Fun game. Reminds me a lot of Asteroids.
The mechanic is really interesting, but it did feel like the alien to asteroid ratio wasn't always right, and there were times where I was shot without any asteroids on the screen to use. This might just be a skill issue on my part, but if you were to work on this more, maybe if the asteroids are hit by a laser, they could break into smaller asteroids which would even out the ratio a bit? This could give the player more options for how to use the asteroids, since there would be more incentive to use them like shields.