It's a good start
Your graphic quality and overall presentation are pretty good. The music and the opening come off quite professional.
There's quite a bit of room for improvement, though. You need a better feedback when you destroy an asteroid than just a flashing "Score +10". An explosion would be nice, for example. The game needs some dimension, some variation, such as having asteroids come in at different angles. And why have the user double-click, rather than just clicking on the target? That made the game needlessly awkward, in my opinion.
Also, I'm not sure what caused this bug but partway through my game, the cursor vanished, and I couldn't get it back, so my city just got destroyed. I'd move the mouse around, but it would only appear when it left the flash. Maybe I'm the only one this happened to.
Anyway, overall it's got the makings of a decent game. Keep building on it, and you could really have something.