An Earth trojan is an asteroid that orbits the Sun in the vicinity of the Earth–Sun (leading 60°) or (trailing 60°), thus having an orbit similar to Earth's. Only two Earth trojans have so far been discovered. The name "trojan" was first used in 1906 for the , the asteroids that were observed near the Lagrangian points of Jupiter's orbit.
Members
(leading)
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: A 300-metre diameter asteroid, discovered using the Wide-field Infrared Survey Explorer (WISE) satellite in January 2010.
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: Discovered by the Pan-STARRS survey in December 2020 and later recognised as an Earth trojan in January 2021.
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It is 1.2 km in diameter.
(trailing)
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No known objects are currently thought to be trojans of Earth.
Searches
An Earth-based search for objects was conducted in 1994, covering 0.35 square degrees of sky, under poor observing conditions.
[
]Received 24 November 1997; revised 13 April 1998.
That search failed to detect any objects:
- "The limiting sensitivity of this search was magnitude ~22.8, corresponding to ~350 m in diameter, or ~175 m in diameter."
In February 2017, the OSIRIS-REx spacecraft performed a search from within the region on its way to asteroid Bennu.
In April 2017, the Hayabusa2 spacecraft searched the region while proceeding to asteroid Ryugu,
Giant-impact hypothesis
A hypothetical planet-sized Earth trojan the size of
Mars, given the name Theia, is thought by proponents of the giant-impact hypothesis to be the origin of the
Moon. The hypothesis states that the Moon formed after Earth and Theia collided,
showering material from the two planets into space. This material eventually
Accretion disk around Earth and into a single orbiting body, the Moon.
At the same time, material from Theia mixed and combined with Earth's mantle and core. Supporters of the giant-impact hypothesis theorise that Earth's large core in relation to its overall volume is as a result of this combination.
Continuing interest in near-Earth asteroids
Astronomy continues to retain interest in the subject. A publication
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describes these reasons thus:
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
Libration around a definite Sun–Earth Lagrangian point, neither nor .
Earth has another noted companion, asteroid 3753 Cruithne. About 5 km across, it has a peculiar type of orbital resonance called an Horseshoe orbit, and is probably only a temporary liaison.[
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469219 Kamoʻoalewa, an asteroid discovered on 27 April 2016, is possibly the most stable quasi-satellite of Earth.[
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Gallery
File:Illustration of Trojan Asteroid 2020 XL5.jpg|
See also
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2003 YN107
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2006 RH120
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3753 Cruithne
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6Q0B44E
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Claimed moons of Earth
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Kordylewski cloud
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Natural satellite
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Quasi-satellite
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Theia / giant-impact hypothesis
