Wow - very polished looking game - the graphics look like a real arcade game. Right from the view in Itch.io to the game itself.
Not a big deal, but given how beautiful the game is, one place that was just a little inconsistent was the use of the alert box - it didn't have the same look. Not a big deal, but if you wanted, you could create your own Div overlay and then style it.
It would be nice to have a little feedback on the screen when you hit a UFO. With the star, for instance, you have a little sound. You could do that for the UFO too and it wouldn't hald gameplay.
The movement was smooth and the speed of movement seemed just right.