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 A geosynchronous orbit has the following properties: Period: 1436 minutes (one sidereal day) Semi-major axis: 42,164 km Period All geosynchronous orbits have an orbital period equal to exactly one sidereal day. This means that the satellite will return to the same point above the Earth's surface every (sidereal) day, regardless of other orbital properties. This orbital period, T, is directly related to the semi-major axis of the orbit through the formula: where: is the length of the orbit's semi-major axis is the standard gravitational parameter of the central body Inclination A geosynchronous orbit can have any inclination. Satellites commonly have an inclination of zero, ensuring that the orbit remains over the equator at all times, making it stationary with respect to latitude from the point of view of a ground observer (and in the ECEF reference frame). Another popular inclinations is 63.4° for a Tundra orbit, which ensures that the orbit's argument of perigee doesn't change over time. Ground track In the special case of a geostationary orbit, the ground track of a satellite is a single point on the equator. In the general case of a geosynchronous orbit with a non-zero inclination or eccentricity, the ground track is a more or less distorted figure-eight, returning to the same places once per sidereal day. See also Geostationary orbit Geosynchronous satellite Graveyard orbit High Earth orbit List of orbits List of satellites in geosynchronous orbit Low Earth orbit Medium Earth orbit Molniya orbit Subsynchronous orbit Supersynchronous orbit Synchronous orbit References External links 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