"Material shielding may be partially effective against galactic cosmic rays in certain energy ranges, but may actually make the problem worse for some of the higher energy rays, because more shielding causes an increased amount of secondary radiation. The aluminum walls of the ISS, for example, are believed to have a net beneficial effect. In interplanetary space, however, it is believed that aluminum shielding would have a negative net effect.
Several strategies are being studied for ameliorating the effects of this radiation hazard for planned human interplanetary spaceflight:
* Spacecraft can be constructed out of hydrogen-rich plastics, rather than aluminum.
* Material shielding has been considered. Liquid hydrogen, which would be brought along as fuel in any case, tends to give relatively good shielding, while producing relatively low levels of secondary radiation. Therefore, the fuel could be placed so as to act as a form of shielding around the crew. Water, which is necessary to sustain life, could also contribute to shielding.
* Electromagnetic fields may also be a possibility.
None of these strategies currently provides a method of protection that would be known to be sufficient, while using known engineering principles and conforming to likely limitations on the mass of the payload."