Great execution on the theme, complete with music, sound effects, and a totally brutal final boss level.
As has already been mentioned, having the player fall downwards goes against the player's expectation, but I also agree that it adds to the challenge. Should you continue development, you might consider having each asteroid apply light gravitational forces to the player, so that they fly where they expect for the most part, but will still veer off into the abyss if they're completely off the mark.
I nevertheless enjoyed hurling through space and getting eviscerated by lasers.
Well done! :D