An Earth trojan is an asteroid that orbits the Sun in the vicinity of the Earth–Sun Lagrangian points L4 (leading 60°) and L5 (trailing 60°), which means they have an orbit similar to Earth's. Only one Earth trojan has so far been discovered. The name trojan was first used in 1906 for the Jupiter trojans, the asteroids that were observed near the Lagrangian points of Jupiter's orbit.
- 2010 TK7: A 300-meter-diameter asteroid, discovered using the Wide-field Infrared Survey Explorer (WISE) satellite. It is the only confirmed Earth trojan.
- No known objects are currently thought to be L5 trojans of Earth.
- Theia: A hypothesized ancient planetary-mass object in the early Solar System that, according to the giant impact hypothesis, collided with the Early Earth around 4.533 billion years ago (BYa).
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The orbits of any Earth trojans could make them less energetically costly to reach than the Moon, even though they will be hundreds of times more distant. Such asteroids could one day be useful as sources of elements that are rare near Earth's surface. On Earth, siderophiles such as iridium are difficult to find, having largely sunk to the core of the planet shortly after its formation. A small asteroid could be a rich source of such elements even if its overall composition is similar to Earth's; because of their small size, such bodies would lose heat much more rapidly than a planet once they had formed, and so would not have melted, a prerequisite for differentiation. Their weak gravitational fields also would have inhibited significant separation of denser and lighter material; a mass the size of 2010 TK7 would exert a surface gravitational force of less than 0.00005 times that of Earth.
Other companions of Earth
Several other small objects have been found on an orbital path associated with Earth. Although these objects are in 1:1 orbital resonance, they are not Earth trojans because they do not librate around a definite Sun–Earth Lagrangian point, either L4 or L5.
- Reilly, M. (27 July 2011). "Earth Stalker Found in Eternal Twilight". New Scientist. Retrieved 2014-02-21.
- Choi, C. Q. (27 July 2011). "First Asteroid Companion of Earth Discovered at Last". Space.com. Retrieved 2011-07-27.
- "The Theia Hypothesis: New Evidence Emerges that Earth and Moon Were Once the Same". The Daily Galaxy. 2007-07-05. Retrieved 2013-11-13.
- Murray, C. (1997). "The Earth's secret companion". Nature 387 (6634): 651–652. Bibcode:1997Natur.387..651M. doi:10.1038/42585.