Low Earth orbit
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A low Earth orbit (LEO) is a circular orbit around Earth between the atmosphere and the Van Allen radiation belt, with a low angle of inclination. These boundaries are not firmly defined, but are typically around 350 - 1400 km above the Earth's surface, with inclination angles less than 60 degrees from the equator. This is generally below intermediate circular orbit (ICO) 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 are called polar orbits. Most artificial satellites are placed in LEO. Satellites in a LEO travel at ca. 27,400 km/h (8 km/s), that is one revolution in circa 90 minutes.
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.
A low earth orbit is a stepping-stone to travel beyond orbit but it is also very useful for communication satellites because of its proximity to Earth. It requires less energy to place a satellite into a LEO and the satellite needs less powerful transmitters for data transfer, the downside is that a network of satellites are required to provide continuous coverage. Remote sensing satellites are also often in LEOs because of the added detail that can be gained.
Most manned spaceflights are in LEO. The only exceptions have been:
- In Project Apollo there were six manned space missions to the Moon, and three more to lunar orbit (including one during which the landing was cancelled)
The LEO environment is becoming congested, not least with space junk. The United States Space Command tracks more than 8,000 objects larger than 10cm in LEO.
Although gravity at the height concerned is by itself not much less than on the surface of the Earth (it reduces 1% every 30 km), people and objects in orbit are weightless (see article).
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.
Alternative
Airships have been proposed to hover above the Earth at an altitude of around 13 miles (20 kilometres) as communication stations, to provide cellular voice and data service. Solar-powered unpiloted aircraft (UAVs) are also projected for this purpose.