One of them are cosmic rays come from outside the heliosphere and make their way, and they are very energetic and they are hazardous for astronauts. If you are sitting on Earth, we have the Earth's magnetic fields to shield us from energetic particles and they are deflected around. As soon as we leave Earth, send astronauts to moon, to Mars, to beyond, they are not protected. We have to have an idea how large and how intense those cosmic rays are going to be and when and how they are going affect astronauts for example.