[one guy in new jersey]

To leave Earth’s protective magnetic shield behind is to step into a constantly-running cosmic ray microwave.

Once you’re cooked, you’re cooked.

[Campion]

It's not quite that bad. (Just don't go anywhere close to Jupiter or Saturn.) And the spacecraft provides some shielding. It may be possible to build it to provide more.

For example, you probably have to carry water along. Put the water in a tank surrounding the living area of the spacecraft and you get considerable rad shielding, basically for free.

However, it is entirely true that we have no experience with long-term human habitation outside LEO (and none at all outside earth orbit). A long-term moonbase is probably a logical next step. If someone can spend a year on the moon without major ill effects, the three-year roundtrip to Mars is not unreasonable.