the perimeter. The semi-major axis (major semiaxis) is the longe ...

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 orbit A highly elliptical orbit (HEO) is an elliptic orbit with high eccentricity, usually referring to one around Earth. Examples of inclined HEO orbits include Molniya orbits, named after the Molniya Soviet communication satellites which used them, ...

s may pass through the LEO region near their lowest altitude (or

perigee An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. The line of apsides (also called apse line, or major axis of the orbit) is the line connecting the two extreme values. Apsides perta ...

) but are not in a LEO orbit because their highest altitude (or

) exceeds .

Sub-orbital A sub-orbital spaceflight is a spaceflight in which the spacecraft reaches outer space, but its trajectory intersects the surface of the gravitating body from which it was launched. Hence, it will not complete one orbital revolution, will no ...

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 Delta-''v'' (also known as "change in velocity"), symbolized as and pronounced , as used in spacecraft flight dynamics, is a measure of the impulse per unit of spacecraft mass that is needed to perform a maneuver such as launching from or l ...

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 Newton's law of universal gravitation describes gravity as a force by stating that every particle attracts every other particle in the universe with a force that is proportional to the product of their masses and inversely proportional to the sq ...

and the

centrifugal force Centrifugal force is a fictitious force in Newtonian mechanics (also called an "inertial" or "pseudo" force) that appears to act on all objects when viewed in a rotating frame of reference. It appears to be directed radially away from the axi ...

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 Weightlessness is the complete or near-complete absence of the sensation of weight, i.e., zero apparent weight. It is also termed zero g-force, or zero-g (named after the g-force) or, incorrectly, zero gravity. Weight is a measurement of the fo ...

. Objects in LEO orbit Earth between the denser part of the atmosphere and below the inner

Van Allen radiation belt The Van Allen radiation belt is a zone of energetic charged particles, most of which originate from the solar wind, that are captured by and held around a planet by that planet's magnetosphere. Earth has two such belts, and sometimes others ma ...

. They encounter atmospheric drag from

gases Gas is a state of matter that has neither a fixed volume nor a fixed shape and is a compressible fluid. A ''pure gas'' is made up of individual atoms (e.g. a noble gas like neon) or molecules of either a single type of atom ( elements such ...

in the

thermosphere The thermosphere is the layer in the Earth's atmosphere directly above the mesosphere and below the exosphere. Within this layer of the atmosphere, ultraviolet radiation causes photoionization/photodissociation of molecules, creating ions; the ...

(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 Decay may refer to: Science and technology * Bit decay, in computing * Decay time (fall time), in electronics * Distance decay, in geography * Software decay, in computing Biology * Decomposition of organic matter * Mitochondrial decay, in g ...

quickly due to atmospheric drag. Equatorial low Earth orbits (ELEO) are a subset of LEO. These orbits, with low

orbital inclination Orbital inclination measures the tilt of an object's orbit around a celestial body. It is expressed as the angle between a reference plane and the orbital plane or axis of direction of the orbiting object. For a satellite orbiting the Earth ...

, allow rapid revisit times over low-latitude locations on Earth. Prograde equatorial LEOs also have lower

launch requirements because they take advantage of the Earth's rotation. Other useful LEO orbits including

s 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 Starlink is a satellite internet constellation operated by Starlink Services, LLC, an international telecommunications provider that is a wholly owned subsidiary of American aerospace company SpaceX, providing coverage to around 130 countries ...

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

(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 Regulation is the management of complex systems according to a set of rules and trends. In systems theory, these types of rules exist in various fields of biology and society, but the term has slightly different meanings according to context. Fo ...

filings. These orbits, below about , require the use of novel technologies for

orbit raising In celestial mechanics, an orbit (also known as orbital revolution) is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an ...

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 satellite A communications satellite is an artificial satellite that relays and amplifies radio telecommunication signals via a transponder; it creates a communication channel between a source transmitter and a receiver at different locations on Earth. ...

s 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 A geosynchronous satellite is a satellite in geosynchronous orbit, with an orbital period the same as the Earth's rotation period. Such a satellite returns to the same position in the sky after each sidereal day, and over the course of a day tra ...

, 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 Orbital decay is a gradual decrease of the distance between two orbiting bodies at their closest approach (the periapsis) over many orbital periods. These orbiting bodies can be a planet and its satellite, a star and any object orbiting it, or ...

, 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 The stratosphere () is the second-lowest layer of the atmosphere of Earth, located above the troposphere and below the mesosphere. The stratosphere is composed of stratified temperature zones, with the warmer layers of air located higher ...

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 satellite An Earth observation satellite or Earth remote sensing satellite is a satellite used or designed for Earth observation (EO) from orbit, including spy satellites and similar ones intended for non-military uses such as environmental monitoring, me ...

s, 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 satellite A reconnaissance satellite or intelligence satellite (commonly, although unofficially, referred to as a spy satellite) is an Earth observation satellite or communications satellite deployed for military or intelligence applications. The ...

s and other

Earth imaging Satellite images (also Earth observation imagery, spaceborne photography, or simply satellite photo) are images of Earth collected by imaging satellites operated by governments and businesses around the world. Satellite imaging companies sell i ...

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 Envisat ("Environmental Satellite") is a large Earth-observing satellite which has been inactive since 2012. It is still in orbit and considered space debris. Operated by the European Space Agency (ESA), it was the world's largest civilian Ear ...

(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 constellation A satellite internet constellation is a constellation of artificial satellites providing satellite internet service. In particular, the term has come to refer to a new generation of very large constellations (sometimes referred to as megacon ...

s such as

. * The Chinese

Tiangong space station Tiangong (), officially the ''Tiangong'' space station (), is a permanently crewed space station constructed by China and operated by China Manned Space Agency. Tiangong is a modular design, with modules docked together while in low Earth o ...

was launched in April 2021 and currently orbits between above the Earth's surface. * The

gravimetry Gravimetry is the measurement of the strength of a gravitational field. Gravimetry may be used when either the magnitude of a gravitational field or the properties of matter responsible for its creation are of interest. The study of gravity c ...

mission GRACE-FO orbits at about as did its predecessor,

GRACE Grace may refer to: Places United States * Grace, Idaho, a city * Grace (CTA station), Chicago Transit Authority's Howard Line, Illinois * Little Goose Creek (Kentucky), location of Grace post office * Grace, Carroll County, Missouri, an uni ...

Former

GOCE The Gravity Field and Steady-State Ocean Circulation Explorer (GOCE) was the first of European Space Agency, ESA's Living Planet Programme satellite, heavy satellites intended to map in unprecedented detail the Earth's gravity field. The spacecr ...

(2009-2013), an

ESA The European Space Agency (ESA) is a 23-member international organization devoted to space exploration. With its headquarters in Paris and a staff of around 2,547 people globally as of 2023, ESA was founded in 1975 in the context of European ...

gravimetry mission, orbited at about 255 km (158 mi). *

Super Low Altitude Test Satellite Super Low Altitude Test Satellite (SLATS) or Tsubame was a JAXA satellite intended to demonstrate operations in very low Earth orbit (VLEO, below 200 km), using ion engines to counteract aerodynamic drag from the Earth's atmosphere which ...

(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 The Kessler syndrome, also known as the Kessler effect, collisional cascading, or ablation cascade, is a scenario proposed by NASA scientists Donald J. Kessler and Burton G. Cour-Palais in 1978. It describes a situation in which the density of o ...

. 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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