To quote NASA associate administrator Jim Reuter, sending crewed missions to Mars by 2040 is an "audacious goal." The challenges include the distance involved, which can take up to six months to traverse using conventional propulsion methods. Then there's the hazard posed by radiation, which includes increased exposure to solar particles, flares, and galactic cosmic rays (GCRs). And then there's the time the crews will spend in microgravity during transits, which can take a serious toll on human health, physiology, and psychology.