Hey, i played your game and is working well! Here is my feedback...
- pretty good graphics, maybe make the stars in the back a bit brighter with movement to add more variety
- tough gameplay!
- could use some powerups, maybe when you destory asteroids they drop some stuff you can use to buy/build upgrades.. i got temporaty homing missiles but not sure how
- needs sound, you can add sound effects easily without needing wave files with zzfx!
- maybe let player bullets go farther, the enemies go across the screen.
- enemies that also fly like the player would be fun to dog fight