A Low Earth Orbit (LEO) is an Earth-centered orbit with an altitude of 2,000 km (1,200 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. Most of the manmade objects in space are in LEO. A histogram of the mean motion of the cataloged objects shows that the number of objects drops significantly beyond 11.25.
A geocentric orbit or Earth orbit involves any object orbiting Planet Earth, such as the Moon or artificial satellites. In 1997 NASA estimated there were approximately 2,465 artificial satellite payloads orbiting the Earth and 6,216 pieces of space debris as tracked by the Goddard Space Flight Center. Over 16,291 previously launched objects have decayed into the Earth's atmosphere.
Altitude or height is defined based on the context in which it is used. As a general definition, altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The reference datum also often varies according to the context. Although the term altitude is commonly used to mean the height above sea level of a location, in geography the term elevation is often preferred for this usage.
Earth radius is the distance from the center of Earth to a point on its surface. Its value ranges from 6,378 kilometres at the equator to 6,357 kilometres at a pole.
There is a large variety of other sources 30 km (19 mi) (especially for polar orbits) due to the oblateness of Earth's spheroid figure and local topography. While definitions in terms of 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 limits in the LEO definitions in terms of altitude.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
In astrodynamics or celestial mechanics, an elliptic orbit or elliptical orbit is a Kepler orbit with an eccentricity of less than 1; this includes the special case of a circular orbit, with eccentricity equal to 0. In a stricter sense, it is a Kepler orbit with the eccentricity greater than 0 and less than 1. In a wider sense, it is a Kepler orbit with negative energy. This includes the radial elliptic orbit, with eccentricity equal to 1.
A circular orbit is the orbit with a fixed distance around the barycenter, that is, in the shape of a circle.
A polar orbit is one in which a satellite passes above or nearly above both poles of the body being orbited on each revolution. It therefore has an inclination of 90 degrees to the body's equator. A satellite in a polar orbit will pass over the equator at a different longitude on each of its orbits.
The LEO region is defined by some sources as the region in space that LEO orbits occupy. 2,000 km (1,200 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.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
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, and tundra orbits.
The term apsis refers to an extreme point in the orbit of an object. It denotes either the points on the orbit, or the respective distance of the bodies. The word comes via Latin from Greek, there denoting a whole orbit, and is cognate with apse. Except for the theoretical possibility of one common circular orbit for two bodies of equal mass at diametral positions, there are two apsides for any elliptic orbit, named with the prefixes peri- and ap-/apo-, added in reference to the body being orbited. All periodic orbits are, according to Newton's Laws of motion, ellipses: either the two individual ellipses of both bodies, with the center of mass of this two-body system at the one common focus of the ellipses, or the orbital ellipses, with one body taken as fixed at one focus, and the other body orbiting this focus. All these ellipses share a straight line, the line of apsides, that contains their major axes, the foci, and the vertices, and thus also the periapsis and the apoapsis. The major axis of the orbital ellipse is the distance of the apsides, when taken as points on the orbit, or their sum, when taken as distances.
A sub-orbital spaceflight is a spaceflight in which the spacecraft reaches outer space, but its trajectory intersects the atmosphere or surface of the gravitating body from which it was launched, so that it will not complete one orbital revolution.
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 1,374.1 km (853.8 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.
The International Space Station (ISS) is a space station, or a habitable artificial satellite, in low Earth orbit. Its first component was launched into orbit in 1998, with the first long-term residents arriving in November 2000. It has been inhabited continuously since that date. The last pressurised module was fitted in 2011, and an experimental inflatable space habitat was added in 2016. The station is expected to operate until 2030. Development and assembly of the station continues, with several new elements scheduled for launch in 2019. The ISS is the largest human-made body in low Earth orbit and can often be seen with the naked eye from Earth. The ISS consists of pressurised habitation modules, structural trusses, solar arrays, radiators, docking ports, experiment bays and robotic arms. ISS components have been launched by Russian Proton and Soyuz rockets and American Space Shuttles.
A space station, also known as an orbital station or an orbital space station, is a spacecraft capable of supporting crewmembers, which is designed to remain in space for an extended period of time and for other spacecraft to dock. A space station is distinguished from other spacecraft used for human spaceflight by lack of major propulsion or landing systems. Instead, other vehicles transport people and cargo to and from the station. As of 2018, one fully functioning space station is in Earth orbit: the International Space Station. Various other components of future space stations, such as Japan's space elevator and U.S. inflatable modules, are also being tested in orbit. Previous stations include the Almaz and Salyut series, Skylab, Mir, and Tiangong-1 and Tiangong-2. China, Russia, the U.S., as well as a few private companies are all planning other stations for the coming decades.
In the context of spaceflight, a satellite is an artificial object which has been intentionally placed into orbit. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as Earth's Moon.
The mean orbital velocity needed to maintain a stable low Earth orbit is about 7.8 km/s, but reduces with increased orbital altitude. Calculated for circular orbit of 200 km it is 7.79 km/s and for 1500 km it is 7.12 km/s. 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 to the launch vehicle delta-v required to reach normal LEO orbital velocity of around 7.8 km/s (28,080 km/h).
Delta-v, symbolised as ∆v and pronounced delta-vee, as used in spacecraft flight dynamics, is a measure of the impulse that is needed to perform a maneuver such as launch from, or landing on a planet or moon, or in-space orbital maneuver. It is a scalar that has the units of speed. As used in this context, it is not the same as the physical change in velocity of the vehicle.
In astrodynamics and rocketry, gravity drag is a measure of the loss in the net performance of a rocket while it is thrusting in a gravitational field. In other words, it is the cost of having to hold the rocket up in a gravity field.
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.
Weightlessness is the complete or near-complete absence of the sensation of weight. This is also termed zero-g, although the more correct term is "zero g-force." It occurs in the absence of any contact forces upon objects including the human body.
Objects in LEO encounter atmospheric drag from gases in the thermosphere (approximately 80–500 km above the surface) or exosphere (approximately 500 km and up), depending on orbit height. Due to atmospheric drag, satellites do not usually orbit below 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.
In 2017, a 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.
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.
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.
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. However, a limited Arecibo Observatory study suggested there could be approximately one million objects larger than 2 millimeters, which are too small to be visible from Earth-based observatories.
A space elevator is a proposed type of planet-to-space transportation system. The main component would be a cable anchored to the surface and extending into space. The design would permit vehicles to travel along the cable from a planetary surface, such as the Earth's, directly into space or orbit, without the use of large rockets. An Earth-based space elevator would consist of a cable with one end attached to the surface near the equator and the other end in space beyond geostationary orbit. The competing forces of gravity, which is stronger at the lower end, and the outward/upward centrifugal force, which is stronger at the upper end, would result in the cable being held up, under tension, and stationary over a single position on Earth. With the tether deployed, climbers could repeatedly climb the tether to space by mechanical means, releasing their cargo to orbit. Climbers could also descend the tether to return cargo to the surface from orbit.
A geosynchronous orbit is an orbit around Earth of a satellite with an orbital period that matches Earth's rotation on its axis, which takes one sidereal day. The synchronization of rotation and orbital period means that, for an observer on Earth's surface, an object in geosynchronous orbit returns to exactly the same position in the sky after a period of one sidereal day. Over the course of a day, the object's position in the sky traces out a path, typically in a figure-8 form, whose precise characteristics depend on the orbit's inclination and eccentricity. Satellites are typically launched in an eastward direction. A circular geosynchronous orbit is 35,786 km (22,236 mi) above the Earth's surface. Those closer to Earth orbit faster than Earth rotates, so from Earth, they appear to move eastward while those that orbit beyond geosynchronous distances appear to move westward.
A geostationary orbit, often referred to as a geosynchronous equatorial orbit (GEO), is a circular geosynchronous orbit 35,786 km (22,236 mi) above Earth's equator and following the direction of Earth's rotation. An object in such an orbit appears motionless, at a fixed position in the sky, to ground observers. Communications satellites and weather satellites are often placed in geostationary orbits, so that the satellite antennae that communicate with them do not have to rotate to track them, but can be pointed permanently at the position in the sky where the satellites are located. Using this characteristic, ocean-color monitoring satellites with visible and near-infrared light sensors can also be operated in geostationary orbit in order to monitor sensitive changes of ocean environments.
A communications satellite is an artificial satellite that relays and amplifies radio telecommunications signals via a transponder; it creates a communication channel between a source transmitter and a receiver at different locations on Earth. Communications satellites are used for television, telephone, radio, internet, and military applications. There are 2,134 communications satellites in Earth’s orbit, used by both private and government organizations. Many are in geostationary orbit 22,200 miles (35,700 km) above the equator, so that the satellite appears stationary at the same point in the sky, so the satellite dish antennas of ground stations can be aimed permanently at that spot and do not have to move to track it.
Spaceflight is ballistic flight into or through outer space. Spaceflight can occur with spacecraft with or without humans on board. Yuri Gagarin of the Soviet Union was the first human to conduct a spaceflight. Examples of human spaceflight include the U.S. Apollo Moon landing and Space Shuttle programs and the Russian Soyuz program, as well as the ongoing International Space Station. Examples of unmanned spaceflight include space probes that leave Earth orbit, as well as satellites in orbit around Earth, such as communications satellites. These operate either by telerobotic control or are fully autonomous.
In orbital mechanics, the Hohmann transfer orbit is an elliptical orbit used to transfer between two circular orbits of different radii in the same plane. In general a Hohmann transfer orbit uses the lowest possible amount of energy in traveling between two objects orbiting at these radii, and so is used to send the maximum amount of mission payload with the fixed amount of energy that can be imparted by a particular rocket. Non-Hohmann transfer paths may have other advantages for a particular mission such as shorter transfer times, but will necessarily require a reduction in payload mass and/or use of a more powerful rocket.
A geosynchronous transfer orbit or geostationary transfer orbit (GTO) is a Hohmann transfer orbit—an elliptical orbit used to transfer between two circular orbits of different radii in the same plane—used to reach geosynchronous or geostationary orbit using high-thrust chemical engines.
Initially, the term space debris referred to the natural debris found in the solar system: asteroids, comets, and meteoroids. However, with the 1979 beginning of the NASA Orbital Debris Program, the term also refers to the debris from the mass of defunct, artificially created objects in space, especially Earth orbit. These include old satellites and spent rocket stages, as well as the fragments from their disintegration and collisions.
The Kármán line, or Karman line, is an attempt to define a boundary between Earth's atmosphere and outer space. This is important for legal and regulatory measures; aircraft and spacecraft fall under different jurisdictions and are subject to different treaties.
An orbital spaceflight is a spaceflight in which a spacecraft is placed on a trajectory where it could remain in space for at least one orbit. To do this around the Earth, it must be on a free trajectory which has an altitude at perigee above 100 kilometers (62 mi); this is, by at least one convention, the boundary of space. To remain in orbit at this altitude requires an orbital speed of ~7.8 km/s. Orbital speed is slower for higher orbits, but attaining them requires greater delta-v.
In astrodynamics and aerospace, a delta-v budget is an estimate of the total delta-v required for a space mission. It is calculated as the sum of the delta-v required for the propulsive maneuvers during the mission, and as input to the Tsiolkovsky rocket equation, determines how much propellant is required for a vehicle of given mass and propulsion system.
A space gun, sometimes called a Verne gun because of its appearance in From the Earth to the Moon by Jules Verne, is a method of launching an object into space using a large gun- or cannonlike structure. Space guns could thus potentially provide a method of non-rocket spacelaunch. It has been conjectured that space guns could place satellites into Earth's orbit, and could also launch spacecraft beyond Earth's gravitational pull and into other parts of the Solar System by exceeding Earth's escape velocity of about 11.2 km/s or 40,320 km/h (25,050 mph). However, these speeds are too far into the hypersonic range for most practical propulsion systems and also would cause most objects to burn up due to aerodynamic heating or be torn apart by aerodynamic drag. Therefore, a more likely future use of space guns would be to launch objects into near Earth orbit, from where attached rockets could be fired or the objects could be "collected" by maneuverable orbiting satellites.
A graveyard orbit, also called a junk orbit or disposal orbit, is an orbit that lies away from common operational orbits. One significant graveyard orbit is a supersynchronous orbit well above geosynchronous orbit. Satellites are typically moved into such orbits at the end of their operational life to reduce the probability of colliding with operational spacecraft and generating space debris.
The Kessler syndrome, proposed by the NASA scientist Donald J. Kessler in 1978, is a scenario in which the density of objects in low Earth orbit (LEO) is high enough that collisions between objects could cause a cascade where each collision generates space debris that increases the likelihood of further collisions. One implication is that the distribution of debris in orbit could render space activities and the use of satellites in specific orbital ranges impractical for many generations.
Medium Earth orbit (MEO), sometimes called intermediate circular orbit (ICO), is the region of space around Earth above low Earth orbit and below geosynchronous orbit.
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
LEO: Mean Motion > 11.25 & Eccentricity < 0.25
LEO refers to orbits that are typically less than 2,400 km (1,491 mi) in altitude.
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.
LEO is the first 100 to 200 miles (161 to 322 km) of space.
LEO region (100 kms [sic] to 1000 kms)
the low-Earth orbit (LEO) environment, defined as 200–1,000 km above Earth's surface