To quote this page -
"""Manned missions to planets such as Mars require extended missions that will expose astronauts to harmful radiation in the form of energetic particles from solar and galatic sources. Traditional methods for protecting spacecraft and occupants from these forms of radiation involve some configuration of a massive material shield to absorb the energy of incoming particles. For the high energy galactic cosmic rays (GCRs) that astronauts will be exposed to, these so-called passive shields are too massive to be practical and will likely produce showers of secondary radiation that could be more harmful than the GCRs themselves."""
https://engineering.dartmouth.edu/~d76205x/research/Shieldin...
It's concerning that so many people that are interested in space exploration are unaware of this.
The primary enabler for actual space exploration is most likely superconducting magnetic fields but this is an unsolved problem, though the European Space Agency did complete some work in 2016 towards it by reusing technology from CERN
https://home.cern/about/updates/2015/08/superconducting-shie...
It's unfortunate that propulsion is where everyone seems to focus their attention when the shielding is so much more important to human settlement.