Quasi-satellite

A quasi- satellite is a small koorbitaler companion of a larger celestial body (such as a planet ) that it orbits at a greater distance with the same orbital period at which it moves around a central star (the sun).

Mostly it involves asteroids residing in the vicinity of a planet. Your orbit around the sun has therefore the same orbital period and almost the same path axis as the planet, but acts " as seen from the Sun" somewhat eccentric (see also Lagrange points and restricted three-body problem ).

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Quasi-satellite are mainly under the gravitational influence of the common central body - and not like a moon under the influence of the planet. In the rest frame the orbit of quasi-satellite looks like a normal Kepler, elliptical orbit. However, taking as a reference system, the movement of the larger object (planet ) around the central star (in which the planet appears to rest ), one sees only the relative motion of the quasi-satellite. He describes in this reference system within an orbit around the central star of a loop around the larger celestial body - that has a slightly different orbit with respect to eccentricity and orbital inclination to the ecliptic. The radial component of motion is directly caused by the difference of the eccentricities between planet and quasi-satellite, while the component of motion along the planet's orbit is caused by the slightly different speed at perihelion and aphelion. At perihelion it overtakes the planet from the inside, while the outside falls back at aphelion further in terms of the planet. In the course of a year it becomes a complete orbit around the planet. This is on a highway, the ride side by side in roughly analogous to two cars at the same speed and overtake mutually, but are not tied to each other.

Stability

Although quasi-satellite are primarily subject to the gravity of the central body ( the sun ), they are still influenced by the planet, near which they are located. Since the ratio of the orbital periods of planet and quasi satellite is exactly 1:1, so they always meet in the same constellation, the interference by the planet always acts in the same way and can thus building-up and throw the quasi-satellite off course.

Computer simulations show that the orbits of quasi-satellite of the outer planets Uranus and Neptune are so stable that quasi- satellites would still stay there forever since the formation of the solar system 4.5 billion years ago. For Jupiter the amount of time until unstable, however, only 10 million years, and for Saturn with less than 100,000 years ago is even lower. If the orbital eccentricity of the quasi-satellite is in a particular area ( for Uranus and Neptune between 0.1 to 0.15 ) and the lower the orbital inclination of its orbit is against the orbital plane of the planet, the more stable are the paths of these quasi-satellite. Because of Uranus and Neptune should have original quasi-satellite, while Jupiter and Saturn should have only temporarily trapped quasi-satellite.

Examples

Earth

So far, a quasi- satellite of the Earth has been observed, the small asteroid 2003 YN107 in the years 1996 to 2006, which since then describes a horseshoe orbit along the Earth's orbit. In 2066 he is again a quasi- satellite of the Earth. At the following meeting in the year 2120, it is even probable that he is captured by the earth, and so for the second real Earth's Moon.

Another object koorbitales Earth, the asteroid 2002 AA29, changes almost cyclically between a horseshoe orbit and a quasi-satellite path, said he is the next time around the year 2600 be a quasi- satellite for 45 years.

Other planets

The Venus has a quasi-satellite, the asteroid 2002 VE68. This is the first discovered this type, and so far the only known co-orbital companion of Venus. According to calculations, since 7,000 years he is already on its current orbit and will follow her even for another 500 years. Due to the high eccentricity of about 0.4 and the inclination of around 9 ° its maximum distance from the sun is almost as large as the Earth and its minimum distance is less than the aphelion of Mercury; it approaches the earth also. Presumably, the asteroid was a near-Earth asteroid that was brought from the earth itself on its current track about 7,000 years ago.

Simulations suggest that may exist about 4.5 billion years quasi-satellite to the planet Uranus and Neptune since the formation of the solar system. It is therefore assumed that these planets have quasi-satellite, they have not lost again since their approach. So far, no quasi- satellites were discovered by gas planet.