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.
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, 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 describes these reasons thus:
The survival to the present day of an ancient [Earth Trojan] population is reasonably assured provided Earth's orbit itself was not strongly perturbed since its formation. It is therefore pertinent to consider that modern theoretical models of planet formation find strongly chaotic orbital evolution during the final stages of assembly of the terrestrial planets and the Earth–Moon system.
Such chaotic evolution may at first sight appear unfavorable to the survival of a primordial population of [Earth trojans]. However, during and after the chaotic assembly of the terrestrial planets, it is likely that a residual planetesimal population, of a few percent of Earth's mass, was present and helped to damp the orbital eccentricities and inclinations of the terrestrial planets to their observed low values, as well as to provide the so-called "late veneer" of accreting planetesimals to account for the abundance patterns of the highly siderophile elements in Earth's mantle.
Such a residual planetesimal population would also naturally lead to a small fraction trapped in the Earth's Trojan zones as Earth's orbit circularized. In addition to potentially hosting an ancient, long-term stable population of asteroids, Earth's Trojan regions also provide transient traps for NEOs that originate from more distal reservoirs of small bodies in the solar system like the main asteroid belt.
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, neither L4 nor 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.
|Discoverer||Year of Discovery||Type||Current Type|
|Moon||0.055||3474800||?||?||Natural satellite||Natural satellite|
|1913 Great Meteor Procession||?||?||?||9 February 1913||Possible Temporary satellite||Destroyed|
|3753 Cruithne||0.515||5000||Duncan Waldron||10 October 1986||Quasi-satellite||Horseshoe orbit|
|1991 VG||0.053||5–12||Spacewatch||6 November 1991||Temporary satellite||Apollo asteroid|
|(85770) 1998 UP1||0.345||210–470||Lincoln Lab's ETS||18 October 1998||Horseshoe orbit||Horseshoe orbit|
|54509 YORP||0.230||124||Lincoln Lab's ETS||3 August 2000||Horseshoe orbit||Horseshoe orbit|
|2001 GO2||0.168||35–85||Lincoln Lab's ETS||13 April 2001||Possible Horseshoe orbit||Possible Horseshoe orbit|
|2002 AA29||0.013||20–100||LINEAR||9 January 2002||Quasi-satellite||Horseshoe orbit|
|2003 YN107||0.014||10–30||LINEAR||20 December 2003||Quasi-satellite||Horseshoe orbit|
|(164207) 2004 GU9||0.136||160–360||LINEAR||13 April 2004||Quasi-satellite||Quasi-satellite|
|(277810) 2006 FV35||0.377||140–320||Spacewatch||29 March 2006||Quasi-satellite||Quasi-satellite|
|2006 JY26||0.083||6–13||Catalina Sky Survey||6 May 2006||Horseshoe orbit||Horseshoe orbit|
|2006 RH120||0.024||2–3||Catalina Sky Survey||14 September 2006||Temporary satellite||Apollo asteroid|
|(419624) 2010 SO16||0.075||357||WISE||17 September 2010||Horseshoe orbit||Horseshoe orbit|
|2010 TK7||0.191||150–500||WISE||1 October 2010||Earth trojan||Earth trojan|
|2013 BS45||0.083||20–40||Spacewatch||20 January 2010||Horseshoe orbit||Horseshoe orbit|
|2013 LX28||0.452||130–300||Pan-STARRS||12 June 2013||Quasi-satellite temporary||Quasi-satellite temporary|
|2014 OL339||0.461||70–160||EURONEAR||29 July 2014||Quasi-satellite temporary||Quasi-satellite temporary|
|2015 SO2||0.108||50–110||Črni Vrh Observatory||21 September 2015||Quasi-satellite||Horseshoe orbit temporary|
|2015 XX169||0.184||9–22||Mount Lemmon Survey||9 December 2015||Horseshoe orbit temporary||Horseshoe orbit temporary|
|2015 YA||0.279||9–22||Catalina Sky Survey||16 December 2015||Horseshoe orbit temporary||Horseshoe orbit temporary|
|2015 YQ1||0.404||7–16||Mount Lemmon Survey||19 December 2015||Horseshoe orbit temporary||Horseshoe orbit temporary|
|469219 Kamoʻoalewa||0.104||40-100||Pan-STARRS||27 April 2016||Quasi-satellite stable||Quasi-satellite stable|
|DN16082203||?||?||?||22 August 2016||Possible Temporary satellite||Destroyed|
|2020 CD3||0.017||1–6||Mount Lemmon Survey||15 February 2020||Temporary satellite||Temporary satellite|
|2020 PN1||0.127||10–50||ATLAS-HKO||12 August 2020||Horseshoe orbit temporary||Horseshoe orbit temporary|
|2020 PP1||0.074||10–20||Pan-STARRS||12 August 2020||Quasi-satellite stable||Quasi-satellite stable|
|2020 XL5||0.387||1180±80||Pan-STARRS||12 December 2020||Earth trojan||Earth trojan|