<p>Though it's probably very unlikely in a solar system as old as ours (~5 billion years), I would not put it out of the realm of possibilities. Most of the collisions of this scale and magnitude have already happened - the early days of the formation of the solar system was violent and full of collisions between  protoplanets. </p>

<p>In fact, one hypothesis for the <a href="http://en.wikipedia.org/wiki/Giant_impact_hypothesis">formation of the moon</a> was that it formed from the debris of just such a collision. </p>

<p>Additionally, the <a href="http://en.wikipedia.org/wiki/Uranus#Axial_tilt">axial tilt of Uranus</a> could also be explained by the collision of the planet with a protoplanet billions of years ago.</p>

<p>Is the likelihood zero? No. But I would bet money that collisions between planets and asteroids/comets are much more probable.  </p>

CC BY-SA 3.0
2013-10-04T01:02:39.083Z
2013-10-04T01:14:20.807Z
2013-10-04T01:14:20.807Z
astromax
<p>There is one known pair of moons in the solar system that seemingly swap orbits every couple of years. That's Saturn's <a href="http://en.wikipedia.org/wiki/Epimetheus_%28moon%29#Orbital_relationship_between_Epimetheus_and_Janus" rel="noreferrer">Epimetheus and Janus</a>. Their orbits are so close together that they interact gravitationally every couple of years (when the inner moon catches up to the outer moon), so that the outer moon is slowed down, and the inner moon is accelerated.</p>

<p>Each moon changed direction from prograde to retrograde and vice-versa in two co-orbital horseshoe orbits, never crossing paths as they exchange a bit of momentum when closer but Saturn's gravitational pull prevails and pulls each of these two moons back in the opposite direction where they came from, swapping from lower to higher orbit. It's a bit tricky to explain in words:</p>

<p><img src="https://i.sstatic.net/SdnjI.png" alt="enter image description here"></p>

<p>[From: <a href="http://inspirehep.net/record/818547/plots" rel="noreferrer">The Rotation of Janus and Epimetheus - Tiscareno, Matthew S. et al. Icarus 204 (2009)</a>.]</p>

CC BY-SA 3.0
2013-12-03T23:35:57.980Z
There is NO retrograde movement! The image is misleading if you do not take into account that it represents a rotating frame. Moons interchange orbits while rotating on the same direction!
Envite
CC BY-SA 3.0
2013-12-03T23:38:42.170Z
Please add this to your answer: http://upload.wikimedia.org/wikipedia/commons/9/99/JanusEpimetheus.gif
Envite
CC BY-SA 3.0
2013-10-04T12:36:27.253Z
The [image](http://en.wikipedia.org/wiki/File:Epimetheus-Janus_Orbit.png) on the Wikipedia page goes a long way to explain what's happening here.
ChrisF
CC BY-SA 3.0
2013-10-04T09:13:04.040Z
2013-10-04T16:24:31.463Z
2013-10-04T16:24:31.463Z
Arne
<p>Some planets, such as Mars, have 2 or more moons. Giants, like Jupiter of Saturn have <em>a lot</em> of moons!</p>

<p>How likely the orbit of one crosses another one's orbit? Is it possible for two moons to collide? If yes, what would exactly happen?</p>

<planet><orbit><natural-satellites>
Moon orbits crossing each other
Zoltán Schmidt
# Moon orbits crossing each other

(Asked by: username_0 on 2013-10-03)

Some planets, such as Mars, have 2 or more moons. Giants, like Jupiter of Saturn have *a lot* of moons!

How likely the orbit of one crosses another one's orbit? Is it possible for two moons to collide? If yes, what would exactly happen?

**Tags:** planet, orbit, natural-satellites

---

## 2 answers

### Answer by username_2 on 2013-10-04. Score: 14 | Accepted answer

There is one known pair of moons in the solar system that seemingly swap orbits every couple of years. That's Saturn's [Epimetheus and Janus](http://en.wikipedia.org/wiki/Epimetheus_%28moon%29#Orbital_relationship_between_Epimetheus_and_Janus). Their orbits are so close together that they interact gravitationally every couple of years (when the inner moon catches up to the outer moon), so that the outer moon is slowed down, and the inner moon is accelerated.

Each moon changed direction from prograde to retrograde and vice-versa in two co-orbital horseshoe orbits, never crossing paths as they exchange a bit of momentum when closer but Saturn's gravitational pull prevails and pulls each of these two moons back in the opposite direction where they came from, swapping from lower to higher orbit. It's a bit tricky to explain in words:

![enter image description here](https://i.sstatic.net/SdnjI.png)

[From: [The Rotation of Janus and Epimetheus - Tiscareno, Matthew S. et al. Icarus 204 (2009)](http://inspirehep.net/record/818547/plots).]

> **Comments:**
> **username_4**: The [image](http://en.wikipedia.org/wiki/File:Epimetheus-Janus_Orbit.png) on the Wikipedia page goes a long way to explain what's happening here.  
> **username_3**: There is NO retrograde movement! The image is misleading if you do not take into account that it represents a rotating frame. Moons interchange orbits while rotating on the same direction!  
> **username_3**: Please add this to your answer: http://upload.wikimedia.org/wikipedia/commons/9/99/JanusEpimetheus.gif  

---

### Answer by username_1 on 2013-10-04. Score: 7

Though it's probably very unlikely in a solar system as old as ours (~5 billion years), I would not put it out of the realm of possibilities. Most of the collisions of this scale and magnitude have already happened - the early days of the formation of the solar system was violent and full of collisions between protoplanets.

In fact, one hypothesis for the [formation of the moon](http://en.wikipedia.org/wiki/Giant_impact_hypothesis) was that it formed from the debris of just such a collision.

Additionally, the [axial tilt of Uranus](http://en.wikipedia.org/wiki/Uranus#Axial_tilt) could also be explained by the collision of the planet with a protoplanet billions of years ago.

Is the likelihood zero? No. But I would bet money that collisions between planets and asteroids/comets are much more probable.


---

