This is very well done, but also very hard. I can describe it as First 2 Newton's laws of motion: The Game! The graphics are well done, the variety of asteroids are especially impressive. And gameplay is super solid, easy to understand, but hard to master. The music is suitably chosen and i didn't find sfx grading, surprisingly it's not annoying, despite all you do is fire thrusters (including rotational ones) so sfx could have been annoying easily, but they are not in my opinion. But perhaps it's down to audio situation of specific players.
Nice idea with mission starting with 57 and then incrementing.
Parallax background would be welcomed indeed. And I also like the idea (by @antti-haavikko) of structuring game with missions to have a more graduate difficulty curve.
The strong force applied to the rocket after the impact feels a bit strong and for me it often lead to more collisions and fast demise. Also I sometimes forget which way rocket is facing and it's hard to tell at a glance. While nose is rounder and it has thrusters (but they are dark on dark background) i think something like a colored stripe near nose (or back) of the rocket would enhance reliability.
I enjoyed the game it's fun and have a nice audio visual appeal, but difficulty is a bit high I personally feel. But still fun, I feel i will replay it from time to time, it has good vibes from flash era of games!