What are cosmic rays and where do they come from?

The answer

Cosmic rays are high-energy particles (mostly protons) that travel through space at nearly the speed of light. Low-energy cosmic rays come from our Sun, while higher-energy ones originate from supernova remnants within our galaxy. The most extreme cosmic rays - ultra-high-energy cosmic rays with energies exceeding 10²⁰ eV - likely come from active galactic nuclei or other exotic sources outside our galaxy. About 10,000 cosmic ray particles hit every square meter of Earth's atmosphere per second, where they create cascading showers of secondary particles. They pose a significant radiation hazard for astronauts on deep-space missions.