low Earth orbit (LEO)

A spacecraft orbit about the Earth typically used for manned missions and for Earth remote-sensing missions; the minimum altitude above the surface is c.200 km/125 mi to minimize drag effects of the Earth's atmosphere. The inclination of orbit is chosen to allow the ground track of the spacecraft to pass over regions of interest; polar inclination orbits are needed for complete global coverage. Typical orbital periods are c.100 min; circular velocity c.7·8 km/s (4·9 mi/s). Depending on altitude, the orbit may eventually decay, causing the spacecraft to re-enter Earth's atmosphere and burn up; because of the occasional destruction of spacecraft in LEO there is an increasing accumulation of tiny debris particles there, a new hazard to spacecraft.

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A low Earth orbit (LEO) is generally defined as an orbit within the locus extending from the Earth’s surface up to an altitude of 2,000 km. Given the rapid orbital decay of objects below approximately 200 km, the commonly accepted definition for LEO is between 200 - 2000 km (124 - 1240 miles) above the Earth's surface. Objects in LEO encounter atmospheric drag in the form of gases in the thermosphere (approximately 80-500 km up) or exosphere (approximately 500 km and up), depending on orbit height. LEO is an orbit around Earth between 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 of any orbit. Orbits with a high inclination angle 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 earlier failure of electronic components due to intense radiation and charge accumulation, while commercial devices such as laptops have been used successfully in LEO during manned flight. The primary exception are communication satellites that require geostationary orbit, and which move as the same angular velocity as the Earth rotates. However, it requires less energy to place a satellite into a LEO and the LEO satellite needs less powerful amplifiers for successful transmission, so LEO is still used for many communication applications. Because these LEO orbits are not geostationary, a network (or "constellation") of satellites is required to provide continuous coverage. Remote sensing satellites can also take advantage of sun-synchronous LEO orbits at an altitude of about 800km and near polar inclination.

Although the Earth's pull due to gravity in LEO is not much less than on the surface of the Earth, people and objects in orbit experience weightlessness due to the effects of freefall.