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.
An Earth-based search for L5 objects was conducted in 1994, covering 0.35 square degrees of sky, under poor observing conditions. That search failed to detect any objects:
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 Hayabusa2 spacecraft searched the L5 region while proceeding to asteroid Ryugu, but did not find any asteroids there.
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 publicationdescribes 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.
469219 Kamoʻoalewa, an asteroid discovered on 27 April 2016, is possibly the most stable quasi-satellite of Earth.
|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|
An asteroid is a minor planet of the inner Solar System. Historically, these terms have been applied to any astronomical object orbiting the Sun that did not resolve into a disc in a telescope and was not observed to have characteristics of an active comet such as a tail. As minor planets in the outer Solar System were discovered that were found to have volatile-rich surfaces similar to comets, these came to be distinguished from the objects found in the main asteroid belt. Thus the term "asteroid" now generally refers to the minor planets of the inner Solar System, including those co-orbital with Jupiter. Larger asteroids are often called planetoids.
In celestial mechanics, the Lagrange points are points of equilibrium for small-mass objects under the influence of two massive orbiting bodies. Mathematically, this involves the solution of the restricted three-body problem in which two bodies are very much more massive than the third.
The Jupiter trojans, commonly called trojan asteroids or simply trojans, are a large group of asteroids that share the planet Jupiter's orbit around the Sun. Relative to Jupiter, each trojan librates around one of Jupiter's stable Lagrange points: either L4, existing 60° ahead of the planet in its orbit, or L5, 60° behind. Jupiter trojans are distributed in two elongated, curved regions around these Lagrangian points with an average semi-major axis of about 5.2 AU.
3753 Cruithne is a Q-type, Aten asteroid in orbit around the Sun in 1:1 orbital resonance with Earth, making it a co-orbital object. It is an asteroid that, relative to Earth, orbits the Sun in a bean-shaped orbit that effectively describes a horseshoe, and that can change into a quasi-satellite orbit. Cruithne does not orbit Earth and at times it is on the other side of the Sun, placing Cruithne well outside of Earth's Hill sphere. Its orbit takes it near the orbit of Mercury and outside the orbit of Mars. Cruithne orbits the Sun in about one Earth year, but it takes 770 years for the series to complete a horseshoe-shaped movement around Earth.
The giant-impact hypothesis, sometimes called the Big Splash, or the Theia Impact, suggests that the Moon formed from the ejecta of a collision between the proto-Earth and a Mars-sized planet, approximately 4.5 billion years ago, in the Hadean eon. The colliding body is sometimes called Theia, from the name of the mythical Greek Titan who was the mother of Selene, the goddess of the Moon. Analysis of lunar rocks, published in a 2016 report, suggests that the impact might have been a direct hit, causing a thorough mixing of both parent bodies.
A natural satellite is, in the most common usage, an astronomical body that orbits a planet, dwarf planet, or small Solar System body. Natural satellites are often colloquially referred to as moons, a derivation from the Moon of Earth.
Planetesimals are solid objects thought to exist in protoplanetary disks and debris disks. Per the Chamberlin–Moulton planetesimal hypothesis, they are believed to form out of cosmic dust grains. Believed to have formed in the Solar System about 4.6 billion years ago, they aid study of its formation.
2002 AA29 (also written 2002 AA29) is a small near-Earth asteroid that was discovered on January 9, 2002 by the LINEAR (Lincoln Near Earth Asteroid Research) automatic sky survey. The diameter of the asteroid is only about 20–100 metres (70–300 ft). It revolves about the Sun on an almost circular orbit very similar to that of the Earth. This lies for the most part inside the Earth's orbit, which it crosses near the asteroid's furthest point from the Sun, the aphelion. Because of this orbit, the asteroid is classified as Aten type, named after the asteroid 2062 Aten.
A quasi-satellite is an object in a specific type of co-orbital configuration with a planet where the object stays close to that planet over many orbital periods.
In celestial mechanics, a horseshoe orbit is a type of co-orbital motion of a small orbiting body relative to a larger orbiting body. The osculating (instantaneous) orbital period of the smaller body remains very near that of the larger body, and if its orbit is a little more eccentric than that of the larger body, during every period it appears to trace an ellipse around a point on the larger object's orbit. However, the loop is not closed but drifts forward or backward so that the point it circles will appear to move smoothly along the larger body's orbit over a long period of time. When the object approaches the larger body closely at either end of its trajectory, its apparent direction changes. Over an entire cycle the center traces the outline of a horseshoe, with the larger body between the 'horns'.
In astronomy, a trojan is a small celestial body (mostly asteroids) that shares the orbit of a larger one, remaining in a stable orbit approximately 60° ahead of or behind the main body near one of its Lagrangian points L4 and L5. Trojans can share the orbits of planets or of large moons.
Claims of the existence of other moons of Earth—that is, of one or more natural satellites with relatively stable orbits of Earth, other than the Moon—have existed for some time. Several candidates have been proposed, but none has been confirmed. Since the 19th century, scientists have made genuine searches for more moons, but the possibility has also been the subject of a number of dubious non-scientific speculations as well as a number of likely hoaxes.
In astronomy, a co-orbital configuration is a configuration of two or more astronomical objects orbiting at the same, or very similar, distance from their primary, i.e. they are in a 1:1 mean-motion resonance..
2006 RH120 is a tiny near-Earth asteroid and fast rotator with a diameter of approximately 2–3 meters that ordinarily orbits the Sun but makes close approaches to the Earth–Moon system around every twenty years, when it can temporarily enter Earth orbit through temporary satellite capture (TSC). Most recently, it was in Earth orbit from September 2006 to June 2007. As a consequence of its temporary orbit around the Earth, it is currently the smallest asteroid in the Solar System with a well-known orbit.
The Nicemodel is a scenario for the dynamical evolution of the Solar System. It is named for the location of the Observatoire de la Côte d'Azur — where it was initially developed in 2005 — in Nice, France. It proposes the migration of the giant planets from an initial compact configuration into their present positions, long after the dissipation of the initial protoplanetary disk. In this way, it differs from earlier models of the Solar System's formation. This planetary migration is used in dynamical simulations of the Solar System to explain historical events including the Late Heavy Bombardment of the inner Solar System, the formation of the Oort cloud, and the existence of populations of small Solar System bodies such as the Kuiper belt, the Neptune and Jupiter trojans, and the numerous resonant trans-Neptunian objects dominated by Neptune.
(419624) 2010 SO16 is a sub-kilometer asteroid in a co-orbital configuration with Earth, classified as near-Earth object and potentially hazardous asteroid of the Apollo group. It was discovered by the Wide-field Infrared Survey Explorer space telescope (WISE) on 17 September 2010.
2010 TK7 is a sub-kilometer Near-Earth asteroid and the first Earth trojan discovered; it precedes Earth in its orbit around the Sun. Trojan objects are most easily conceived as orbiting at a Lagrangian point, a dynamically stable location (where the combined gravitational force acts through the Sun's and Earth's barycenter) 60 degrees ahead of or behind a massive orbiting body, in a type of 1:1 orbital resonance. In reality, they oscillate around such a point. Such objects had previously been observed in the orbits of Mars, Jupiter, Neptune, and the Saturnian moons Tethys and Dione.
2013 BS45 (also written 2013 BS45) is a horseshoe companion to the Earth like 3753 Cruithne. Like Cruithne, it does not orbit the Earth in the normal sense and at times it is on the other side of the Sun, yet it still periodically comes nearer to the Earth in sort of halo orbit before again drifting away. While not a traditional natural satellite, it does not quite have normal heliocentric orbit either and these are sometimes called quasi-satellties or horseshoe orbits.
Theia is a hypothesized ancient planet in the early Solar System that, according to the giant-impact hypothesis, collided with the early Earth around 4.5 billion years ago, with some of the resulting ejected debris gathering to form the Moon.