There is a problem however, and that is that is not so much a puzzle game but more so a game of trial and error with physics. Don't get me wrong, it was a very enjoyable game, but I found myself just having to adjust my aim by the most minuscule amount in order to take out the Alien. It went from fun to frustrating quite quickly, I will cut you some slack however, as it is very hard for most games to escalate the difficulty without being needlessly cheap.
And regarding the art style, I really didn't like it, it certainly doesn't look bad, but rather unpolished. Some of the colors were somewhat grainy and bland, maybe a use of brighter colors could improve this.
The sound was good, I enjoyed the applause when I made the alien explode into pieces.
I liked how you say "fail" if I hit the Human, and "epic fail :)" if I hit the Alien then the Human, it would normally make me mad when that happened, but that little comment put a smile on my face.
Overall it was very good, physics games cannot be easy to make, and I like the amount of creativity you put in it, the problem is that before long it becomes somewhat redundant.
Anyway, keep it up, and congrats on the front page!