Not a bad project so far! Seems like you have a pretty solid concept down, interested in seeing it grow.
Some constructive criticism I would have would be on your art style. I am no artist myself so not like I could do better but the art style seems kind of miss matched. The background with the planets is very detailed and Hi-Rez looking, while the asteroids and ships look kind of plain. If nothing else, just look at touching up the asteroids a bit, maybe to look less like metal.
Also some performance issues I noticed you may want to know about. First, when firing with the left mouse, it seems like it doesn't register every time, sometimes it will do three shot bursts and other times it will continuously fire. I noticed that this firing is effected by when the ship is moving, as in when the ship is moving firing is less consistent with the mouse clicking.
If a enemy is destroyed and I still had launched rockets fired, they seems to bog down the game a noticeable bit, doesn't really effect anything now but may want to look into it as you add more.
Still, good game so far had fun tearing everyone to shreds <img src="smileys/smiley2.gif" border="0" align="middle" />