Comets are commonly called icy dust balls or dirty snowballs and as such rather light:

Known comets have been estimated to have an average density of $0.6 {\rm g}/{\rm cm}^3$. Because of their low mass, comet nuclei do not become spherical under their own gravity and therefore have irregular shapes.

I read about their live times, I learned that they can rotate, how they can be classified by their type. NASA summarizes the key facts as follows

Comets are frozen leftovers from the formation of the solar system composed of dust, rock and ices. They range from a few miles to tens of miles wide, but as they orbit closer to the sun, they heat up and spew gases and dust into a glowing head that can be larger than a planet. This material forms a tail that stretches millions of miles.

What interests me has to do with their formation. I know that comets are not spherical and rather small in size. I am now wondering how comets' nuclei stick together, more specifically if there is some empiral knowledge or even some theory I should know about. Obviously, gravitation is not strong enough, so it must be merely the structural stability of the material, but how can that be estimated?


  • $\begingroup$ One word: "poorly". $\endgroup$ – Mark Feb 5 at 23:35

Van-der-Waals forces and gravitation are the main answer.

The main property keeping together bodies smaller than a few $100 {\rm km}$ are their constituents surface forces between the grains (van-der-Waals) and chemical/ molecular bonding within the grains themselves. It's the same mechanic which makes the dust and grease in your flat stick to surfaces and the fluffy dirt clumps under the bed stick together, thus cohesion/ adhesion.

The typical internal strength of such dirty iceball as comets are, or rubble piles as some minor asteroids like Ruygu seem to be (e.g. Wada et al, 2018), does not need to be large - there is nothing in deep space which pulls it apart. And even the few µg of self-gravitation help to keep smaller fragments and ejecta to return to the parent body when hit by the inevitable micro-meteorites after their intial formation (Blum et al, 2017) (so yes, there is still self-gravitation involved which helps form them; they are just too small to achieve hydrostatic equilibrium and differentiation).

67P Churyumov-Gerasimenke provided a nice cometary physics lab which allowed in-situ investigation. The strength of the material could be analysed directy (Heinisch et al, 2018), (Grousin et al, 2018) with the lander Philae, even though it landed somewhat precariously (see also this overview by Blum et al (2018) focussing on planet formation and lessons learned from 67P in particular).

In summary: most measurement, both in-situ and lab experiments, indicate that the internal strength of comets is within the range of $1 {\rm Pa} \ldots ... 10 {\rm kPa}$ and most likely at the lower end of this range. Their strength is thus determined by their small gravity and the inter-particle forces, thus van-der-Waals which in part is possibly enhanced a bit by sintering together the different constituents grains.

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    $\begingroup$ Wow, you are quick. The essential part I overlooked is that nothing is really pulling comets appart, usually, when being far away from e.g. Jupiter. Also, I like the comparison to the fluffy dust balls. $\endgroup$ – B--rian Feb 5 at 12:06
  • $\begingroup$ I added two more wiki-links and fixed a typo and some typesetting, hope you are ok with it. $\endgroup$ – B--rian Feb 5 at 19:45
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    $\begingroup$ Just to add to how little you need to hold comets together, the OSIRIS-Rex spacecraft that recently visited Bennu had to slow its orbital speed to a paltry 5 cm/sec to not escape the gravity. The real question here, to me, is what keeps them stuck together immediately after an impact, i.e., why don't they just crash and bounce off? $\endgroup$ – zephyr Feb 5 at 20:04

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