NASA's Vision for Space Exploration calls for a return to the Moon as preparation for even longer journeys to Mars and beyond. But there's a potential showstopper: radiation.

Space beyond low-Earth orbit is awash with intense radiation from the Sun and from deep galactic sources such as supernovas. Astronauts en route to the Moon and Mars are going to be exposed to this radiation, increasing their risk of getting cancer and other maladies. Finding a good shield is important.

The most common way to deal with radiation is simply to physically block it, as the thick concrete around a nuclear reactor does. But making spaceships from concrete is not an option. (Interestingly, it might be possible to build a moonbase from a concrete mixture of moondust and water, if water can be found on the Moon, but that's another story.) NASA scientists are investigating many radiation-blocking materials such as aluminum, advanced plastics and liquid hydrogen. Each has its own advantages and disadvantages.