An interesting design. The only bad thing about the missile control mechanic is that sometimes it doesn't feel like the player has total control over it. It might be intentional but is a bit frustrating (is the same feeling of Asteroids, but the limits on the screen borders make it worse).
Good adherence to the theme by making the player use just one button. Having some of the enemy ships to trigger powerups was very clever. Maybe using that same button to restart, pause and quit the game will make it even more "only one".
It is an original idea that makes you change the mindset in relation to similar games like Space Invaders and Missile Command because you have more control over the missile, not only its trajectory.
Bug: Sometimes the missile is not shot despite I click. It happens when I try to make quick shots. But might be that it has a cooldown.