When on the Earth, atmosphere and magnetosphere reliably protect people from the solar cosmic rays ruinous for all living beings. Out in the interplanetary space cosmonauts are vulnerable, unless special protection measures are involved. To fully shield human beings from space radiation the spaceship body should be about three meters thick, but the engine able to launch such a huge spaceship has not been invented yet. Certainly, the cosmonauts are shielded both by the environmental suits and by the spaceship body, but the protection is not sufficient. So, what can be the solution to the problem?

NASA are working on it too:

"On a trip to Mars and back, without the appropriate shielding, about every cell in the body would be traversed by one of these heavier articles."

Radiation Hazards to Crews of Interplanetary Missions: Biological Issues and Research Strategies (1997)

Space Daily