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Binary comets, like 67P/Churyumov–Gerasimenko and MU69 Ultima Thule and at least a couple of others, have been observed. How likely would it be for a binary comet to again join with another object to form a trinary comet, i.e. three comets stuck together?

Could such non-destructive slow merger of comets explain extreme elongation such as that of the interstellar object 1I/Oumuamua? It might've been around for longer than the age of the Solar System, having time for more unlikely events. Is there a feasible process that lines up more than two bodies together?

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  • $\begingroup$ It's a little early to assume Ultima Thule is a comet, is it not? The composition and presence/absence of a coma are not yet known. $\endgroup$ – called2voyage Jan 3 at 21:23
  • $\begingroup$ @called2voyage Okay, nomenclature here. It is certainly a proto-comet. It would form a coma if it came close enough to the Sun. Kuiper Belt objects are comets as far as I'm concerned. 69/P doesn't cease to be a comet when it is too far away to have a coma. $\endgroup$ – LocalFluff Jan 3 at 21:25
  • $\begingroup$ Again, the composition is not known. We might assume based on other Kuiper Belt objects that it is icy enough to be a comet, but we don't have data for that yet, do we? $\endgroup$ – called2voyage Jan 3 at 21:26
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    $\begingroup$ @uhoh True, I just think calling it a "comet" without qualifier is unnecessary for the question and untechnical. $\endgroup$ – called2voyage Jan 4 at 2:47
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    $\begingroup$ Elongated asteroids like Itokawa, Eros and others are most probably several big bodies in contact, covered by regolith. I suppose they are elongated (not clumped compactly by gravity) because of rotation. Because elongated objects make up significant amount of observed asteroids/comets, we can suppose "ternary" and more complex systems should not be rare. But there is some problem if masses of bodies are similar - at collision the momentum will be tranfered most efficiently, like in Newton's cradle. So if masses are close, there is more probable some of contact parts will be ejected. $\endgroup$ – Heopps Jan 5 at 10:43

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