I'm still not very good at it.

Collision detection now works properly, but it still uses square hitboxes because I honestly just can't be bothered with per-pixel collision detection. Hitboxes are Good Enough™. The bonus enemy has been shrunk down to 8x8, and I've added a new red asteroid to complement the green one:

I now have plenty of time left for polishing, testing, and bug-fixing, before releasing it later tonight.