Nothing.
You have only ONE life to live. I prefer not to live mine in fear or let other's fears rule my life.
The issue is that you're combining your preferred action based result ("I'm not afraid from doing X") with the actual ethics of the matter, which is nearly an entirely different matter. For example, whether you're afraid of attacking an enemy in war or not is only one part of the examination of the situation of you attacking an opponent. Similarly, whether you're "afraid of going to mars" or not is not examining the whole mission in question here. Man, your average college undergrad psych experiment needs ethics approval, meanwhile many posters here can't see any extraordinary questions about a never-undertaken one way trip to another planet compared to crossing a street. This reveals not my "fear", but rather the glancing superficiality with which our fellow posters are thinking about the matter at hand.