An Earth trojan is an

asteroid An asteroid is a minor planet of the inner Solar System. Sizes and shapes of asteroids vary significantly, ranging from 1-meter rocks to a dwarf planet almost 1000 km in diameter; they are rocky, metallic or icy bodies with no atmosphere. ...

that orbits the Sun in the vicinity of the Earth–Sun

Lagrangian point In celestial mechanics, the Lagrange points (; also Lagrangian points or libration points) are points of equilibrium for small-mass objects under the influence of two massive orbiting bodies. Mathematically, this involves the solution of th ...

s (leading 60°) or (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 trojan 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 poin ...

s, the asteroids that were observed near the Lagrangian points of Jupiter's orbit.

Members

(leading)

* : A 300-metre diameter asteroid, discovered using the Wide-field Infrared Survey Explorer (WISE) satellite in January 2010. * : Discovered by the Pan-STARRS survey in December 2020 and later recognized as an Earth trojan in January 2021.

(trailing)

* No known objects are currently thought to be trojans of Earth.

Searches

An Earth-based search for objects was conducted in 1994, covering 0.35 square degrees of sky, under poor observing conditions. Received 24 November 1997; revised 13 April 1998. 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." In February 2017, the OSIRIS-REx spacecraft performed a search from within the region on its way to asteroid Bennu. No additional Earth trojans were discovered. In April 2017, the ''

Hayabusa2 is an asteroid sample-return mission operated by the Japanese state space agency JAXA. It is a successor to the ''Hayabusa'' mission, which returned asteroid samples for the first time in June 2010. ''Hayabusa2'' was launched on 3 December 2 ...

'' spacecraft searched the 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 (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 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 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 ...

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.

Continuing interest in near-Earth asteroids

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, neither nor . Earth has another noted companion, asteroid

3753 Cruithne 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 ...

. 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 469219 Kamoʻoalewa (), provisionally designated , is a very small asteroid, fast rotator and near-Earth object of the Apollo group, approximately in diameter. At present it is a quasi-satellite of Earth, and currently the smallest, closest, ...

, an

discovered on 27 April 2016, is possibly the most stable quasi-satellite of Earth.

References

{{Portal bar, Stars, Spaceflight, Outer space, Science Trojans 3