Very neat graphically, the super-slow dust effect when you hit an asteroid is especially adorable.
As for the rest - it feels a little sterile, small things like minor animation or just a sine movement on the astronaut would liven it up a lot. Also I missed some sort of interaction between bullets and aliens greatly. The slow-motion particles show a great example here, I think... you could have something explode on the aliens and then they could slowly drift out into space while rotating, or something like that. There's great potential here.
But overall, one of the better entries, keep it up.