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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 , existing 60° ahead of the planet in its orbit, or , 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. Like main belt asteroids, Jupiter trojans form families. The term "Trojan asteroid" is normally understood to specifically mean the Jupiter trojans because the first Trojans were discovered near Jupiter's orbit and Jupiter currently has by far the most known Trojans. Barnard believed he had seen the recently discovered Saturnian satellite Phoebe, which was only two arc minutes away in the sky at the time, or possibly an asteroid. The object's identity was not understood until its orbit was calculated in 1999.

Estimates of the total number of Jupiter trojans are based on deep surveys of limited areas of the sky. The binary's orbit is extremely close, at 650 km, compared to 35,000 km for the primary's Hill sphere. whereas the distribution for main belt asteroids was found to be non Maxwellian, with a deficit of periods in the range 8–10 hours.

Spectroscopically, the Jupiter trojans mostly are D type asteroids, which predominate in the outer regions of the asteroid belt. The Jupiter trojans' spectra are similar to those of the irregular moons of Jupiter and, to a certain extent, comet nuclei, though Jupiter trojans are spectrally very different from the redder Kuiper belt objects. A team from the Keck Observatory in Hawaii announced in 2006 that it had measured the density of the binary Jupiter trojan 617 Patroclus as being less than that of water ice (0.8 g/cm3), suggesting that the pair, and possibly many other Trojan objects, more closely resemble comets or Kuiper belt objects in composition—water ice with a layer of dust—than they do the main belt asteroids. In a variation of this theory Jupiter captures trojans during its initial growth then migrates as it continues to grow. During Jupiter's migration the orbits of objects in horseshoe orbits are distorted causing the L4 side of these orbits to be over occupied. As a result, an excess of trojans is trapped on the L4 side when the horseshoe orbits shift to tadpole orbits as Jupiter grows. This model also leaves the Jupiter trojan population 3–4 orders of magnitude too large. The second theory proposes that the Jupiter trojans were captured during the migration of the giant planets described in the Nice model. In the Nice model the orbits of the giant planets became unstable 500–600 million years after the Solar System's formation when Jupiter and Saturn crossed their 1:2 mean motion resonance. Encounters between planets resulted in Uranus and Neptune being scattered outward into the primordial Kuiper belt, disrupting it and throwing millions of objects inward. This process can also occur later when Jupiter and Saturn cross weaker resonances. In a revised version of the Nice model Jupiter trojans are captured when Jupiter encounters an ice giant during the instability. In this version of the Nice model one of the ice giants (Uranus, Neptune, or a lost fifth planet) is scattered inward onto a Jupiter crossing orbit and is scattered outward by Jupiter causing the orbits of Jupiter and Saturn to quickly separate. When Jupiter's semi major axis jumps during these encounters existing Jupiter trojans can escape and new objects with semi major axes similar to Jupiter's new semi major axis are captured. Following its last encounter the ice giant can pass through one of the libration points and perturb their orbits leaving this libration point depleted relative to the other. After the encounters end some of these Jupiter trojans are lost and others captured when Jupiter and Saturn are near weak mean motion resonances such as the 3:7 resonance via the mechanism of the original Nice model. Levison et al. believe that roughly 200 ejected Jupiter trojans greater than 1 km in diameter might be travelling the Solar System, with a few possibly on Earth crossing orbits. Some of the escaped Jupiter trojans may become Jupiter family comets as they approach the Sun and their surface ice begins evaporating. Lucy is set to explore seven Jupiter trojans. It was launched on October 16, 2021, and will arrive at the Trojan cloud in 2027 after two Earth gravity assists and a fly by of a main belt asteroid. It will then return to the vicinity of Earth for another gravity assist to take it to Jupiter's Trojan cloud where it will visit 617 Patroclus.