Radiation Dangers to Astronauts
Astronauts are exposed to many different types of dangerous radiation in space. Space agencies, like NASA, must carefully monitor the radiation exposure of astronauts to make sure they remain safe and healthy.
Earth's atmosphere and magnetic field both serve as radiation shields for those of us who are on Earth's surface. Most piloted space missions (ones with astronauts aboard) "fly" in Low Earth Orbit (LEO), slightly above Earth's atmosphere. Astronauts in LEO are still within the protective bubble of Earth's magnetosphere, which deflects many types of particle radiation. Astronauts are outside of the protection of our atmosphere, however, and are thus at greater risk of exposure to high-energy electromagnetic radiation including ultraviolet "light", X-rays, and gamma rays. Even in LEO, astronauts must take precautions to deal with radiation, especially when they are outside on spacewalks or when "solar storms" are brewing.
Trips by astronauts to the Moon, Mars, and asteroids will provide us with bigger challenges protecting astronauts from radiation as they leave the protection of Earth's magnetosphere behind. The Moon offers almost no protection, as it lacks both an atmosphere and a magnetic field. Mars has a very thin atmosphere and a weak, regional magnetic field in some locations. Early Mars bases may be built at low elevation (at the bottom of the "deepest" parts of the atmosphere) locations that are also within a regional magnetic field, in order to take advantage of as much natural radiation shielding as possible.
Astronauts are exposed to many different types of radiation. Particle radiation includes high-speed protons and electrons as well as cosmic rays made from the nuclei of heavier atoms. Much of the particle radiation comes from the Sun and energetic solar storms, but some cosmic rays come from distant supernova explosions. Disturbances of Earth's magnetosphere also generate radiation, including the particles swirling around in Earth's inner and outer radiation belts. Most of the high-energy electromagnetic radiation, including ultraviolet "light", X-rays, and gamma rays, comes from the Sun and solar flares.