<p>Van-der-Waals forces and gravitation are the main answer.</p>
<p>The main property keeping together bodies smaller than a few <span class="math-container">$100 {\rm km}$</span> 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.</p>
<p>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. <a href="https://arxiv.org/abs/1804.03734" rel="nofollow noreferrer">Wada et al, 2018</a>), 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 <a href="https://arxiv.org/abs/1710.07846" rel="nofollow noreferrer">their intial formation (Blum et al, 2017)</a> (so yes, there is still self-gravitation involved which helps form them; they are just too small to achieve hydrostatic equilibrium and differentiation).</p>
<p><a href="https://en.wikipedia.org/wiki/67P/Churyumov%E2%80%93Gerasimenko" rel="nofollow noreferrer">67P Churyumov-Gerasimenke</a> provided a nice cometary physics lab which allowed in-situ investigation. The strength of the material could be <a href="https://www.researchgate.net/publication/328945555_Compressive_strength_of_comet_67PChuryumov-Gerasimenko_derived_from_Philae_surface_contacts" rel="nofollow noreferrer">analysed directy (Heinisch et al, 2018)</a>, (<a href="https://link.springer.com/article/10.1007/s11214-019-0594-x" rel="nofollow noreferrer">Grousin et al, 2018</a>) with the lander <a href="https://en.wikipedia.org/wiki/Philae_(spacecraft)" rel="nofollow noreferrer">Philae</a>, even though it landed somewhat precariously (see also this overview by <a href="https://link.springer.com/article/10.1007/s11214-018-0486-5" rel="nofollow noreferrer">Blum et al (2018)</a> focussing on planet formation and lessons learned from 67P in particular).</p>
<p>In summary: most measurement, both in-situ and lab experiments, indicate that the internal strength of comets is within the range of <span class="math-container">$1 {\rm Pa} \ldots  ... 10 {\rm kPa}$</span> 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.</p>

CC BY-SA 4.0
2021-02-05T12:06:41.717Z
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.
B--rian
CC BY-SA 4.0
2021-02-05T19:45:28.023Z
I added two more wiki-links and fixed a typo and some typesetting, hope you are ok with it.
B--rian
CC BY-SA 4.0
2021-02-05T20:04:35.320Z
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](https://www.asteroidmission.org/asteroid-operations/) 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?
zephyr
CC BY-SA 4.0
2021-09-20T09:10:33.920Z
@zephyr see https://www.aanda.org/articles/aa/pdf/2017/07/aa30345-16.pdf plot 13 (which an updated plot from some way earlier Weidenschilling work): relative collision velocities for particles of that size in a solar nebula are mm/s. So it's far from 'smashing'. Yet the 'bouncing barrier' at slightly larger (and faster) particles is something only overcome by turbulent disks and gas-solid interactions in things like eddies to create local over-densities of solids and enhance their self-gravity
planetmaker
CC BY-SA 4.0
2021-02-05T11:55:59.987Z
2021-02-08T13:58:58.903Z
2021-02-08T13:58:58.903Z
planetmaker
<p><a href="https://en.wikipedia.org/wiki/Comet" rel="nofollow noreferrer">Comets</a> are commonly called <em>icy dust balls</em> or <em>dirty snowballs</em> and as such rather light:</p>
<blockquote>
<p>Known comets have been estimated to have an average density of <span class="math-container">$0.6 {\rm g}/{\rm cm}^3$</span>. Because of their low mass, comet nuclei do not become spherical under their own gravity and therefore have irregular shapes.</p>
</blockquote>
<p>I read about their <a href="https://astronomy.stackexchange.com/questions/18551/how-long-does-a-comet-live">live times</a>, I learned that <a href="https://astronomy.stackexchange.com/questions/41214/do-some-comets-spin-if-so-how-fast">they can rotate</a>, how they can be <a href="https://en.wikipedia.org/wiki/List_of_comets_by_type" rel="nofollow noreferrer">classified by their type</a>. <a href="https://solarsystem.nasa.gov/asteroids-comets-and-meteors/comets/overview/" rel="nofollow noreferrer">NASA</a> summarizes the key facts as follows</p>
<blockquote>
<p>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.</p>
</blockquote>
<p>What interests me has to do with their formation. I know that comets are not spherical and rather small in size. <strong>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.</strong> Obviously, gravitation is not strong enough, so it must be merely the structural stability of the material, but how can that be estimated?</p>
<h3>References</h3>
<ul>
<li>Press Release of the Max Planck Society: <a href="https://www.mpg.de/513711/pressRelease20051017" rel="nofollow noreferrer">Are comets more like &quot;Icy Dust Balls&quot; or &quot;Dirty Snowballs&quot;?</a> (2005)</li>
<li><a href="https://astronomy.stackexchange.com/questions/40435/the-creation-of-comets">The creation of comets</a></li>
<li><a href="https://astronomy.stackexchange.com/questions/5903/can-two-comets-travel-together-as-one">Can two comets travel together as one?</a></li>
<li><a href="https://astronomy.stackexchange.com/questions/14256/how-big-do-comets-get">How big do comets get?</a></li>
<li><a href="https://astronomy.stackexchange.com/questions/490/why-did-the-comet-shoemaker-levy-9-fragments-cause-such-large-explosions-on-jupi">Why did the Comet Shoemaker-Levy-9 fragments cause such large explosions on Jupiter?</a></li>
<li>The article from the Scientific American <a href="https://www.scientificamerican.com/article/if-comets-melt-why-do-the/" rel="nofollow noreferrer"><em>If comets melt, why do they seem to last for long periods of time?</em></a></li>
</ul>

CC BY-SA 4.0
2021-02-05T23:35:53.157Z
One word: "poorly".
Mark
<comets><modeling><theories>
How do comets' nuclei stick together?
B--rian
# How do comets' nuclei stick together?

(Asked by: username_0 on 2021-02-05)

[Comets](https://en.wikipedia.org/wiki/Comet) 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](https://astronomy.stackexchange.com/questions/18551/how-long-does-a-comet-live), I learned that [they can rotate](https://astronomy.stackexchange.com/questions/41214/do-some-comets-spin-if-so-how-fast), how they can be [classified by their type](https://en.wikipedia.org/wiki/List_of_comets_by_type). [NASA](https://solarsystem.nasa.gov/asteroids-comets-and-meteors/comets/overview/) 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?

### References

* Press Release of the Max Planck Society: [Are comets more like "Icy Dust Balls" or "Dirty Snowballs"?](https://www.mpg.de/513711/pressRelease20051017) (2005)
* [The creation of comets](https://astronomy.stackexchange.com/questions/40435/the-creation-of-comets)
* [Can two comets travel together as one?](https://astronomy.stackexchange.com/questions/5903/can-two-comets-travel-together-as-one)
* [How big do comets get?](https://astronomy.stackexchange.com/questions/14256/how-big-do-comets-get)
* [Why did the Comet Shoemaker-Levy-9 fragments cause such large explosions on Jupiter?](https://astronomy.stackexchange.com/questions/490/why-did-the-comet-shoemaker-levy-9-fragments-cause-such-large-explosions-on-jupi)
* The article from the Scientific American [*If comets melt, why do they seem to last for long periods of time?*](https://www.scientificamerican.com/article/if-comets-melt-why-do-the/)

**Tags:** comets, modeling, theories
> **Comments:**
> **username_1**: One word: "poorly".  

---

## 1 answers

### Answer by username_2 on 2021-02-05. Score: 10 | Accepted answer

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](https://arxiv.org/abs/1804.03734)), 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)](https://arxiv.org/abs/1710.07846) (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](https://en.wikipedia.org/wiki/67P/Churyumov%E2%80%93Gerasimenko) provided a nice cometary physics lab which allowed in-situ investigation. The strength of the material could be [analysed directy (Heinisch et al, 2018)](https://www.researchgate.net/publication/328945555_Compressive_strength_of_comet_67PChuryumov-Gerasimenko_derived_from_Philae_surface_contacts), ([Grousin et al, 2018](https://link.springer.com/article/10.1007/s11214-019-0594-x)) with the lander [Philae](https://en.wikipedia.org/wiki/Philae_(spacecraft)), even though it landed somewhat precariously (see also this overview by [Blum et al (2018)](https://link.springer.com/article/10.1007/s11214-018-0486-5) 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.

> **Comments:**
> **username_0**: 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.  
> **username_0**: I added two more wiki-links and fixed a typo and some typesetting, hope you are ok with it.  
> **username_3**: 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](https://www.asteroidmission.org/asteroid-operations/) 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?  
> **username_2**: @zephyr see https://www.aanda.org/articles/aa/pdf/2017/07/aa30345-16.pdf plot 13 (which an updated plot from some way earlier Weidenschilling work): relative collision velocities for particles of that size in a solar nebula are mm/s. So it's far from 'smashing'. Yet the 'bouncing barrier' at slightly larger (and faster) particles is something only overcome by turbulent disks and gas-solid interactions in things like eddies to create local over-densities of solids and enhance their self-gravity  

---

