But the old "They'd never get through the Van Allen Belts alive" stuff is complete hooey. The belts are extremely attenuated; the astronauts are not in them for very long; and they actively fly a path through the thinnest parts. They also shield the capsules as best they can, based on a very short flight duration.
Similarly cosmic rays: short duration, low risk. Longer future missions: potentially big problems (as I see I wrote about on this thread quite a while back); but water itself is a great shield, so you store water around the outer walls of the vessel and that helps a lot. When you get to the Moon or Mars, you don't build shiny white science fiction habitation structures on the surface; you go underground, ideally using already existing lava tubes, because you want as much insulating dirt between you and the cosmic rays as possible. Sadly, the reality will be so much duller than the thrilling space art we've been producing for a century and more.
Same with temperature extremes: yes, they're huge in and out of the Sun (and that's why spaceships roll slowly to even out the exposure), but space is almost a complete vacuum so individual molecules have almost no effect. Think of sticking your hand into an oven at 250C. It's comfortable (for a while) because the air is so thin (but still enormously denser than space). Touch the metal and you get a very nasty burn because it's so dense and so contains vastly more heat energy.
So space deniers, go do one! (The very polite version.)
