An Earth trojan is an asteroid that orbits the Sun in the vicinity of the Earth–Sun Lagrangian points L4 (leading 60°) or L5 (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 Jupiter trojans, the asteroids that were observed near the Lagrangian points of Jupiter's orbit.
Members
- Current
L4 (leading)
- 2010 TK7: A 300 metre diameter asteroid, discovered using the Wide-field Infrared Survey Explorer (WISE) satellite, is the only confirmed Earth trojan as of 2017.[1][2][3]
L5 (trailing)
- No known objects are currently thought to be L5 trojans of Earth. A search was conducted in 1994 covering 0.35°2 of sky under poor observing conditions [4] which failed to detect any objects "The limiting sensitivity of this search was magnitude ~22.8, corresponding to C-type asteroids ~350m in diameter or S-type asteroids ~175m in diameter."[4]
- Proposed
L4
- 2020 XL5: In 2021 it was discovered that asteroid 2020 XL5 appears to be librating around L4, making it another Earth Trojan if confirmed. Subsequent analysis confirmed modeling stability for at least several thousand years into the future based on existing orbital parameters.[5][6] This would make 2020 XL5 more stable than 2010 TK7, which is potentially unstable of time scales of less than 2,000 years.[7]
Discovery
2010 TK7 was discovered using the Wide-field Infrared Survey Explorer (WISE) satellite, on January 25, 2010.
In February 2017, the OSIRIS-REx spacecraft performed a search from within the L4 region on its way to asteroid Bennu.[8] No additional Earth trojans were discovered.[9]
In April 2017, the Hayabusa2 spacecraft searched the L5 region while proceeding to asteroid Ryugu,[10] but did not find any asteroids there.[11]
Significance
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 (even if they differentiated, the core would still be within reach). 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 (although the asteroid's rotation could cause separation).
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,[12] showering material from the two planets into space. This material eventually accreted 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.
Astronomy continues to retain interest in the subject. A publication[13] 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 librate around a definite Sun–Earth Lagrangian point, either L4 or L5.
Earth has another noted companion, asteroid 3753 Cruithne. About 5 km across, it has a peculiar type of orbital resonance called an overlapping horseshoe, and is probably only a temporary liaison.[14]
469219 Kamoʻoalewa, an asteroid discovered on 27 April 2016, is possibly the most stable quasi-satellite of Earth.[15]
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