Cosmic rays are fast-moving elementary particles, such as protons, electrons and atomic nuclei, that strike the Earth from outer space. They travel almost at the speed of light. Scientists believe they originate from supernovas, black holes and quasars.
The Earth's atmosphere provides protection from cosmic rays. With no atmosphere, these particles would pass through humans and damage their DNA.
Prolonged exposure leads to radiation poisoning and development of medical conditions such as cancer, brain damage and cataracts. During long space missions, like a manned mission to Mars, it is still necessary to find a way to shield travelers from the adverse effects of cosmic rays.