Trojan (asteroid)

An object located at either of the stable Lagrangian points (L4 or L5) of a larger object's orbit. The term comes from the theme of the names given to the first asteroids to be found at Jupiter's Lagrangian points, Achilles, Patroclus, and Hektor – characters in Homer's epic poem about the Trojan War, the Iliad.

Jupiter's Trojans occupy two banana-shaped regions, centered 60° ahead and behind the planet, which can be considered the "Greek" and "Trojan" nodes, respectively; only Hektor in the Greek node and Patroclus in the Trojan node are, so to speak, in the camps of the enemy. Perturbations by other planets, principally Saturn, cause the Trojans to oscillate about the Lagrangian points in an arc about 45-80° from Jupiter with a period of 150-200 years. Several hundred Jupiter Trojans are known out of a total population that includes an estimated 2,300 objects bigger than 15 km across and many more of smaller size; most do not move in the plane of the planet's orbit but rather in orbits inclined by up to 40°.

Unless qualified, "Trojan" is assumed to mean "Jupiter Trojan." There is one known Earth Trojan (the 300m-wide 2010TK7), three known Mars Trojans, and four known Neptune Trojans. Searches have so far failed to uncover any similar objects in the orbits of any of the other planets. Saturn's satellites Helene, Calypso, and Telesto are also sometimes called Trojans because they lie at the leading Lagrangian point of Dione, and at the trailing and leading Lagrangian points of Tethys, respectively.

Origin of the Jupiter Trojans

The traditional theory about where the Jupiter Trojans came from argues that they formed near Jupiter while the planet itself was coming together, some 4.6 billion years ago. If this were true, the Trojans would be expected to have a rock and ice composition similar to the Jovian moons Callisto and Ganymede. However, recent observations are consistent with a newer theory put forward in May 2005 by an international team of researchers.¹ This theory suggests that Jupiter, Saturn, Uranus and Neptune originally formed in a once-densely-filled area close to the Sun, in a great disk of innumerable small bodies, called planetesimals. This circling disc can be likened to a kind of primordial Kuiper Belt – the region of objects which now lies beyond Neptune. According to the group's computer simulations, it was not until 650 million years after the planets formed that they arrived in their current orbits, having undergone a chaotic migration outwards, flinging planetesimals as they went. Some of these icy bodies, they suggested, were captured by Jupiter as Trojans rather than migrating to the Kuiper Belt. This new theory is supported, for example, by observations of the Trojan Patroclus that were announced in February 2006.

Reference

A. Morbidelli, H. F., Levison, K. Tsiganis, and R. Gomes. "Chaotic capture of Jupiter's Trojan asteroids in the early Solar System." Nature 435, 462-465, 2005.

Related category

• ASTEROIDS AND OTHER MINOR PLANETS: TYPES AND GROUPS

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