When astronauts fly beyond the Earth's magnetosphere, they're exposed to hundreds of times more radiation than they would experience on Earth. The easiest way to protect them is to hide behind a wall of protons; rock or water will do nicely. Researchers are testing hydrogels, which can absorb hundreds of times their weight in water, as a potential radiation barrier. The gels could be 3D printed on Earth and then mixed with water from the Moon to form a radiation shield.
