The Low Earth Orbit, LEO is used for the vast majority of satellites.
As the names imply, Low Earth Orbit is relatively low in altitude; the definition of LEO stating that the altitude range is between 200 and 1200 km above the Earth's surface
With Low Earth Orbit extending from 200 km to 1200 km it means that it is relatively low in altitude, although well above anything that a conventional aircraft can reach.
However LEO is still very close to the Earth, especially when compared to other forms of satellite orbit including geostationary orbit.
The low orbit altitude of leads to a number of characteristics:
- Orbit times are much less than for many other forms of orbit. The lower altitude means higher velocities are required to balance the earth's gravitational field. Typical velocities are very approximately around 8 km/s, with orbit times sometimes of the order of 90 minutes, although these figures vary considerably with the exact details of the orbit.
- The lower orbit means the satellite and user are closer together and therefore path losses a less than for other orbits such as GEO
- The round trip time, RTT for the radio signals is considerably less than that experienced by geostationary orbit satellites. The actual time will depend upon factors such as the orbit altitude and the position of the user relative to the satellite.
- Radiation levels are lower than experienced at higher altitudes.
- Less energy is expended placing the satellites in LEO than higher orbits.
- Some speed reduction may be experienced as a result of friction from the low, but measurable levels of gasses, especially at lower altitudes. An altitude of 300 km is normally accepted as the minimum for an orbit as a result of the increasing drag from the presence of gasses at low altitudes.
Applications for LEO satellites
A variety of different types of satellite use the LEO orbit levels. These include different types and applications including:
- Communications satellites - some communications satellites including the Iridium phone system use LEO.
- Earth monitoring satellites use LEO as they are able to see the surface of the Earth more clearly as they are not so far away. They are also able to traverse the surface of the Earth.
- The International Space Station is in an LEO that varies between 320 km (199 miles) and 400 km (249 miles) above the Earth's surface. It can often be seen from the Earth's surface with the naked eye.
Space debris in LEO
Apart from the general congestion experienced in Low Earth Orbit, the situation is made much worse by the general level of space debris that exists.
There is a real and growing risk of collision and major damage - any collisions themselves are likely to create further space debris.
The US Joint Space Operations Center currently tracks over 8 500 objects that have dimensions larger than 10 centimetres. However debris with smaller dimensions can also cause significant damage and could render a satellite unserviceable after a collision.