Low Earth orbit
From Wikipedia, the free encyclopedia
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)[1][2] 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.
Most manned spaceflights have been in LEO, including all Space Shuttle and various space station missions; the only exceptions have been suborbital test flights such as the early Project Mercury missions and the flights of the X-15 rocket plane (which was not intended to reach LEO), and the Project Apollo missions to the Moon.
Most artificial satellites are placed in LEO, where they travel at about 27,400 km/h (8 km/s), making one complete revolution around the Earth in about 90 minutes. 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. Lower orbits also aid remote sensing satellites because of the added detail that can be gained. Remote sensing satellites can also take advantage of sun-synchronous LEO orbits at an altitude of about 800km and near polar inclination. ENVISAT is one example of an Earth observation satellite that makes use of this particular type of LEO.
The LEO environment is becoming congested, not least with space debris. The Space Control Center[1], part of United States Strategic Command (formerly the United States Space Command), tracks more than 8,500 objects larger than 10cm in LEO.
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.
Atmospheric and gravity drag associated with launch typically add 1,500-2,000 m/s to the 
(delta-V) required to reach normal LEO orbital velocity of around 7,800 m/s.
