Really like the sense of depth by having the planet in the background! Though the game was pretty quiet. It would be nice to have some sound effects or background music added in to fill out the environment. I would also suggest adding health bars on the different things. I spent a lot of time shooting the little space boxes but never knew how close they were to being destroyed as they were floating away.
And maybe it was just me, but I didn't see a way to restart when I game overed besides refreshing? Adding a button for that would be helpful (unless I just missed it, in which case, my bad :P)