# Possible criteria or parameters for planetary collision?

1. Aug 4, 2013

### Positron137

Possible "criteria" or parameters for planetary collision?

Suppose a computer model were created to simulate two proto-type planets on a collision course (for example, the Earth-Moon collision in the Hartman theory of the Moon formation). We know that the Moon is spiraling outwards (tidal acceleration). The first question I have, is why do certain moons of planets spiral outwards or inwards (into a main planet)? Second question: is the tendency of these moons to spiral outward or inward dependent on a "criteria: of some sort of the formation of the moon (so if a moon, or moons were created by a proto-planet collision, or by gravitational attraction past the Roche radius)? WOuld there have to be several parameters regarding the collision to determine the behavior of the moon's orbit? And if there are parameters, what would they be? Thanks!

Last edited: Aug 4, 2013
2. Aug 5, 2013

### Staff: Mentor

I think you are mixing two different things here.

- the collision event and moon formation. That happens in (relatively) short timescales.
- orbital changes long afterwards. The moon spirals out for billions of years now.

The orbital radius of a moon increases if the orbital period is longer than a day of the planet, and if the planet shows significant tides due to this moon.

3. Aug 5, 2013

### Positron137

Ah ok. I understand. So I guess the proper thing to ask is, after the collision though, what determines whether the moon will tidally accelerate (orbit outward) or decelerate (spiral inward) after billions of years?

4. Aug 5, 2013

### Staff: Mentor

If there are no other objects influencing this:
The periods of one moon orbit compared to the rotation period of the planet determines the sign (neglecting the rotation of the moon here), the size of tides on the planet (and moon) determines the magnitude.

5. Aug 5, 2013

### Positron137

Ok. What do mean by "sign" exactly?

6. Aug 5, 2013

### Staff: Mentor

Outwards (like our moon: one orbit takes longer than a day) or inwards (like Phobos at Mars).

7. Aug 5, 2013

### Positron137

Ah ok. So is it "positive" if it orbits outwards, and "negative" if it orbits inwards?

8. Aug 5, 2013

### Staff: Mentor

That depends on your arbitrary definition of "positive" and "negative".

9. Aug 5, 2013

### Positron137

I see. Alright.