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!