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Cosmic rays from space hit Earth's atmosphere all the time. When a high-energy cosmic ray enters the atmosphere, it can cause an "air shower". The cosmic ray hits a molecule in the atmosphere and "breaks up", producing lots more sub-atomic particles. A real air shower can make millions of particles. This picture shows a simple version of an air shower. The cosmic ray (in red, at the top) makes lots of other particles, many with odd names. The sub-atomic particles shown here include protons (green), neutrons (orange), pions (yellow), muons (purple), photons (blue), and electrons & positrons (pink).
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Windows to the Universe original artwork by Randy Russell using a photo courtesy UCAR (Nicole Gordon).

Cosmic Rays

Cosmic rays are a type of radiation that comes from space. Cosmic rays aren't really "rays". T hey are particles (mostly protons) with very high energies. Cosmic rays come from various places, including the Sun, supernova explosions, and extremely distant sources such as radio galaxies and quasars. Because of their high energy, this type of particle radiation can be dangerous to people and to machines. On Earth we are mostly shielded from them by our planet's magnetic field and atmosphere.

Cosmic rays were discovered by the Austrian-American physicist Victor Hess. Hess discovered this new type of radiation in 1912. He won the 1936 Nobel Prize in Physics for this discovery.

There are several different types of cosmic rays. Solar cosmic rays come from the Sun. They have less energy than most other types of cosmic rays. Solar flares and other explosions on the Sun make this type of cosmic ray. A second type is the galactic cosmic ray. Galactic cosmic rays have more energy than solar cosmic rays. Astronomers think they come from supernova explosions, black holes, and neutron stars within our own Milky Way galaxy. A third type is the rare extragalactic cosmic ray. Scientist think these particles come from somewhere outside of our galaxy. However, they aren't exactly sure where they come from. Extragalactic cosmic rays have even more energy than galactic cosmic rays. Extragalactic cosmic rays may come from quasars or from the nucleus of an active galaxy. Maybe they are made when galaxies crash into each other. Nobody knows for sure. Finally, there are the strange anomalous cosmic rays (ACR). ACRs have lower energies than other types. Astronomers think they might come from the edge of our Solar System. The Sun's magnetic field makes a gigantic "bubble" in space called the heliosphere. ACRs may come from the edge of the heliosphere, where the Sun's magnetic field bumps into interstellar space.

What kinds of particles make up cosmic rays? Cosmic rays are made from different kinds of subatomic particles. That means they are particles that are parts of atoms or are smaller than atoms. Most cosmic rays are protons. Others are made from the nucleus of some kind of atom, so they have protons and neutrons in them. The most common is the nucleus of a helium atom, which has 2 protons and 2 neutrons (and is also called an alpha particle). Others are nuclei of carbon, oxygen, iron, calcium, and other types of atoms. A small number of cosmic rays are electrons. No matter what they are made of, cosmic rays move really fast and have a lot of energy!

Since cosmic rays are a kind of radiation, they can hurt people and machines. Lucky for us, Earth's magnetic field and atmosphere protect us from most cosmic rays. On average, people get hit with about 2.3 millisieverts of radiation each year. A millisievert is a unit for measuring radiation. It is abbreviated mSv. Cosmic rays make up about 0.2 mSv of the radiation we get each year. That isn't very much; less than 10% of the total. Astronauts do have to worry about cosmic rays, though. If astronauts travel away from Earth (to the Moon or Mars, for example), they aren't protected by Earth's magnetic field any more. They could get hit by as much as 900 mSv of cosmic ray radiation in a year! Cosmic rays can damage our DNA and cause cancer and radiation sickness. Scientists will have to figure out how to protect astronauts from cosmic rays before we can send a mission to Mars.

When cosmic rays hit Earth's atmosphere, they crash into atoms and molecules of gas. That usually makes even more cosmic ray particles! Since there are more particles, the energy from the cosmic ray from space is spread out. The new cosmic ray particles often hit other gas molecules. That makes still more cosmic rays, but with lower energies. The collisions between cosmic rays and gases in the atmosphere can happen many times. In the end, there might be thousands or millions of "secondary" cosmic rays. This is called an "air shower" of cosmic rays.

Earth doesn't always get hit by the same number of cosmic rays. Strangely, cosmic rays are less of a problem when the Sun is most active. Sometimes there are more solar flares and other "space weather storms"; sometimes there are fewer. The Sun has a cycle that is 11 years long. At "solar max" the Sun is very active; at "solar min" there are very few "storms" on the Sun. Since some cosmic rays come from the Sun, you might think that there is more danger from cosmic rays when the Sun is active. Good guess; but wrong! When the Sun is active, it "puffs up" its heliosphere. Like Earth's magnetic field, the Sun's magnetic field helps shield us from galactic and extragalactic cosmic rays. So an active Sun means better shielding! So, if you're an astronaut, the best time to be going on a long trip in space is when the Sun is most active.

Have you ever hear of carbon-14 dating? It's something archeologists use to help figure out how old things are. Living things have small amounts of radioactive carbon in them. The radioactive carbon comes from carbon dioxide gas in the atmosphere. The radioactive type of carbon is a special type of carbon, called an isotope, named carbon-14 (or 14C for short). How does radioactive carbon get into our atmosphere? You guessed it - cosmic rays! Sometimes when cosmic rays hit nitrogen, the most common gas in our atmosphere, they change the nitrogen atoms into radioactive 14C atoms. Later, that 14C ends up in living creatures.

What else do cosmic rays do? Scientist aren't completely sure, but they think they might help set off some lightning strikes. They also might help cause clouds to form. Scientists aren't quite sure whether cosmic rays help cause lightning or help produce clouds, but they might. The scientists are still studying this part of the cosmic ray story.

Last modified January 23, 2008 by Randy Russell.

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Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part by the National Science Foundation and NASA, our Founding Partners (the American Geophysical Union and American Geosciences Institute) as well as through Institutional, Contributing, and Affiliate Partners, individual memberships and generous donors. Thank you for your support! NASA AGU AGI NSF