thumb|upright=1.3|Animation (not to scale) showing geosynchronous satellite orbiting the Earth

A geosynchronous orbit (sometimes abbreviated GEO) is an Earth-centered orbit with an orbital period that matches Earth's rotation on its axis, 23 hours, 56 minutes, and 4 seconds (one sidereal day). The synchronization of rotation and orbital period means that, for an observer on Earth's surface, an object in geosynchronous orbit returns to exactly the same position in the sky after a period of one sidereal day. Over the course of a day, the object's position in the sky may remain still or trace out a path, typically in a figure-8 form, whose precise characteristics depend on the orbit's inclination and eccentricity. A circular geosynchronous orbit has a constant altitude of .

A special case of geosynchronous orbit is the geostationary orbit (often abbreviated GSO), which is a circular geosynchronous orbit in Earth's equatorial plane with both inclination and eccentricity equal to 0. A satellite in a geostationary orbit remains in the same position in the sky to observers on the surface. The first appearance of a geosynchronous orbit in popular literature was in October 1942, in the first Venus Equilateral story by George O. Smith, but Smith did not go into details. British science fiction author Arthur C. Clarke popularised and expanded the concept in a 1945 paper entitled Extra-Terrestrial Relays – Can Rocket Stations Give Worldwide Radio Coverage?, published in Wireless World magazine. Clarke acknowledged the connection in his introduction to The Complete Venus Equilateral. is sometimes called the Clarke Orbit. Similarly, the collection of artificial satellites in this orbit is known as the Clarke Belt.

[[Syncom#Syncom 2|Syncom 2: The first functional geosynchronous satellite|thumb|left]]

In technical terminology, the geosynchronous orbits are often referred to as geostationary if they are roughly over the equator, but the terms are used somewhat interchangeably. Specifically, geosynchronous Earth orbit (GEO) may be a synonym for geosynchronous equatorial orbit, or geostationary Earth orbit.

Conventional wisdom at the time was that it would require too much rocket power to place a satellite in a geosynchronous orbit and it would not survive long enough to justify the expense, so early efforts were put towards constellations of satellites in low or medium Earth orbit. Although these projects had difficulties with signal strength and tracking that could be solved through geosynchronous satellites, the concept was seen as impractical, so Hughes often withheld funds and support. In August 1961, they were contracted to begin building the working satellite.

Today, hundreds of geosynchronous satellites provide remote sensing, navigation and communications. some rural and remote areas in developed countries still rely on satellite communications.

Types

Geostationary orbit

thumb|The geostationary satellite (green) always remains above the same marked spot on the equator (brown).

A geostationary orbit (GSO) is a circular (zero eccentricity) geosynchronous orbit in the plane of the Earth's equator (zero inclination) with a radius of approximately (measured from the center of the Earth).

A perfectly stable geostationary orbit is an ideal that can only be approximated. In practice the satellite drifts out of this orbit because of perturbations such as the solar wind, radiation pressure, variations in the Earth's gravitational field, and the gravitational effect of the Moon and Sun, and thrusters are used to maintain the orbit in a process known as station-keeping.

Eventually, without the use of thrusters, the orbit becomes inclined, oscillating between 0° and 15° every 55 years. At the end of the satellite's lifetime, when fuel approaches depletion, satellite operators may decide to omit these expensive manoeuvres to correct inclination and only control eccentricity. This prolongs the life-time of the satellite as it consumes less fuel over time, but the satellite can then only be used by ground antennas capable of following the N-S movement. At least two satellites are needed to provide continuous coverage over an area. It was used by Sirius XM Satellite Radio to improve signal strength in the northern US and Canada.

Quasi-zenith orbit

The Quasi-Zenith Satellite System (QZSS) is a four-satellite system that operates in a geosynchronous orbit at an inclination of 42° and a 0.075 eccentricity. Each satellite dwells over Japan, allowing signals to reach receivers in urban canyons, then passes quickly over Australia.

Launch

Geosynchronous satellites are launched to the east into a prograde orbit that matches the rotation rate of the equator. The smallest inclination that a satellite can be launched into is that of the launch site's latitude, so launching the satellite from close to the equator limits the amount of inclination change needed later.

Most launch vehicles place geosynchronous satellites directly into a geosynchronous transfer orbit (GTO), an elliptical orbit with an apogee at GSO height and a low perigee. On-board satellite propulsion is then used to raise the perigee, circularise and reach GSO.

Once in a viable geostationary orbit, spacecraft can change longitudinal position by adjusting their semi-major axis such that the new period is shorter or longer than a sidereal day to effect an apparent "drift" eastward or westward, respectively. Once at the desired longitude, the spacecraft's period is restored to geosynchronous.

Proposed orbits

Statite proposal

A statite is a hypothetical satellite that uses radiation pressure from the Sun against a solar sail to modify its orbit.

Space elevator

Another form of geosynchronous orbit is the theoretical space elevator. If a mass orbiting above the geostationary belt is tethered to the earth’s surface, and the mass is accelerated to maintain an orbital period equal to one sidereal day then since the orbit now requires more downward force than gravity alone supplies, the tether is tensioned by the extra centripetal force required, and this tension keeps the tether structure stable as a crawler carries objects up and down along it.

Retired satellites

thumb|alt=Earth from space, surrounded by small white dots|A computer-generated image of space debris. Two debris fields are shown: around geosynchronous space and low Earth orbit.

Geosynchronous satellites require some station-keeping to remain in position, and once they run out of thruster fuel and are no longer useful they are moved into a higher graveyard orbit. It is not feasible to deorbit geosynchronous satellites, for to do so would take far more fuel than would be used by slightly elevating the orbit; and atmospheric drag is negligible, giving GSOs lifetimes of thousands of years.

The retirement process is becoming increasingly regulated and satellites must have a 90% chance of moving over 200 km above the geostationary belt at end of life.

Space debris

Space debris in geosynchronous orbits typically has a lower collision speed than at LEO (Low Earth Orbit) since most GSO satellites orbit in the same plane, altitude and speed; however, the presence of satellites in eccentric orbits allows for collisions at up to 4 km/s. Although a collision is comparatively unlikely, GSO satellites have a limited ability to avoid any debris.

Debris less than 10 cm in diameter cannot be seen from the Earth, making it difficult to assess their prevalence.

Despite efforts to reduce risk, spacecraft collisions have occurred. The European Space Agency telecom satellite Olympus-1 was struck by a meteoroid on August 11, 1993, and eventually moved to a graveyard orbit, and in 2006 the Russian Express-AM11 communications satellite was struck by an unknown object and rendered inoperable, although its engineers had enough contact time with the satellite to send it into a graveyard orbit. In 2017 both AMC-9 and Telkom-1 broke apart from an unknown cause.

Properties

thumb|The orbit of a geosynchronous satellite at an inclination, from the perspective of an off-Earth observer ([[Earth-centered inertial|ECI) and of an observer rotating around the Earth at its spin rate (ECEF).]]

A geosynchronous orbit has the following properties:

  • Period: 1436 minutes (one sidereal day)
  • Semi-major axis: 42,164 km This means that the satellite returns to the same point above the Earth's surface every (sidereal) day, regardless of other orbital properties.
  • Satellites currently in Geosynchronous Orbit, list updated daily
  • Science@NASA – Geosynchronous Orbit
  • NASA – Planetary Orbits
  • Science Presse data on Geosynchronous Orbits (including historical data and launch statistics)
  • Orbital Mechanics (Rocket and Space Technology)