Its a good start, but it needs work
Not a bad Idea for a game (In fact, it was also not a bad idea when they made it into Asteroids), but it definately needs some work.
Why not add some explosion effects when your/enemy ships die, or provide a two-second cooldown between losing a craft and it respawning. Also, if the enemy crafts wrap around the edge of the screen (leave on the left and come in on the right), then your bullets should do the same thing.
Good luck!