Despite the name, cosmic rays are not rays of light. They are high-energy particles, mostly atomic nuclei, that travel through space at nearly the speed of light and constantly hit Earth’s atmosphere.
How cosmic rays were discovered
In 1912, Austrian physicist Victor Hess carried instruments up in a balloon and found that radiation increased with altitude. It had to come from space. He received the Nobel Prize in Physics in 1936.
What they are made of
Primary cosmic rays are about 90 percent protons (hydrogen nuclei), about 9 percent helium nuclei, and a small share of heavier nuclei and electrons.
Types by origin and energy
Solar energetic particles. Released by the Sun during solar flares and coronal mass ejections. They have relatively low energies but can arrive in large numbers, see solar flares.
Galactic cosmic rays. Most cosmic rays come from inside the Milky Way. Supernova shock waves are thought to accelerate many of them, see exploding stars. Our galaxy’s magnetic fields scramble their paths, so most cannot be traced back to a source.
Ultra-high-energy cosmic rays. Rare particles with energies millions of times higher than anything human-made accelerators can produce. The “Oh-My-God particle” detected in 1991 carried roughly the energy of a fast-thrown baseball in a single subatomic particle. These probably come from outside our galaxy, possibly from active galaxies, see quasars and active galaxies, but their sources are still debated.
Air showers
When a primary cosmic ray hits a molecule high in the atmosphere, it creates a cascade of secondary particles, an air shower, that can spread over kilometers. Observatories such as the Pierre Auger Observatory in Argentina use arrays of detectors on the ground to reconstruct them. Muons from these showers pass through your body all the time.
Effects on people and technology
- On the ground: Earth’s atmosphere and magnetic field block most cosmic rays. They contribute only part of normal background radiation.
- In aircraft: exposure increases with altitude and latitude. Frequent flyers and aircrew receive more, which aviation authorities monitor.
- In space: cosmic rays are one of the biggest health risks for astronauts on long missions, see the challenges of space exploration.
- Electronics: particles can flip bits in computer memory, so spacecraft use radiation-hardened electronics.
Earth’s magnetic field also steers charged particles toward the poles, see planetary magnetic fields.
