Atmosphere conversion?

[Emperor's Child]

Why 120? If memory serves, you only need 10 troops to eliminate the turn to turn natural decrease that non-native population will see. In my games I make sure I build 15+ to ensure a small upswing and over time it will be enough. Only for colonies that get isolated by enemy colonies (and their associated ship traffic in system) will I bump the number higher to 30 or so, and that seems to be sufficient.

And don’t forget the trick of building “security troops.” This is a small troop with only a cockpit, but nothing else. This is the cheapest (and fastest) way to keep those folks happy since you can build them almost twice as fast as regular troops, but they won’t do anything if you are invaded.