# What is the smallest natural sphere in our solar system?
**URL:** https://boards.straightdope.com/t/what-is-the-smallest-natural-sphere-in-our-solar-system/671945
**Category:** Factual Questions
**Created:** [October 22, 2013, 4:50pm UTC](https://boards.straightdope.com/t/what-is-the-smallest-natural-sphere-in-our-solar-system/671945 "2013-10-22T16:50:01Z")
**Posts on this page:** 20
**Page:** 1
### Author:  [@gytalf2000](https://boards.straightdope.com/u/gytalf2000)
#### Post date: [October 22, 2013, 4:50pm UTC](https://boards.straightdope.com/t/what-is-the-smallest-natural-sphere-in-our-solar-system/671945/1 "2013-10-22T16:50:01Z")
I know that some of the asteroids are quite large (hundreds of miles across? I forget how big) – at what point do they get big enough to start looking like planets? Taking on the form of a sphere? Or is the material that they are composed of preventing them from being spherical?
Aren’t some of the moons in our solar system pretty tiny? Are some asteroids bigger than the smallest moons? What is the smallest natural sphere that we have found?
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### Author:  [@scr4](https://boards.straightdope.com/u/scr4)
#### Post date: [October 22, 2013, 5:02pm UTC](https://boards.straightdope.com/t/what-is-the-smallest-natural-sphere-in-our-solar-system/671945/2 "2013-10-22T17:02:32Z")
[Wikipedia’s list of solar system objects by size](http://en.wikipedia.org/wiki/List_of_Solar_System_objects_by_size).
Of course there’s no clear dividing line between “spherical” and “non-spherical”. But looking at that list, [Mimas](http://en.wikipedia.org/wiki/Mimas_(moon)) at 198.3 km radius seems to be the smallest object that looks nearly spherical, except for the huge crater. But there are a couple of objects that are slightly larger, but non-spherical. Most notably [Vesta](http://en.wikipedia.org/wiki/4_Vesta) at 263 km mean radius.
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### Author:  [@Exapno\_Mapcase](https://boards.straightdope.com/u/Exapno_Mapcase)
#### Post date: [October 22, 2013, 5:08pm UTC](https://boards.straightdope.com/t/what-is-the-smallest-natural-sphere-in-our-solar-system/671945/3 "2013-10-22T17:08:24Z")
This may come down to an opinion on what is considered spherical.
As a rough rule of thumb, objects smaller than 300 km in diameter don’t collapse in on themselves. That leaves Ceres, Pallas, and Vesta in the asteroids as large enough. But even Ceres has [two different diameters](http://nssdc.gsfc.nasa.gov/planetary/factsheet/asteroidfact.html): it’s 960 x 932 km. Does that make it spherical?
There are huge numbers of moons smaller than Vesta, which has a diameter on that page of 530 km. This [Wikipedia page](http://en.wikipedia.org/wiki/List_of_Solar_System_objects_by_size) lists all of those between 200 and 400 km radius (400 - 800 km diameter) and claims that 200 km radius is the cutoff.
> [@](#):
>
> Most objects in this size range are expected to be round. All the satellites except Proteus are round. The asteroid 10 Hygiea is not, and 2 Pallas and 4 Vesta are borderline. Like the satellites, TNOs in this size range are expected to be round (assuming the estimated size is correct).
That would put Proteus, a moon of Neptune, as the smallest spherical moon and a few trans-Neptunian objects as slightly smaller. They also list Vesta as the smallest asteroid.
I’m pretty sure that the cutoff is arbitrary, since they go from a radius of 199 to one of 200.5. Not enough to make the body behave differently. If Vesta is borderline, then most of those below it also are.
Slightly ninjaed.
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### Author:  [@Gagundathar](https://boards.straightdope.com/u/Gagundathar)
#### Post date: [October 22, 2013, 5:35pm UTC](https://boards.straightdope.com/t/what-is-the-smallest-natural-sphere-in-our-solar-system/671945/4 "2013-10-22T17:35:17Z")
Ceres might be better described as an oblate spheroid.
That applies to most, if not all, sizable objects in our system.
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### Author:  [@gytalf2000](https://boards.straightdope.com/u/gytalf2000)
#### Post date: [October 22, 2013, 5:56pm UTC](https://boards.straightdope.com/t/what-is-the-smallest-natural-sphere-in-our-solar-system/671945/5 "2013-10-22T17:56:06Z")
> [@scr4](#):
>
> [Wikipedia’s list of solar system objects by size](http://en.wikipedia.org/wiki/List_of_Solar_System_objects_by_size).
>
> Of course there’s no clear dividing line between “spherical” and “non-spherical”. But looking at that list, [Mimas](http://en.wikipedia.org/wiki/Mimas_(moon)) at 198.3 km radius seems to be the smallest object that looks nearly spherical, except for the huge crater. But there are a couple of objects that are slightly larger, but non-spherical. Most notably [Vesta](http://en.wikipedia.org/wiki/4_Vesta) at 263 km mean radius.
Fascinating! Thanks for the link and the info.
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### Author:  [@Asympotically\_fat](https://boards.straightdope.com/u/Asympotically_fat)
#### Post date: [October 22, 2013, 6:36pm UTC](https://boards.straightdope.com/t/what-is-the-smallest-natural-sphere-in-our-solar-system/671945/6 "2013-10-22T18:36:24Z")
I was going to answer “my balls”, but thought better of it.
Obviously there are a lot of objects that are almost, but not quite spherical due to their rotation (e.g. oblate spherioids such as the Earth and the other planets) and there are also some objects that don’t even look spherical, but would be if they weren’t spinning so fast (e.g. the dwarf planet Haumea). Objects that are in hydrostatic equilibrium will settle down to be very close to spherical if they are not rotating and the smallest known object in the solar system to be in hydrostatic equilibrium is Mimas and certainly describing it as spherical would certainly be no great liberty.
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### Author:  [@Leo\_Bloom](https://boards.straightdope.com/u/Leo_Bloom)
#### Post date: [October 22, 2013, 6:39pm UTC](https://boards.straightdope.com/t/what-is-the-smallest-natural-sphere-in-our-solar-system/671945/7 "2013-10-22T18:39:45Z")
What’s a sphere, anyway?
Ask Science!
[Computing the Sphericity and Roundness of Rocks
Heather Dunlop
Dec. 1, 2005](http://dunlop1.net/doc/sphericity_roundness_presentation.pdf)
(Interestingly, for a robotics study. Mars, I guess.)
Math, charts, for the taking.
See also Wiki “[Sphericity](https://en.wikipedia.org/wiki/Sphericity).” Wiki “Sphericity scale” is empty; use Dunlop for leads.
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### Author:  [@Senegoid](https://boards.straightdope.com/u/Senegoid)
#### Post date: [October 22, 2013, 7:22pm UTC](https://boards.straightdope.com/t/what-is-the-smallest-natural-sphere-in-our-solar-system/671945/8 "2013-10-22T19:22:28Z")
From the context of the question, OP is apparently referring to astronomical objects.
Otherwise, I was about to mention that I have a small jar of protons on my desk, that I bought as a souvenir at a bazaar I visited in Marrakech a while back. I was wondering if those are actually considered sperical?
They’re made of three quarks, as we now understand. Are quarks spherical? I know they’re really really small.
And that’s assuming we actually have a meaningful definition of “size” at those quarcical scales. Isn’t there a law that, while you can measure an object’s size _or_ shape, you can never determine both at once?
(ETA: I can’t picture how you would put together three spherical objects to make a larger spherical object. Try it will billiard balls sometime. It doesn’t work.)
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### Author:  [@Leo\_Bloom](https://boards.straightdope.com/u/Leo_Bloom)
#### Post date: [October 22, 2013, 8:15pm UTC](https://boards.straightdope.com/t/what-is-the-smallest-natural-sphere-in-our-solar-system/671945/9 "2013-10-22T20:15:30Z")
I asked this a while back…
Can’t remember search terms or I’d find it…
The Usual Suspects get this question a lot.
Also, **Senegoid** , that’s a fine question but a textbook example of what should be a freestanding thread. I’ll leave the honors to you…
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### Author:  [@DrDeth](https://boards.straightdope.com/u/DrDeth)
#### Post date: [October 22, 2013, 8:21pm UTC](https://boards.straightdope.com/t/what-is-the-smallest-natural-sphere-in-our-solar-system/671945/10 "2013-10-22T20:21:02Z")
> [@Senegoid](#):
>
> They’re made of three quarks, as we now understand.
Hey, those are mine!
Signed:
Muster Mark
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### Author:  [@Lumpy](https://boards.straightdope.com/u/Lumpy)
#### Post date: [October 23, 2013, 1:36am UTC](https://boards.straightdope.com/t/what-is-the-smallest-natural-sphere-in-our-solar-system/671945/11 "2013-10-23T01:36:24Z")
Could arbitrarily small objects have formed from collisions that produced melts that slowly cooled?
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### Author:  [@dtilque](https://boards.straightdope.com/u/dtilque)
#### Post date: [October 23, 2013, 8:52am UTC](https://boards.straightdope.com/t/what-is-the-smallest-natural-sphere-in-our-solar-system/671945/12 "2013-10-23T08:52:41Z")
> [@Gagundathar](#):
>
> Ceres might be better described as an oblate spheroid.
> That applies to most, if not all, sizable objects in our system.
Which means that the OP should have been “What’s the smallest object in dynamic equilibrium?” That is, the shape is determined mainly by the effects of gravity and rotation.
It turns out that the answer depends on the composition of the object. Rock is stronger than ice, so rocky bodies have a larger diameter at which they can be in non-dynamic equilibrium.
Take Vesta, for example. It’s not in dynamic equilibrium or else it would be spherical. Instead, it’s definitely oblate, but that oblateness does not come from rotation. Originally it was spherical, but that was when it was warmer and the rocks a bit softer. After it cooled off, it took a couple large hits from other objects that happened to be at almost the exact same place. These hits knocked off about 1% of Vesta’s mass and left it out of round. The craters are centered at the south pole, although the south pole was not necessarily where the hits took place.
So Mimas is probably about as small as an icy body can be and be in dynamic equilibrium, while rocky bodies will have to be somewhat larger than Vesta.
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### Author:  [@Superhal](https://boards.straightdope.com/u/Superhal)
#### Post date: [October 23, 2013, 9:32am UTC](https://boards.straightdope.com/t/what-is-the-smallest-natural-sphere-in-our-solar-system/671945/13 "2013-10-23T09:32:30Z")
Hmm, the context of your question is questionable. If it orbits the sun, its a planet. If it orbits a planet, its a moon. Size doesn’t matter. This is essentially the crux of the argument.
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### Author:  [@Chronos](https://boards.straightdope.com/u/Chronos)
#### Post date: [October 23, 2013, 3:42pm UTC](https://boards.straightdope.com/t/what-is-the-smallest-natural-sphere-in-our-solar-system/671945/14 "2013-10-23T15:42:12Z")
Replacing “is spherical” with “is in dynamic equilibrium” doesn’t help much, since just in the same way we can dispute how spherical is “spherical enough”, we can also dispute just how close to dynamic equilibrium is close enough. After all, an object with a mountain can’t be perfectly in dynamic equilibrium.
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### Author:  [@DrDeth](https://boards.straightdope.com/u/DrDeth)
#### Post date: [October 23, 2013, 3:49pm UTC](https://boards.straightdope.com/t/what-is-the-smallest-natural-sphere-in-our-solar-system/671945/15 "2013-10-23T15:49:11Z")
> [@Superhal](#):
>
> Hmm, the context of your question is questionable. If it orbits the sun, its a planet. If it orbits a planet, its a moon. Size doesn’t matter. This is essentially the crux of the argument.
There are many objects, some fairly large, that orbit the sun and are not planets.
And, altho there’s no agreed upon defining line\* in theory, there are millions of objects that orbit planets that are not “moons”- for example the Rings of Saturn.
- but it seems to be about 1/2 KM in practice.
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### Author:  [@DrDeth](https://boards.straightdope.com/u/DrDeth)
#### Post date: [October 23, 2013, 3:58pm UTC](https://boards.straightdope.com/t/what-is-the-smallest-natural-sphere-in-our-solar-system/671945/16 "2013-10-23T15:58:59Z")
Some searching on wiki seems to show the moon Methone as the smallest that _may_ be in hydrostatic equalibrium.
> **[Methone (moon)](https://en.wikipedia.org/wiki/Methone_(moon))**
>
> Methone /mɛˈθoʊniː/ is a small, egg-shaped moon of Saturn that orbits out past Saturn's ring system, between the orbits of Mimas and Enceladus. It was discovered in 2004, though it wasn't until 2012 that it was imaged in detail by the Cassini spacecraft.
> Methone was first discovered by the Cassini Imaging Team and given the temporary designation S/2004 S 1. Methone is also named Saturn XXXII (32). The Cassini spacecraft made two visits to Methone, and its closest approach was made on May 20, 201...
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### Author:  [@Gagundathar](https://boards.straightdope.com/u/Gagundathar)
#### Post date: [October 23, 2013, 6:13pm UTC](https://boards.straightdope.com/t/what-is-the-smallest-natural-sphere-in-our-solar-system/671945/17 "2013-10-23T18:13:32Z")
Too bad we don’t have a better picture of Polydueces, but it is smaller at 1.3 ± 0.4 km.
I didn’t really realize how many moons were in the Saturnian system.
> **[Polydeuces (moon)](https://en.wikipedia.org/wiki/Polydeuces_(moon))**
>
> Polydeuces /ˌpɒlɪˈdjuːsiːz/, also designated Saturn XXXIV, is a small trojan moon of Saturn occupying the trailing L5 Lagrange point of Dione. It was discovered by the Cassini Imaging Science Team in images taken by the Cassini space probe on 21 October 2004. With a mean diameter of about 3 km (1.9 mi), Polydeuces is thought to have a smooth surface coated with fine, icy particles accumulated from the cryovolcanic plumes of Enceladus. In its orbit around Saturn, Polydeuces periodically drifts a...
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### Author:  [@dtilque](https://boards.straightdope.com/u/dtilque)
#### Post date: [October 24, 2013, 4:03am UTC](https://boards.straightdope.com/t/what-is-the-smallest-natural-sphere-in-our-solar-system/671945/18 "2013-10-24T04:03:54Z")
> [@DrDeth](#):
>
> Some searching on wiki seems to show the moon Methone as the smallest that _may_ be in hydrostatic equalibrium.
> [Methone (moon) - Wikipedia](http://en.wikipedia.org/wiki/Methone_(moon))
Only about a mile across. Normally such a small object would not be in equilibrium, but if its interior has been melted by tidal heating, then of course it would be.
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### Author:  [@md2000](https://boards.straightdope.com/u/md2000)
#### Post date: [October 24, 2013, 5:54am UTC](https://boards.straightdope.com/t/what-is-the-smallest-natural-sphere-in-our-solar-system/671945/19 "2013-10-24T05:54:24Z")
As we can see by comparing glaciers to mountains (one moves, the other does not) It would seem there are two different answers depending on the composition. Ice would have a substantially lower mass of spherocity. If we add gas giants into the mix, there’s another critical mass, at which a ball of gas will coalesce into a sphere rather than dissipate into space.
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### Author:  [@Lumpy](https://boards.straightdope.com/u/Lumpy)
#### Post date: [October 24, 2013, 12:06pm UTC](https://boards.straightdope.com/t/what-is-the-smallest-natural-sphere-in-our-solar-system/671945/20 "2013-10-24T12:06:17Z")
> [@Gagundathar](#):
>
> I didn’t really realize how many moons were in the Saturnian system.
I think we’re going to have to dump the idea of planets having a discrete number of moons, and simply accept that many planets are surrounded by rubble of all sizes.
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