<p><a href="https://astronomy.stackexchange.com/a/34853/7982">This answer</a> to <em>Are Saturn's rings stable?</em> begins with:</p>

<blockquote>
  <p>Most of Saturn's rings are inside it's <a href="https://en.wikipedia.org/wiki/Roche_limit" rel="noreferrer">Roche limit</a>, which means they will never clump together. Tidal forces prevent this from happening.</p>
</blockquote>

<p>I always confuse Roche limit and <a href="https://en.wikipedia.org/wiki/Hill_sphere" rel="noreferrer">Hill sphere</a>, so I thought I'd ask a question about both.</p>

<p><strong>Question:</strong> Under what conditions could a solar system body orbiting the Sun have a Roche limit larger than its Hill sphere?</p>

CC BY-SA 4.0
2020-01-28T08:49:10.567Z
Another word for Hill sphere is Roche sphere, so . . . some confusion is understandable.    https://en.wikipedia.org/wiki/Hill_sphere
userLTK
CC BY-SA 4.0
2020-01-28T09:19:44.933Z
@userLTK that may be the original source of my trouble. It's possible that I learned it that way long ago.
uhoh
CC BY-SA 4.0
2020-01-29T00:22:37.287Z
@userLTK yes now I remember, "Roche lobe" (mentioned in [this answer](https://astronomy.stackexchange.com/a/34866/7982)) is exactly the term I learned to use for what we also call the Hill sphere.
uhoh
CC BY-SA 4.0
2020-01-29T03:01:13.873Z
As a sidebar, Phobos is pretty close to qualifying as a real example of this.   A calculation of Phobos' hill sphere puts it about 16 km from Phobos center, less than 1 radius away from it's surface.  Of-course, an orbit in those conditions becomes close to impossible, so the Roche limit isn't all that relevant.
userLTK
<planetary-science><roche-limit><hill-sphere>
When could the Roche limit be larger than the Hill sphere?
uhoh
# When could the Roche limit be larger than the Hill sphere?

(Asked by: username_0 on 2020-01-28)

[This answer](https://astronomy.stackexchange.com/a/34853/7982) to *Are Saturn's rings stable?* begins with:

> Most of Saturn's rings are inside it's [Roche limit](https://en.wikipedia.org/wiki/Roche_limit), which means they will never clump together. Tidal forces prevent this from happening.

I always confuse Roche limit and [Hill sphere](https://en.wikipedia.org/wiki/Hill_sphere), so I thought I'd ask a question about both.

**Question:** Under what conditions could a solar system body orbiting the Sun have a Roche limit larger than its Hill sphere?

**Tags:** planetary-science, roche-limit, hill-sphere

> **Comments:**
> **username_1**: Another word for Hill sphere is Roche sphere, so . . . some confusion is understandable.    https://en.wikipedia.org/wiki/Hill_sphere  
> **username_0**: @username_1 that may be the original source of my trouble. It's possible that I learned it that way long ago.  
> **username_0**: @username_1 yes now I remember, "Roche lobe" (mentioned in [this answer](https://astronomy.stackexchange.com/a/34866/7982)) is exactly the term I learned to use for what we also call the Hill sphere.  
> **username_1**: As a sidebar, Phobos is pretty close to qualifying as a real example of this.   A calculation of Phobos' hill sphere puts it about 16 km from Phobos center, less than 1 radius away from it's surface.  Of-course, an orbit in those conditions becomes close to impossible, so the Roche limit isn't all that relevant.  

---

## 0 answers

