The controls need work, improve the graphics, add more content, and add sound. You should always have a play tester before submitting a game.

I'm getting the impression that you're relatively new to this, which is fine. So I hope you read these reviews to learn a few things about game design that will help you to grow and flourish as a game designer.
First) Test your game! There are a lot of bugs in this game that suggest that it wasn't thoroughly tested before release. For example, my health dropped into the negative values, and my game did not end. Unless you intended for this to game to last for all of eternity, this would be a rather glaring bug, that should have been caught before the game was exposed to the public.
2) Have other people test your controls: The idea of having the movement of the gun based on the distance from the mouse to the gun seems like a good idea at first (and a lot of games use this very well), because it allows the player to control the speed that the gun moves by "stretching" farther away from it. HOWEVER, this only works if you code your program to recognize mouse positions outside of the Flash itself (which is a bit more difficult). If your flash can only see the mouse within its borders, then the gun will never be able to reach the edges (in fact, it loses about 20% of the screen on either side, meaning that a good 2/5 of the playing area cannot be reached by the gun. And it seems the comets know this, because in the later levels they were almost exclusively sticking to the edges. If you had had more people testing this game, that would have very quickly become apparent.
C) Balance testing (notice the theme? Lots of testing!). In the early levels, you are forced to put your minerals into speed to have any hope of hitting the comets as they come in (though this would probably be easier if the controls didn't inhibit the movement of the gun so badly). Once you survive a few levels, because all of your minerals went into speed, your gun is too weak, and it takes so many shots to destroy each comet that you barely get any minerals anymore, and at that point it's just a rapid descent to death... oh wait, negative health doesn't kill you, so it's just a descent to feeling like a loser.
Long story short (too late), there are definitely some things you can learn from this game. Some of them are things you could fix in the game and re-release it to see if people enjoy it more. Others are points you can learn from as you design and develop your next game. Best wishes to you in your future endeavours.

Your mouse controls, to put it frankly, suck. Gun movement doesn't line up with the mouse properly. Moving the mouse all the way to the edge of the screen doesn't make your gun move there.

Additionally, being thrown right into the game with no instructions is a big flaw. It's basic game design that you include proper instructions on how to play your game.

Also, no music?

fun game but the gun moves to slow..

your basic space invaders style game not great graphics difficult to upgrade.