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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$\begingroup$ Thee largest Trojan asteroid, 624 Hektor, is larger than all of Jupiter's moons except the Galilean four. The largest hundred trojans are all larger than most of Jupiter's known moons. $\endgroup$– notovnyOct 19, 2020 at 14:14
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$\begingroup$ There might be related questions here that can shed some light on this. There are several factors that come into play. 1) space is big, gravity is weak, time is short, there's just not much dynamical "ooomph" to drive this, 2) to bring things together and coalesce you need a dissipation mechanism. If two asteroids drift together somehow they may just bounce (recoil) without losing enough energy in order to stick. Of course it does happen there are "stuck-together binary" asteroids (New Horizons photographed a Kuiper belt one as it flew past) and close-orbiting pairs so it can happen sometimes. $\endgroup$– uhohOct 20, 2020 at 1:11
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$\begingroup$ related: What process shaped Ultima Thule? and Is 486958 Arrokoth (2014 MU69 aka Ultima Thule) the only solar-system object determined to be binary by occultation? and this answer and Binary asteroid and read about 67P/Churyumov-Gerasimenko another famous "stuck-together binary". $\endgroup$– uhohOct 20, 2020 at 1:23
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$\begingroup$ L4 & L5 are kind of stable, but they're like a hilltop rather than a valley, so stuff at those points doesn't naturally come together. $\endgroup$– PM 2RingOct 20, 2020 at 7:58
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1 Answer
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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.
answered Oct 20, 2020 at 9:30