[citation][nom]animal_chin[/nom]One thing that the article failed to mention was that during a trip to Mars the astronauts are bombarded with space radiation from galactic cosmic rays. These astronauts only have a few centimeters of aluminum protecting themselves, which is not enough to stop the rays. So having a shorter flight greatly reduces their chance of getting cancer, which is a huge deal for anyone going into space.[/citation]
A few centimeters of aluminum is all you need to stop the most damaging "cosmic rays". "Cosmic rays" are high energy protons (easily stopped by a sheet of paper), high energy electrons (4 to 5 sheets of paper will stop them), neutrons (high speed neutrons are less of a worry than low speed as they will likely pass through something with little damage compared to low speed neutrons, light materials here can also be used as shielding like parafin wax and even water), and gamma/x-rays (denser materials are need here, but you don't need a large amount). You're bigger problem is space flight is ionization of the hull from the high energy protons and electrons which can wreak havoc with electronics and the like.