I really like the art style and theme of the game as a whole. The concept is simple but exciting. I think one area for improvement is the difficulty scaling. The rate of increase in difficulty is pretty one-dimensional - as more time passes, the frequency of asteroids increases. It'd be great to see a wider scope of obstacles - maybe new kinds of asteroids or some events that can trigger changes in the change rates of variables like movement speed, your radius, asteroid trajectory, etc.
I understand your desire to keep a simple game simple, and it totally works as it is, but for this to be a game people want to come back to and play, i think adding multi-dimensional changes in play over the play duration of play would help add some excitement and allow players to wonder what might happen next, without changing the core 'one-button' mechanic. Really fun, clean game - well done