At the moment, there is no incentive to fire a larger projectile at a planet. I'm sure that will change and justify the larger investment. Any impact kills all units on both bodies. I suspect this will change going forward. For example - a 3 Hally asteroid might only put a hole in a larger planet while a 25 Halley one will destroy it outright.
Does anyone know how to make a moon that only requires 5 halley's? (for balance reasons) I can only ever get them to require 3, or 25 regardless of how much I tweak the radius of the moon.
In one game I had 4500 metal income from one planet. That would be one Halley every 13th second. 25 Halleys' would be like 335 seconds or 6 and a half minute.
I can't remember. It's since been edited to 3. Anything over 240 radius or something like that. Just up the mass and radius, similar to the default planets.