Earth trojan

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Animation of 2010 TK7.gif
Lagrange points2.svg
The orbit of 2010 TK7 , the first Earth trojan to be discovered (left). Lagrangian points L4 and L5. Lines around the blue triangles represent tadpole orbits (right)

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.



2010 TK7, one of the two known Earth trojans, is located at the lower right, circled by a small green ring. Asteroid 2010 TK7.jpg
2010 TK7 , one of the two known Earth trojans, is located at the lower right, circled by a small green ring.

L4 (leading)

L5 (trailing)


An Earth-based search for L5 objects was conducted in 1994, covering 0.35 square degrees of sky, under poor observing conditions. [5] That search failed to detect any objects:

"The limiting sensitivity of this search was magnitude ~22.8, corresponding to C-type asteroids ~350 m in diameter, or S-type asteroids ~175 m in diameter." [5]

In February 2017, the OSIRIS-REx spacecraft performed a search from within the L4 region on its way to asteroid  Bennu. [6] No additional Earth trojans were discovered. [7]

In April 2017, the Hayabusa2 spacecraft searched the L5 region while proceeding to asteroid Ryugu, [8] but did not find any asteroids there. [9]


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).

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, [10] showering material from the two planets into space. This material eventually accreted around Earth and into a single orbiting body, the Moon. [11]

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 [12] 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.

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, 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. [13]

469219 Kamoʻoalewa, an asteroid discovered on 27 April 2016, is possibly the most stable quasi-satellite of Earth. [14]

Known and suspected companions of Earth
Name Eccentricity Diameter
DiscovererYear of DiscoveryTypeCurrent Type
Moon 0.0553474800 ? ? Natural satellite Natural satellite
1913 Great Meteor Procession  ? ? ?9 February 1913Possible Temporary satellite Destroyed
3753 Cruithne 0.5155000 Duncan Waldron 10 October 1986 Quasi-satellite Horseshoe orbit
1991 VG 0.0535–12 Spacewatch 6 November 1991 Temporary satellite Apollo asteroid
(85770) 1998 UP1 0.345210–470 Lincoln Lab's ETS 18 October 1998 Horseshoe orbit Horseshoe orbit
54509 YORP 0.230124 Lincoln Lab's ETS 3 August 2000 Horseshoe orbit Horseshoe orbit
2001 GO2 0.16835–85 Lincoln Lab's ETS 13 April 2001Possible Horseshoe orbit Possible Horseshoe orbit
2002 AA29 0.01320–100 LINEAR 9 January 2002 Quasi-satellite Horseshoe orbit
2003 YN107 0.01410–30 LINEAR 20 December 2003 Quasi-satellite Horseshoe orbit
(164207) 2004 GU9 0.136160–360 LINEAR 13 April 2004 Quasi-satellite Quasi-satellite
(277810) 2006 FV35 0.377140–320 Spacewatch 29 March 2006 Quasi-satellite Quasi-satellite
2006 JY26 0.0836–13 Catalina Sky Survey 6 May 2006 Horseshoe orbit Horseshoe orbit
2006 RH120 0.0242–3 Catalina Sky Survey 14 September 2006 Temporary satellite Apollo asteroid
(419624) 2010 SO16 0.075357 WISE 17 September 2010 Horseshoe orbit Horseshoe orbit
2010 TK7 0.191150–500 WISE 1 October 2010 Earth trojan Earth trojan
2013 BS45 0.08320–40 Spacewatch 20 January 2010 Horseshoe orbit Horseshoe orbit
2013 LX28 0.452130–300 Pan-STARRS 12 June 2013 Quasi-satellite temporary Quasi-satellite temporary
2014 OL339 0.46170–160 EURONEAR 29 July 2014 Quasi-satellite temporary Quasi-satellite temporary
2015 SO2 0.10850–110 Črni Vrh Observatory 21 September 2015 Quasi-satellite Horseshoe orbit temporary
2015 XX169 0.1849–22 Mount Lemmon Survey 9 December 2015 Horseshoe orbit temporary Horseshoe orbit temporary
2015 YA 0.2799–22 Catalina Sky Survey 16 December 2015 Horseshoe orbit temporary Horseshoe orbit temporary
2015 YQ1 0.4047–16 Mount Lemmon Survey 19 December 2015 Horseshoe orbit temporary Horseshoe orbit temporary
469219 Kamoʻoalewa 0.10440-100 Pan-STARRS 27 April 2016 Quasi-satellite stable Quasi-satellite stable
DN16082203  ? ? ?22 August 2016Possible Temporary satellite Destroyed
2020 CD3 0.0171–6 Mount Lemmon Survey 15 February 2020 Temporary satellite Temporary satellite
2020 PN1 0.12710–50 ATLAS-HKO 12 August 2020 Horseshoe orbit temporary Horseshoe orbit temporary
2020 PP1 0.07410–20 Pan-STARRS 12 August 2020 Quasi-satellite stable Quasi-satellite stable
2020 XL5 0.3871180±80 Pan-STARRS 12 December 2020 Earth trojan Earth trojan

See also

Related Research Articles

Asteroid Astronomical object

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.

Lagrange point Equilibrium points near two orbiting bodies

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.

Jupiter trojan Asteroid sharing the orbit of Jupiter

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 Aten asteroid and a co-orbital object with Earth

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.

Giant-impact hypothesis Theory of the formation of the Moon

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.

Natural satellite Astronomical body that orbits a planet

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.

Planetesimal Solid objects in protoplanetary disks and debris disks

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.

<span class="nowrap">2002 AA<sub>29</sub></span>

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.

Horseshoe orbit Type of co-orbital motion of a small orbiting body relative to a larger orbiting body

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'.

Trojan (celestial body) Objects sharing the orbit of a larger one

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.

Claimed moons of Earth Claims that Earth may have other natural satellites

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.

Nice model

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.

<span class="nowrap">(419624) 2010 SO<sub>16</sub></span>

(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.

<span class="nowrap">2010 TK<sub>7</sub></span> Near-Earth asteroid

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 (planet) Planet that is hypothesized to have impacted Earth and created the Moon

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.


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