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Many asteroids are today known to be binary, to have "moons".

But what comets are known to be binary? Or what fraction of them are theoretically thought to be binary? And if possible, then separate Oort cloud comets from comets with more local asteroid like orbits in this respect.

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None. and that doesn't surprise me much.

Comets are thought to be Oort-cloud objects which by interaction either with other Oort-cloud objects or with objects passing the Solar system have been perturbed to venture into the gravitational reach of the outer planets, whence they are flung into the inner Solar system via close encounters (gravitational slingshots) with one of the outer planets, in particular Jupiter and Saturn. For such a slingshot to be efficient, the encounter must be close when any binary (proto-)comet will most likly be disassociated by the gravitational tides of the sling-shooting planet.

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  • $\begingroup$ Maybe the Oort-clouds are a little bit from the known planets for a slingshot maneuver? $\endgroup$
    – peterh
    Commented Sep 5, 2014 at 10:49
  • $\begingroup$ @PeterHorvath see edit. $\endgroup$
    – Walter
    Commented Sep 5, 2014 at 17:47
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Contact binaries, in which two objects are touching may be fairly common. This is one interpretation of the "rubber duck" shape of the "Rosetta comet" 67P/Churyumov–Gerasimenko, and radar observation of 8P/Tuttle suggest it is also formed of two objects in contact with each other.

The low mass and so weak gravity of comets means that have a very small sphere of influence. It would have a radius of only a few km.

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