This game is already a really good proof-of-concept. There are a lot of little touches that you could add to make it even better. Sound is always a nice feature, and some colors would be nice (if you don't mind rewriting some of the code).
We haven't yet even included two of the primary features of the original game: asteroids breaking up into smaller asteroids, and UFOs that can shoot at you.
Now that you know some of the basic game programing strategies maybe you have some ideas on how to go about making those happen.
Good luck!