Trojan (astronomy)
In astronomy, a Trojan is a minor planet or natural satellite (moon) that shares an orbit with a planet or larger moon, but does not collide with it because it orbits around one of the two Lagrangian points of stability (Trojan points), L4 and L5, which lie approximately 60° ahead of and behind the larger body, respectively. Trojan objects are one type of co-orbital object. In this arrangement, the massive star and the smaller planet orbit about their common barycenter—a location in space where the forces of their mutual gravitational attraction balance each other out. A much smaller mass located at one of the Lagrange points is subject to a combined gravitational force that acts through this barycenter. As a consequence, the mass can follow a circular orbit around this point with the same period as the planet, and the arrangement can remain stable over time.[1]
Trojan asteroids are asteroids that reside in a Trojan point of a planet. A Trojan moon is a moon residing at the Trojan point of another (larger) moon. Trojan planets are theoretical planets that reside at Trojan points of other planets.
Saturn has the most known Trojan satellites: Saturn's moon Tethys has two Trojan moons (Telesto and Calypso), and Dione also has two Trojan moons (Helene and Polydeuces).
In 2011, NASA announced the discovery of the first known Earth Trojan.[2]
[edit] Trojan asteroids
In 1772 the French mathematician and astronomer Joseph-Louis Lagrange predicted the existence and location of two groups of small bodies located near a pair of gravitationally stable points along Jupiter’s orbit. The term originally referred to the Trojan asteroids orbiting around Jupiter's Lagrangian points, which are by convention named after figures from the Trojan War of Greek mythology. By convention, the asteroids orbiting Jupiter's L4 point are named after the heroes from the Greek side of the war, while those at L5 are from the Trojan side. The two exceptions, the Greek-themed 617 Patroclus and the Trojan-themed 624 Hektor, were actually assigned to the wrong sides.[3] Astronomers estimate that the Jupiter Trojans are comparable in number to the asteroids of the asteroid belt.[4]
Subsequently objects have been found orbiting the Lagrangian points of Neptune, Mars, and Earth.[5] Asteroids at the Lagrangian points of planets other than Jupiter may be called Lagrangian asteroids.[6]
- Three Mars trojans are confirmed: 5261 Eureka, 1998 VF31, 1999 UJ7.[7] 2007 NS2 is possibly one.[citation needed]
- Eight Neptune trojans[8] are known, but they may outnumber the Jupiter Trojans by an order of magnitude.[9][10]
- 2010 TK7 was confirmed as the first known Earth trojan in 2011. It is located in the L4 Lagrangian point, which lies ahead of Earth.[11]
[edit] See also
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Look up Trojan, Trojan asteroid, Trojan moon, or Trojan planet in Wiktionary, the free dictionary. |
[edit] References
- ^ Robutel, P.; Souchay, J. (2010), "An introduction to the dynamics of trojan asteroids", in Dvorak, Rudolf; Souchay, Jean, Dynamics of Small Solar System Bodies and Exoplanets, Lecture Notes in Physics, 790, Springer, p. 197, ISBN 3642044573, http://books.google.com/books?id=CLUYgQlWz4IC&pg=PA197
- ^ NASA's WISE Mission Finds First Trojan Asteroid Sharing Earth's Orbit 7.27.11
- ^ Wright, Alison (August 1, 2011). "Planetary science: The Trojan is out there". Nature Physics 7 (8): 592. Bibcode 2011NatPh...7..592W. doi:10.1038/nphys2061. http://www.nature.com/nphys/journal/v7/n8/full/nphys2061.html. Retrieved 2011-08-12.
- ^ Yoshida, F.; Nakamura, T (2005). "Size distribution of faint L4 Trojan asteroids". The Astronomical journal 130 (6): 2900–11. Bibcode 2005AJ....130.2900Y. doi:10.1086/497571.
- ^ Connors, Martin; Wieger, Paul; Veillet, Christian (27 July 2011). "Earth's Trojan asteroid". Nature 475 (7357): 481–483. Bibcode 2011Natur.475..481C. doi:10.1038/nature10233. PMID 21796207. http://www.nature.com/nature/journal/v475/n7357/full/nature10233.html. Retrieved 2011-07-27.
- ^ Robert J. Whiteley and David J. Tholen, "A CCD Search for Lagrangian Asteroids of the Earth–Sun System", Icarus 136:1, November 1998:154–167
- ^ "List of Martian Trojans". http://www.minorplanetcenter.org/iau/lists/MarsTrojans.html. Retrieved 2012-01-31.
- ^ "List of Neptune Trojans". http://www.minorplanetcenter.org/iau/lists/NeptuneTrojans.html. Retrieved 2010-10-27.
- ^ Chiang, E. I. & Lithwick, Y. Neptune Trojans as a Testbed for Planet Formation, The Astrophysical Journal, 628, pp. 520–532 Preprint
- ^ David Powell (30 January 2007). "Neptune May Have Thousands of Escorts". Space.com. http://www.space.com/scienceastronomy/070130_st_neptune_trojans.html. Retrieved 2007-03-08.
- ^ Choi, Charles Q. (27 July 2011). "First Asteroid Companion of Earth Discovered at Last". Space.com. http://www.space.com/12443-earth-asteroid-companion-discovered-2010-tk7.html. Retrieved 2011-07-27.
