Low Earth orbit

A low Earth orbit (LEO) is an orbit around Earth between the atmosphere and the Van Allen radiation belt. The boundaries are not firmly defined but are typically around 200 - 1200 km (124 - 726 miles) above the Earth's surface. Above this are intermediate circular orbit (ICO) and, far above, geostationary orbit. Orbits lower than this are not stable and will decay rapidly because of atmospheric drag. Orbits higher than this are subject to early electronic failure because of intense radiation and charge accumulation. Orbits with a high inclination angle are usually called polar orbits, the properties of this and the sun-synchronous orbit being more significant than their altitude.

Objects in low earth orbit encounter atmospheric gases in the thermosphere (approximately 80-500 km up) or exosphere (approximately 500 km and up), depending on orbit height.

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 (which went beyond LEO).

Most artificial satellites are placed in LEO, where they travel at about 27,400 km/h (8 km/s), making one revolution in about 90 minutes. The primary exception are communication satellites that require geostationary orbit. However, it requires less energy to place a satellite into a LEO and the satellite needs less powerful transmitters for data transfer, so LEO is still used for many communication applications. Because these orbits are not geostationary, a network 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 special type of LEO.

The LEO environment is becoming congested, not least with space debris. The North American Aerospace Defense Command (NORAD), part of the United States Strategic Command (formerly the United States Space Command), tracks more than 8,000 objects larger than 10cm in LEO.

Although gravity in LEO is not much less than on the surface of the Earth (it reduces 1% every 30 km), people and objects in orbit experience weightlessness.

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 7,800 m/s.

Low Earth orbit: Encyclopedia BETA

Low Earth orbit

See also