A medium Earth orbit (MEO) is an Earth-centred orbit with an altitude above a low Earth orbit (LEO) and below a high Earth orbit (HEO) – between 2,000 and 35,786 km (1,243 and 22,236 mi) above sea level.
The boundary between MEO and LEO is an arbitrary altitude chosen by accepted convention, whereas the boundary between MEO and HEO is the particular altitude of a geosynchronous orbit, in which a satellite takes 24 hours to circle the Earth, the same period as the Earth’s own rotation. All satellites in MEO have an orbital period of less than 24 hours, with the minimum period (for a circular orbit at the lowest MEO altitude) about 2 hours.
Satellites in MEO orbits are perturbed by solar radiation pressure, which is the dominating non-gravitational perturbing force. Other perturbing forces include: Earth's albedo, navigation antenna thrust, and thermal effects related to heat re-radiation.
The MEO region includes the two zones of energetic charged particles above the equator known as the Van Allen radiation belts, which can damage satellites’ electronic systems without special shielding.
Two medium Earth orbits are particularly significant. A satellite in the semi-synchronous orbit at an altitude of approximately 20,200 kilometres (12,600 mi) has an orbital period of 12 hours and passes over the same two spots on the equator every day. This reliably predictable orbit is used by the Global Positioning System (GPS) constellation. Other navigation satellite systems use similar medium Earth orbits including GLONASS (with an altitude of 19,100 kilometres, 11,900 mi), Galileo (with an altitude of 23,222 kilometres, 14,429 mi) and BeiDou (with an altitude of 21,528 kilometres, 13,377 mi).
The Molniya orbit has a high inclination of 63.4° and high eccentricity of 0.722 with a period of 12 hours, so a satellite spends most of its orbit above the chosen area in high latitudes. This orbit was used by the (now defunct) North American Sirius Satellite Radio and XM Satellite Radio satellites and the Russian Molniya military communications satellites, after which it is named.
Communications satellites in MEO include the O3b and forthcoming O3b mPOWER constellations for telecommunications and data backhaul to maritime, aero and remote locations (with an altitude of 8,063 kilometres, 5,010 mi).
Communications satellites to cover the North and South Pole are also put in MEO.