> We have no idea how to build a spaceship that can withstand the radiation in space ...
No, that's not true, we know how to build it, but we can't afford it. A fully shielded spacecraft is easy to imagine and draw on a piece of paper, but not so easy to build and launch.
In defence of my comment, we have never tested a human-carrying craft in non-earth-orbit-space, during a solar mass ejection event (or other unexpected radiation event). We may know how to guess at building one, but actually trying it is the only way to know if we're right (i.e. if our model fits the universe.) Maybe I was wrong to say "no idea" - but it's still not a solved, easy problem.
> In defence of my comment, we have never tested a human-carrying craft in non-earth-orbit-space, during a solar mass ejection event ...
That's true, but (a) we know exactly what radiation levels take place during such an event, and (b) we know how to build radiation shields. It's just not practical to launch an adequate shield.
> but actually trying it is the only way to know if we're right ...
No, we have enough experience with radiation to know our model corresponds to reality.
No, that's not true, we know how to build it, but we can't afford it. A fully shielded spacecraft is easy to imagine and draw on a piece of paper, but not so easy to build and launch.