While definitely an enjoyable game the first time, I would have liked to see a more complex physics engine, ie: the enemies taking damage like the player does from colliding into asteroids, and maybe even the asteroids blocking enemies shots?
Also, the dog-fighting aspect of the game is much more enjoyable than the asteroid sweeper aspect.
I found myself seeking out the enemies at the beginning of a level and enjoying the combat with them and then not really enjoying the hordes of asteroids I was left with afterward.
Perhaps the number of enemies could be approximately half the number of current asteroids, and the number of asteroids could be about as many enemies there are now, scaled to each level of course?
I believe those numbers would be much more enjoyable, and present a challenge not too outrageous.