I think the moon gravity is a bit much. It would be cool to use the gravity of the moon in your attack, but I think that would require it moving slower and perhaps some prediction logic where you can see where the moon will be if you fire now?

I've built an iOS game (https://eveofimpact.com) years ago (an earth vs asteroids kind of thing) and I ran into similar problems where gravity was too strong, and trying to use gravity to your advantage was very tricky and unpredictable making it less fun. I eventually decided to just make it play a more "decorative" role.

edit: Forgot to say awesome job :)