Quasi-satellite

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,cite journal | author= M. Connors et al| title= "Discovery of an asteroid and quasi-satellite in an Earth-like horseshoe orbit"| journal= Meteoritics & Planetary Science| year= 2002| volume= 37| pages= 1435| url= http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2002M%26PS...37.1435C&db_key=AST&data_type=HTML&format=&high=444b66a47d23464] and are sometimes confused with them. An example of such an object is mpl|2002 AA|29.

Known quasi-satellites

Earth currently has the known quasi-satellites 3753 Cruithne, mpl|2003 YN|107, and mpl|(164207) 2004 GU|9, and Venus has the quasi-satellite mpl|2002 VE|68. These objects remain in quasi-satellite orbits for times of tens to hundreds of years.

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) cite journal | author= P. Wiegert and K. Innanen| title= "The stability of quasi satellites in the outer solar system"| journal= The Astronomical Journal| year= 2000| volume= 119 | issue=4 | pages= 1978–1984 | url=http://www.iop.org/EJ/article/1538-3881/119/4/1978/990367.html | doi= 10.1086/301291 ] , 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. [http://www.federalspace.ru/Doc1Show.asp?DocID=16 (the orbit is erroneously called "quasi-synchronous")] 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.

ee also

* Co-orbital moon
* Natural satellite

References

External links

* [http://www.astro.uwo.ca/~wiegert/quasi/quasi.html Quasi-satellite Information Page]
* [http://www.space.com/scienceastronomy/earth_asteroid_021021.html Earth's New Travelling Companion: Quasi-Satellite Discovered]
* [http://www.astronomy.com/asy/default.aspx?c=a&id=2060 Astronomy.com: A new "moon" for Earth]
* [http://www.astro.utu.fi/tuorla/new/venusqs.shtml Discovery of the first quasi-satellite of Venus] – University of Turku news release (August 17th, 2004)