Played it, that's a bit of fun, just tried the first two aliens. What other variation is there besides alien size? If this is for touch, how about launching the missiles with an up swipe and the missiles speed are controlled by the speed of the swipe?
I suggest also adding a more interesting 'kill alien' animation (explosion? alien shrink? missiles drill in? etc.)
Make first few aliens very easy so player progresses quickly, maybe just 5 hits at first? Don't want to frustrate the player that they cannot get past the first level (unless you are the arcade game Donkey Kong!)