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.
You understate the scale of the problem.
“An astronaut on a mission to Mars could receive radiation doses up to 700 times higher than on our planet – a major showstopper for the safe exploration of our Solar System. ...
“Earth’s magnetic field and atmosphere protect us from the constant bombardment of galactic cosmic rays – energetic particles that travel at close to the speed of light and penetrate the human body. ...
SCIENCE & EXPLORATION
The radiation showstopper for Mars exploration
31/05/2019
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ESA / Science & Exploration / Human and Robotic Exploration
An astronaut on a mission to Mars could receive radiation doses up to 700 times higher than on our planet – a major showstopper for the safe exploration of our Solar System. A team of European experts is working with ESA to protect the health of future crews on their way to the Moon and beyond.
Earth’s magnetic field and atmosphere protect us from the constant bombardment of galactic cosmic rays – energetic particles that travel at close to the speed of light and penetrate the human body.
“Cosmic radiation could increase cancer risks during long duration missions. Damage to the human body extends to the brain, heart and the central nervous system and sets the stage for degenerative diseases. ...
““One day in space is equivalent to the radiation received on Earth for a whole year,” explains physicist Marco Durante, who studies cosmic radiation on Earth. ...
““The real problem is the large uncertainty surrounding the risks. We don’t understand space radiation very well and the long-lasting effects are unknown,” explains Marco...
“Recent data from ExoMars Trace Gas Orbiter showed that on a six-month journey to the Red Planet an astronaut could be exposed to at least 60% of the total radiation dose limit recommended for their entire career.
““As it stands today, we can’t go to Mars due to radiation. It would be impossible to meet acceptable dose limits,” reminds Marco.”