Earth trojan

An Earth trojan is an asteroid that orbits the Sun in the vicinity of the Earth–Sun Lagrangian points L₄ (leading 60°) and L₅ (trailing 60°), 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

L₄ (leading)

L₅ (trailing)

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 2010 TK₇ would exert a surface gravitational force of less than 0.00005 times that of Earth (although the centrifugal force of rotation could cause separation).

Trojan asteroid

Trojan asteroid refers to asteroids orbiting in the 4th and 5th Lagrange points of major planets in the Solar System. Currently there are six planets known to have trojans:

It is possible Mercury and Saturn have trojan asteroids, but none have been discovered so far due to; Mercury's close proximity to the Sun hinders the observation of such objects, and Saturn's proximity to Jupiter throwing possible would-be trojans out of a stable orbit with Saturn.

Trojan (astronomy)

In astronomy, a trojan is a minor planet or moon that shares the orbit of a planet or larger moon, wherein the trojan remains in the same, stable position relative to the larger object. In particular, a trojan remains near one of the two trojan points of stability – designated L₄ and L₅ – which lie approximately 60° ahead of and behind the larger body, respectively. Trojan points are two of five types of Lagrangian points, and a trojan is a type of Lagrangian object.

They are one type of co-orbital object. In this arrangement, the massive star and the smaller planet orbit about their common barycenter. A much smaller mass located at one of the Lagrangian points is subject to a combined gravitational force that acts through this barycenter. Hence the object can orbit around the barycenter with the same orbital period as the planet, and the arrangement can remain stable over time.

The Jupiter trojans account for most known trojans in the Solar System. They are divided into the Greek camp (L₄) in front of and the Trojan camp (L₅) trailing behind Jupiter in their orbit. More than 6,000 have been found so far and more than a million Jupiter trojans larger than one kilometer are thought to exist, whereas only a few Mars trojans (7) and Neptune trojans (13) have been found to date. Numerical calculations of the orbital dynamics involved indicate that Saturn and Uranus probably do not have any primordial trojans. The discovery of the first Earth trojan, 2010 TK7, was announced by NASA in 2011.