Efficient in its gameplay,
maybe you could add some objectives to reach in some ways,
I like the sound spatialization in asteroids explosions (you may pan it a bit less so both ears can share a bit of the sound for less odd feeling), and music is one sided.
I like the limited Ammo, but maybe you could add a little cooldown to shoot here we can loose thoses ammo so fast lol
You could add differents randoms sprites for the asteroids more fun like a satellite, a ship, a cow (rarer just for fun)... Anything you know :p
Overall good game.