I've noticed that you can never do 0 damage - the lowest is 1 damage per attack. I was wondering why this was in referring to planet invasion. You can have over 20000 defense and attackers will still do 1 damage per attack no matter what their attack rating is. I really think that with very high defense, 0 damage should be dealt, instead of always allowing 1 damage per attack.

Well, I've seen zero damage when the planet attack was zero, thats about it. Those asteroids with 10 attack do 1 damage. It's a rounding thing based on the combat math.