During space segments of a mission and also on planet surface (like Mars) astronauts might not be adequately protected from radiation.
So far the only human mission outside low Earrth orbit have been Apollo missions to the moon. Radiation shields on the vessel might not be feasible. Further on planet like Mars there is not a magnetic field that protect us like Earth's Van Allen radiation belt. There is no sufficioent prediction on the amount of radiation to which astronaut will be exposed
Some reasonable measures can be put in place. Water can be used as a shield on the vessel. On Mars surface astronatus could live in an under-ground habitat and use terrain itself as a shield Some estimation shows radiation to be at an acceptable level.