An Earth trojan is an

asteroid An asteroid is a minor planet—an object larger than a meteoroid that is neither a planet nor an identified comet—that orbits within the Solar System#Inner Solar System, inner Solar System or is co-orbital with Jupiter (Trojan asteroids). As ...

that orbits the Sun in the vicinity of the

Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...

–Sun

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

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 trojans, the asteroids that were observed near the Lagrangian points of

Jupiter Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a Jupiter mass, mass more than 2.5 times that of all the other planets in the Solar System combined a ...

'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 The Panoramic Survey Telescope and Rapid Response System (Pan-STARRS1; List of observatory codes, obs. code: IAU code#F51, F51 and Pan-STARRS2 obs. code: IAU code#F52, F52) located at Haleakala Observatory, Hawaii, US, consists of astronomical ...

survey in December 2020 and later recognised as an Earth trojan in January 2021. It is 1.2 km in diameter.

(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 asteroid C-type (carbonaceous ) asteroids are the most common variety, forming around 75% of known asteroids. They are volatile-rich and distinguished by a very low albedo because their composition includes a large amount of carbon, in addition to rocks ...

s ~350 m in diameter, or S-type asteroids ~175 m in diameter." In February 2017, the

OSIRIS-REx OSIRIS-REx was a NASA asteroid-study and sample-return mission that visited and collected samples from 101955 Bennu, a C-type asteroid, carbonaceous near-Earth object, near-Earth asteroid. The material, returned in September 2023, is expected ...

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'' spacecraft searched the region while proceeding to asteroid Ryugu, but did not find any asteroids there.

Giant-impact hypothesis

A hypothetical planet-sized Earth trojan the size of

Mars Mars is the fourth planet from the Sun. It is also known as the "Red Planet", because of its orange-red appearance. Mars is a desert-like rocky planet with a tenuous carbon dioxide () atmosphere. At the average surface level the atmosph ...

, given the name Theia, is thought by proponents of the giant-impact hypothesis to be the origin of the

Moon The Moon is Earth's only natural satellite. It Orbit of the Moon, orbits around Earth at Lunar distance, an average distance of (; about 30 times Earth diameter, Earth's diameter). The Moon rotation, rotates, with a rotation period (lunar ...

. 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. 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

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

Gallery

File:Illustration of Trojan Asteroid 2020 XL5.jpg,

References

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