low Earth orbit
A low Earth orbit (LEO) is a circular orbit around Earth between the atmosphere and the Van Allen radiation belt. These boundaries are not firmly defined, but are typically around 350 - 800 km above the Earth's surface, with inclination angles less than 60 degrees from the equator. This is generally below intermediate circular orbit (ICO), Sun-synchronous orbit and far below 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 higher inclination angle (>~ 70 degrees) are usually called polar orbits.
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 (which did not reach LEO), and the Project Apollo missions to the Moon (which went beyond LEO).
Most early artificial satellites were placed in LEO. Here they travel at about 27,400 km/h (8 km/s), making one revolution in about 90 minutes. The primary exceptions are communication satellites, now common, that now mostly use geostationary orbit to obviate the requirement for dishes to track the satellite's movement and meteorological satellites. It requires less energy to place a satellite into LEO and the satellite needs less powerful transmitters for data transfer, so LEO is still used for occasional communication applications. Because these orbits are not geostationary, a network of satellites is required to provide continuous coverage. Lower orbits also aid remote sensing because of the added detail that can be gained.
The LEO environment is becoming congested, not least with space debris. 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 any orbit experience weightlessness. This is an effect of freefall and has nothing to do with the strength of the gravitational field.
Atmospheric and gravitational losses 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.