Cosmic rays are a type of radiation that comes from space. Cosmic rays aren't really "rays". They are particles (mostly protons) that have a lot of energy. They have so much energy that they can be dangerous. We are lucky, though. Earth's magnetic field and atmosphere protect us; they block most cosmic rays.
A physicist named Victor Hess discovered cosmic rays in 1912. He won the Nobel Prize in 1936 for his discovery.
There are different kinds of cosmic rays, and they come from different places. Some come from solar flares and other explosions on the Sun. Some cosmic rays come from supernova explosions, black holes, and neutron stars within our own Milky Way galaxy. They are called galactic cosmic rays, and have more energy that the ones that come from the Sun. A third type of cosmic ray has even more energy than galactic cosmic rays. Scientists think they might come from somewhere outside of our galaxy. They are called extragalactic cosmic rays. "Extragalactic" means "outside of our galaxy".
Cosmic rays are made up of tiny particles; pieces of atoms. Most are protons from the nucleus of a hydrogen atom. Some cosmic rays are electrons. Others are from the nucleus of some other kind of atom, like carbon, oxygen, iron, or calcium. All cosmic rays move really, really fast and carry a lot of energy!
Cosmic rays are a kind of particle radiation, so they can be dangerous to people, animals, and machines. Did you know that you are hit with a small amount of radiation from nature every year? Don't worry, though - it is usually such a tiny amount that it doesn't hurt you at all. About one-tenth of the radiation that hits you each year is from cosmic rays. Earth's magnetic field and atmosphere protect us from most cosmic rays. Astronauts in space need to be careful, though, because they get hit by more cosmic rays. Cosmic rays sometimes damage satellites, too.
When cosmic rays hit Earth's atmosphere, they crash into atoms and molecules of gas. That usually makes even more cosmic ray particles! One cosmic ray particle from space with lots of energy can make lots of low-energy cosmic rays in the atmosphere. When this happens, scientists call it an "air shower" of cosmic rays.