Scientists are still searching for a good solution. The trick is to provide adequate shielding without adding lots of extra weight to the spacecraft. Some lightweight radiation-shielding materials are currently being tested in an experiment called MISSE onboard the International Space Station. But these alone won't be enough.
The real bad guy is Galactic Cosmic Radiation (GCR) produced in distant supernova explosions. It consists, in part, of very heavy positive ions - such as iron nuclei - zipping along at great speed. The combination of high mass and high speed makes these little atomic "cannon balls" very destructive. When they pierce through the cells in people's bodies, they can smash apart DNA, leading to illness and even cancer."It turns out that the worst materials you can use for shielding against GCR are metals," Bushnell notes. When a galactic comic ray hits a metallic atom, it can shatter the atom's nucleus - a process akin to the fission that occurs in nuclear power plants. The secondary radiation produced by these collisions can be worse than the GCR that the metal was meant to shield.