I tell my engineering friend to drop his insanely challenging dream of being an astronaut
Why would you discourage your friend from following his dream to be an astronaut.
I think he could spend time doing something better, it is nearly impossible.
It's unfair for you to tell others to give up on their dreams because there is a reason why you would set your goals very high. It's his right to dream big. By discouraging him, others will think that you are probably just jealous of him and wants him to fail.
You worry too much and I was only joking.
If it is his dream maybe you can tell him how people become astronauts. They should be in the air force and need to be able to pass a lot of g-force testing, things like that.