A medium Earth orbit (MEO) is an Earth-centered orbit with an altitude above a low Earth orbit (LEO) and below a high Earth orbit (HEO) – between 2000and above sea level.[1]
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.[2]
Satellites in MEO orbits are perturbed by solar radiation pressure, which is the dominating non-gravitational perturbing force.[3] 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.[4]
A medium Earth orbit is sometimes called mid Earth orbit[1] or intermediate circular orbit (ICO).[2]
Two medium Earth orbits are particularly significant. A satellite in the semi-synchronous orbit at an altitude of approximately has an orbital period of 12 hours and passes over the same two spots on the equator every day.[1] This reliably predictable orbit is used by the Global Positioning System (GPS) constellation.[2] Other navigation satellite systems use similar medium Earth orbits including GLONASS (with an altitude of 19100km (11,900miles)),[5] Galileo (with an altitude of 232220NaN0)[6] and BeiDou (with an altitude of 215280NaN0).[7]
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.[1]
Communications satellites in MEO include the O3b and O3b mPOWER constellations for low-latency broadband and data backhaul to maritime, aero and remote locations (with an altitude of 80630NaN0).[8]
Communications satellites to cover the North and South Pole are also put in MEO.[9]
Telstar 1, an experimental communications satellite launched in 1962, orbited in MEO.[10]
In May 2022, Kazakhstani mobile network operator, Kcell, and satellite owner and operator, SES used SES's O3b MEO satellite constellation to demonstrate that MEO satellites could be used to provide high-speed mobile internet to remote regions of Kazakhstan for reliable video calling, conferencing and streaming, and web browsing, with a latency (delay) five times lower than on the existing platform based on geostationary orbit satellites.[11] [12]
In September 2023, satellite operator SES announced the first satellite internet service to use satellite constellations in both MEO and Low Earth Orbit (LEO). The SES Cruise mPOWERED + Starlink service will use SES's O3b mPOWER MEO satellites and SpaceX's Starlink LEO system to provide cruise ship passengers with internet, social media and video calls at up to 3 Gbps per ship anywhere in the World. Subsequently, in February 2024, SES announced that Virgin Voyages will be the first cruise line to deploy the service.[13] [14] [15]
Space debris in medium Earth orbit stay practically permanently orbiting Earth. Most space debris extends to the lowest high Earth orbits just beyond the edge of medium Earth orbit, where geostationary satellites are and where after their end of use they are parked in similar orbits, so-called graveyard orbits.