Cosmic rays hurt astronauts because the particles can kill their cells. This can kill the astronauts. Too much exposure to cosmic rays can cause leukemia and brain cancer. When cosmic rays hit a person, their DNA can be changed. Cosmic rays will have a larger effect on astronauts on missions to Mars than on people on earth. For example, they well have a 20% to 40% higher rate of cancer.

The astronauts that went to the Moon had protection against cosmic rays and the astronauts that will go to Mars will need more protection. Even if they are a health problem in space they are not problem on earth because the atmosphere protects the earth from a lot of particles. They react with atoms in the atmosphere; they change into muons and pions when they hit the atmosphere. They lose a lot of energy and that means they will do less harm to people. Even though the atmosphere protects us, if we fly over the North Pole, we are exposed to the equivalent in radiation to 10 chest X-rays. Cosmic rays also cause the Aurora Borealis and can cause computer errors.