I am under the impression the moon takes out all units on the planet it hits? I was on the other side of the planet when I smashed a moon into the AI's base and it was a draw. If so, after I lanch a moon at a planet can I send in a construction bot to re build on it after it's destroyed everything?
if the planet is big enough (scale 3+ I believe) the moon wont destroy everything on the planet. Smaller planets may be wiped completely. I suppose the size of the impacting moon also makes a difference on how big the area of effect of the impact is.