Thanks for the feedback! The art assets shouldn't be that high on my to-do list, but I still end up spending a lot of time pushing sprites. Luckily I've managed to work on the gameplay too.
I added multiple spawning enemies, variations on the boulders, power ups and a simple GUI element. I did learn something too; irandom_range and random_range are slightly different functions, and irandom_range(-1,1) also includes the value 0. Invisible boulders were a bit too hardcore obstacles.
It actually starts to feel like a game. I guess it's already more complex than Asteroids...
I'm really liking GameMaker. So far, if I have and idea, I've managed to test it and get it working in under an hour. With Unreal it would take days (mostly because of bad and/or obsolete documentation).
Next, respawing and lives. More enemies. More guns. A shield? Maybe. Cheers mates, have a good one.