I didn't notice at first that the moon could protecting us from the asteroids, it's a cool addition.
The gameplay is very simple but works well, I think I would personally prefer if everything was faster, I can easily anticipate the asteroids and destroy them without any tension or difficulty, if there were more enemies on screen it would be more dynamic to me. But the fact there is a little wait before obtaining a new starship is interesting and force the player to be sure when he want to launch them!
The absence of sounds and music is very unfortunate, it would add more life to the game. Still, it was quite fun! :)