Time travelling isn't possible.
1) You can't time travel into the future. It hasn't happened yet.
2) You can't time travel back into the past, since it has already happened. Can't happen again. Time says so.
3) If you WERE able to time travel, then figure this one out:
You time travel like, a minute back into the past. You see yourself, standing there, preparing to time travel. So, what would happen if you run up to yourself, and stopped yourself from doing it? What would happen to you? If you stopped yourself from time travelling, how could you time travel to stop yourself? Confusing, yes I know.
Time travelling is fun, but only when it comes to Heroes or Back to the Future.
Aliens exist = True
That's all I gotta say about that.