Coordinate object

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A co-ordinate object or co-ordinate companion is a celestial body that is in the same or a very similar orbit around a central star as a second larger celestial body. Due to their very similar orbits, both have the same mean period around the central star. Co-ordinate objects are in gravitational interaction with the larger celestial body in whose orbit they are located, and because of the same mean period of orbit they are in a so-called 1: 1 resonance . This means that such orbits are only stable under certain conditions, of which normally the most important is that the co-ordinate companion has a negligibly small mass in relation to the larger body (so-called restricted three-body problem ).

Trojans

The best-known co-orbital companions are the Trojans of Jupiter . It was discovered for the first time in its Lagrangian points L 4 and L 5 (which are in the orbit of a celestial body 60 ° in front of or behind it). The first Trojan horse discovered was Achilles . Later, Trojan companions were found in the Lagrangian points of other planets and even moons (see Trojan Moon ). In 2010 an earth Trojan ( 2010 TK7 , in Lagrangian point L 4 ) was discovered for the first time.

Companion on horseshoe orbits

Another group of co-orbital objects run in horseshoe orbits within the orbit of the larger celestial body together with it around the central star. From the point of view of the larger body, the co-orbital companions describe a large arc that they periodically swing back and forth. The shape of the arch is reminiscent of a horseshoe , hence the name horseshoe orbit. Viewed from the stationary reference system ( inertial system ), however, they still describe elliptical orbits. So far, only a few objects on horseshoe tracks are known. One of the most notable is the asteroid 2002 AA 29 (an object less than 100 m in diameter), which is a co-ordinate companion of Earth . (419624) 2010 SO 16 behaves similarly to earth.

Another type of co-orbital objects on very unusual horseshoe orbits are the small, almost equally large Saturn moons Janus and Epimetheus , which orbit Saturn as co-orbital moons in very similar orbits and come very close every four years and exchange their orbits.

The transition from a Trojan to an ordinary horseshoe track is fluid: If the distance of a Trojan to the L 4 or L 5 point is too great, then once on the orbit it will cross the point opposite the larger celestial body at L 3 and then into Walk towards the other Lagrange point and swing back and forth in a large arc.

Quasi satellites

A smaller celestial body that orbits the larger celestial body relatively far away, namely within one orbit period of the larger body around the central star, is called a quasi-satellite . Quasi satellites are mainly under the gravitational influence of the central body, which both orbit, and not like a moon under the main gravitational influence of the planet . This is roughly analogous to two cars on a motorway that drive next to each other at the same speed and overtake each other, but are not firmly bound to each other. Between 1996 and 2006, a quasi-satellite of the earth was the small asteroid 2003 YN 107 , which since then has described a horseshoe orbit along the earth's orbit as before.

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