Mars, as a certain pop star once put it, isn't the kind of place where you'd want to raise your kids. Nor is it the kind of place anybody is ever going to visit, as some of the NASA scientists know perfectly well. Even leaving aside the cold, the lack of atmosphere and the absence of water, there's the deadly radiation. If the average person on Earth absorbs about 350 millirems of radiation every year, an astronaut traveling to Mars would absorb about 130,000 millirems of a particularly virulent form of radiation that would probably destroy every cell in his...