ok a few things you shout change
1. i think you should make it so that when the bullet hits the asteroid it explodes or breaks apart or something
2. you should make everything be faster
3. you need to make it so that you cant hold down the space bar and have it shoot just an impenetrable line
4. make enemies/bosses, or at least make different levels
but besides those things its not too bad, and keep workin on it, it has potential