As a static enemy won't scare anyone, let's make enemies move.
With the knowledge you have at the moment, you would suggest creating
spaceship_mc class and using an enter frame listener to update each enemy position. Most scripts relies on
Event.ENTER_FRAME event is simultaneously dispatched to all DisplayObjects listening for it, so you don't have to synchronize animations.
That's true, but with many DisplayObjects to be updated, this technique although being the most correct from a programming point of view can dramatically increase the work of the Flash player.
In this game we won't deal with such an huge number of moving DisplayObjects to represent a risk for your CPU, anyway it's time to learn something new.
Add this new class ...