Cosmic rays are incredibly fast particles, mostly protons, that zoom through space and constantly bombard Earth from all directions. Despite their name, they're not rays of light at all: they're tiny bits of matter traveling at nearly the speed of light, packing enormous energy from their journey across the universe.

These particles come from violent events in space: exploding stars called supernovas, spinning neutron stars, and even distant galaxies. When cosmic rays slam into Earth's atmosphere, they smash into air molecules and create showers of other particles that rain down toward the surface. Our atmosphere acts like a shield, protecting us from most of this radiation, though a few particles make it all the way down.

Scientists discovered cosmic rays over a century ago when they noticed that electroscopes (devices that measure electrical charge) would discharge even inside sealed containers. Something from outside was getting in. This led to the discovery that Earth is constantly being hit by these high-speed space travelers. Today, cosmic rays help scientists study everything from the Sun's activity to the structure of pyramids (by detecting how many cosmic rays pass through the stone). Astronauts in space have to worry about cosmic rays more than we do on Earth, since they don't have our thick atmosphere to protect them from as much radiation.