Fun way of using the theme!
First, I did not understand what I had to do, so I ended up with a 0 score, then something like 18-20 (I was trying to hit the "equal" button when I died so I restarted the game before seeing my last scode). It went pretty hard in a few step after being quite easy on the first 10-15 aliens.
I agree with the other comments, the visual should make the aliens and space most of the screen instead of being in a corner of it, I almost never watched the aliens and beam. Also, maybe using the countdown as a score instead of only aliens count, so the faster you are, the more you earn points.