This is just the kind of game pixel art needs: simple mechanics, a catchy soundtrack, an exotic setting, a high score, and secrets. Two constructive criticisms: 1) Warn the player before entering the shuttle so he doesn't accidentally end the game. 2) Change the colour of the aliens to something easier to spot (unless, of course, they are meant to be a bit tricky to see). Aside from that, I have nothing to add :)