Quasi-satellite

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Diagram of generic quasi-satellite orbit

A quasi-satellite is an object in a 1:1 orbital resonance with its planet that stays close to the planet over many orbital periods.

A quasi-satellite's orbit around the Sun takes exactly the same time as the planet's, but has a different eccentricity (usually greater), as shown in the diagram at right. When viewed from the perspective of the planet the quasi-satellite will appear to travel in an oblong retrograde loop around the planet.

In contrast to true satellites, quasi-satellite orbits lie outside the planet's Hill sphere, and are unstable. Over time they tend to evolve to other types of resonant motion, where they no longer remain in the planet's neighbourhood, then possibly later move back to a quasi-satellite orbit, etc.

Other types of orbit in a 1:1 resonance with the planet, include horseshoe orbits and tadpole orbits around the Lagrangian points, but objects in these orbits do not stay near the planet's longitude over many revolutions about the star. Objects in horseshoe orbits are known to sometimes periodically transfer to a relatively short-lived quasi-satellite orbit,^[1] and are sometimes confused with them. An example of such an object is 2002 AA₂₉.

1 Known quasi-satellites
2 See also
3 References
4 External links

[edit] Known quasi-satellites

Earth currently has the known quasi-satellites 3753 Cruithne, 2003 YN₁₀₇, and (164207) 2004 GU₉, and Venus has the quasi-satellite 2002 VE₆₈. These objects remain in quasi-satellite orbits for times of tens to hundreds of years.^[1]

Based on simulations it is believed that Uranus and Neptune could potentially hold quasi-satellites for the age of the solar system (about 4.5 billion years) ^[2], but a quasi-satellite's orbit would remain stable for only 10 million years around Jupiter and 100,000 years around Saturn. No actual quasi-satellites of these planets are currently known.

In early 1989, the Soviet Phobos 2 spacecraft was injected into a quasi-satellite orbit around the Martian moon Phobos, with a mean orbital radius of about 100 km from the moon.^[3] According to computations, it could have then stayed trapped in the vicinity of Phobos for many months. The spacecraft was lost due to a malfunction of the on-board control system.