At first I didnt understood what the point of the game is, or what I was supossed to do.
And when I realized it, the game gets very hard. The alien falls down with almost anything, and then he falls all the way to the bottom, so you have to return there to try again; the UFO moves way too slow, which also makes the alien falling more frustrating.
But the concept is great, it only need better application :)
PD: I don't see how you applied the limitation