There's much greater radiation exposure away from Earth's magnetic field. I believe this is still an unsolved problem. Proposed solutions for shielding are heavy and expensive.

You just have to keep the lights on and not die of cancer for six months. The radiation isn't that bad.

Edit: The specific number is under 2 millisievert per day. And "One sievert carries with it a 5.5% chance of eventually developing fatal cancer based on the linear no-threshold model."

I though shielding with water was the way forward there.

In science fiction novels...

It's much harder if you include the return trip.

You have to keep things going for 3 years, but I think such a trip is still pretty straightforward with our current abilities.

But wow what a drawn-out load of nothing a mission like that would be.

No resupply, no spares - no chance.

There's room on board for literal tons of spares.