Jupiter's trojans are located in the L4 and L5 Lagrange points.
These two points are stable, so why don't all the trojans have already merged into small moons?
And since it's not the case, what prevents them to do so?
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The Jupiter Trojans are not at the Sun-Jupiter L4 and L5 Lagrange points. They are instead in pseudo orbits about those points. These pseudo orbits appear from the perspective of a frame of reference that rotates at Jupiter's orbital rate (i.e., a frame in which the Sun and Jupiter are more or less fixed). Unlike central body motion, which results in planar orbits, these pseudo orbits in general are not planar, and they librate between in front and behind the L4 or L5 point over a period of about 150 years.
The very large volume means interactions are rare, and the crazy pseudo orbits means collisions are more likely to be violent than not. That said, at least one Trojan asteroid, 624 Hektor, is most likely to be a peanut-shaped contact binary.