I think that the best tutorial is experience, creating a level where the player can get used to the controls and the physics and then testing their knowledge.
And i do completely agree with the backgrounds i think it's cool and i really tried a lot of things with the asteroids/planets so they'd look like they we're on the background and we're not collidable, i think i failed in that department.
Thank you for playing and giving feedback. :)
-Giffi