Re: Radiation
Radiation is far from the only problem. I think provisioning a trip that takes many months with no hope of resupply may be a far greater issue. Back in the days of exploration, ships carried some hand tools, nails, rope, canvas, metal parts, and figured they could find food, water and wood for repairs along the way and make/improvise anything they needed. They didn't always come home.
An interplanetary vessel is going to include huge quantities of sophisticated electronics, composite materials, etc, etc, etc. Fixing anything that breaks en route is likely to be a major challenge. And due to the complexity of providing food, air, and water to the astronauts, there is going to be lots that might need fixing. A one or two year trip to Mars is going to be a far more complex task than a quick round trip to the Moon.
Biting the hand that feeds IT
