Earth trojan
An Earth trojan is an asteroid that orbits the Sun in the vicinity of the Earth–Sun Lagrangian points or , thus having 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.
Members
;Current- : A 150 to 500-metre-diameter asteroid, discovered using the Wide-field Infrared Survey Explorer satellite, is the only confirmed Earth trojan as of 2017.
- No known objects are currently thought to be trojans of Earth. A search was conducted in 1994 covering 0.35°2 of sky under poor observing conditions 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."
Discovery
In February 2017, the OSIRIS-REx spacecraft performed a search from within the L4 region on its way to asteroid Bennu. No additional Earth trojans were discovered.
In April 2017, the Hayabusa 2 spacecraft searched the L5 region while proceeding to asteroid Ryugu, but did not find any asteroids there.
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. Their weak gravitational fields also would have inhibited significant separation of denser and lighter material; a mass the size of would exert a surface gravitational force of less than 0.00005 times that of Earth.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:
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 or.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.
, an asteroid discovered on 27 April 2016, is possibly the most stable quasi-satellite of Earth.
| Name | Eccentricity | Diameter | Discoverer | Year of Discovery | Type | Current Type |
| Moon | 0.055 | 1737400 | ? | ? | Natural satellite | Natural satellite |
| 1913 Great Meteor Procession | ? | ? | ? | 1913 February 9 | Possible Temporary satellite | Destroyed |
| 3753 Cruithne | 0.515 | 5000 | Duncan Waldron | 1986 October 10 | Quasi-satellite | Horseshoe orbit |
| 1991 VG | 0.053 | 5–12 | Spacewatch | 1991 November 6 | Temporary satellite | Apollo asteroid |
| 1998 UP1 | 0.345 | 210–470 | Lincoln Lab's ETS | 1998 October 18 | Horseshoe orbit | Horseshoe orbit |
| 54509 YORP | 0.230 | 124 | Lincoln Lab's ETS | 2000 August 3 | Horseshoe orbit | Horseshoe orbit |
| 2001 GO2 | 0.168 | 35–85 | Lincoln Lab's ETS | 2001 April 13 | Possible Horseshoe orbit | Possible Horseshoe orbit |
| 2002 AA29 | 0.013 | 20–100 | LINEAR | 2002 January 9 | Quasi-satellite | Horseshoe orbit |
| 2003 YN107 | 0.014 | 10–30 | LINEAR | 2003 December 20 | Quasi-satellite | Horseshoe orbit |
| 2004 GU9 | 0.136 | 160–360 | LINEAR | 2004 April 13 | Quasi-satellite | Quasi-satellite |
| 2006 FV35 | 0.377 | 140–320 | Spacewatch | 2006 March 29 | Quasi-satellite | Quasi-satellite |
| 2006 JY26 | 0.083 | 6–13 | Catalina Sky Survey | 2006 May 6 | Horseshoe orbit | Horseshoe orbit |
| 2006 RH120 | 0.024 | 2–3 | Catalina Sky Survey | 2006 September 14 | Temporary satellite | Apollo asteroid |
| 2010 SO16 | 0.075 | 357 | WISE | 2010 September 17 | Horseshoe orbit | Horseshoe orbit |
| 2010 TK7 | 0.191 | 150–500 | WISE | 2010 October 1 | Earth trojan | Earth trojan |
| 2013 BS45 | 0.083 | 20–40 | Spacewatch | 2013 January 20 | Horseshoe orbit | Horseshoe orbit |
| 2013 LX28 | 0.452 | 130–300 | Pan-STARRS | 2013 June 12 | Quasi-satellite temporary | Quasi-satellite temporary |
| 2014 OL339 | 0.461 | 170 | EURONEAR | 2014 July 29 | Quasi-satellite temporary | Quasi-satellite temporary |
| 2015 SO2 | 0.108 | 50–111 | Črni Vrh Observatory | 2015 September 21 | Quasi-satellite | Horseshoe orbit temporary |
| 2015 XX169 | 0.184 | 9–22 | Mount Lemmon Survey | 2015 December 9 | Horseshoe orbit temporary | Horseshoe orbit temporary |
| 2015 YA | 0.279 | 9–22 | Catalina Sky Survey | 2015 December 16 | Horseshoe orbit temporary | Horseshoe orbit temporary |
| 2015 YQ1 | 0.404 | 7–16 | Mount Lemmon Survey | 2015 December 19 | Horseshoe orbit temporary | Horseshoe orbit temporary |
| 469219 Kamoʻoalewa | 0.104 | 41-100 | Pan-STARRS | 2016 April 27 | Quasi-satellite stable | Quasi-satellite stable |
| DN16082203 | ? | 2-3 | Desert Fireball Network | 2016 August 22 | Possible Temporary satellite | Destroyed |
| 2020 CD3 | 0.017 | 1–6 | Mount Lemmon Survey | 2020 February 15 | Temporary satellite | Temporary satellite |