Sorry for the late feedback - I almost missed this one!
The graphics are pretty cool i have to say - the way you have done the asteroids themselves is impressive. I loved asteroids as a kid, I like that your adding new elements to the gameplay.
I know its a wip, so i will just comment on what is there now.
Not sure about other players but I found it too easy - you have given players the shield (which i think is pretty cool actually) so i think you should remove life all together - 1 hit = death, if not make it less hits to be killed.
The ship size was a little uncomfortably big for the screen i felt - also, because of the ship size players are encouraged and rewarded with defensive shooting. (ie - my ship is too big to get out of the road in time - my only option is to destroy the object coming at me).
Dont get me wrong - its cool if you want to leave it that way. Though i think this style of game really shines (and really captures your attention) when the player is forced to move while shooting.
So my vote is; make the ship smaller while keeping the asteroids large.
Reduce that break effect when you hold down - in a strange way the fact that the ship was so hard to control actually made it much more fun. (This is probably because if the ships movement is too controlled it becomes easy again - and therefore not paticularly engaging).
As it is right now though - its still fun. Well done :)