When cosmic rays, especially high-energy galactic cosmic rays, strike Earth's atmosphere, they often produce a cascade of secondary sub-atomic particles called an "air shower". This diagram depitcs an incoming cosmic ray (in red, at the top) and the resulting air shower which includes protons (green), neutrons (orange), pions (yellow), muons (purple), photons (blue), and electrons & positrons (pink). An actual air shower may consist of millions of particles, depending on the energy of the initial cosmic ray. Click on image for full size (43 Kb) Windows to the Universe original artwork by Randy Russell using a photo courtesy UCAR (Nicole Gordon).

Cosmic rays are a type of radiation that comes from space. Cosmic rays aren't really "rays"; they are subatomic 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 equipment, though here 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 in 1912. Hess won the 1936 Nobel Prize in Physics for this discovery. The American physicist Robert Andrews Millikan confirmed Hess discovery in 1925, and coined the term "cosmic rays" for this new type of radiation from space.

There are several different types of cosmic rays, and the radiation comes from various sources. Closest to home are solar cosmic rays, produced on the Sun by solar flares and similar energetic events. Solar cosmic rays have lower energies (up to about 10¹⁰ electron volts per particle) than do other types of cosmic rays. Galactic cosmic rays have higher energies (roughly 10¹⁰ to 10¹⁵ eV) and are believed to come from supernova explosions, black holes, and neutron stars within our own Milky Way galaxy. Even more energetic (10¹⁵ eV or higher) are the rare extragalactic cosmic rays. Astronomers believe these particles come from beyond our galaxy, but are otherwise unsure of their exact origins. They may come from the nuclei of active galaxies, from quasars, or be produced during collisions between galaxies. They might even be left over from exotic particle decay processes that occurred when the Universe was young. A fourth, and also somewhat mysterious, type of cosmic ray is called the anomalous cosmic rays (ACR). ACRs have unexpectedly low energy levels, and may be produced at the edge of the heliosphere, the boundary between the region where the Sun's magnetic field is dominant and interstellar space.

What kinds of particles make up cosmic rays? Most, about 90%, are protons; in other words, the ionized nuclei of normal hydrogen, the most abundant form of matter in the Universe. Helium nuclei (also known as alpha particles), which include 2 protons and 2 neutrons, account for about 9% of cosmic rays. Most of the remaining 1% are electrons. The nuclei of a broad range of other types of atoms (including carbon, oxygen, iron, calcium, lithium, beryllium, boron, gallium and others) are represented in small but measurable amounts in the cosmic ray menagerie. Different types of cosmic rays (solar vs. galactic, for instance) have slightly different abundances of different constituents.

Because of their high energies, cosmic rays can be dangerous to living creatures and to machines (especially those that rely on electronics) alike. Fortunately for us, Earth protects us from most cosmic rays. Because they are electrically charged, cosmic rays interact with Earth's magnetic field. Earth's magnetism deflects away many cosmic rays, and steers much of the remainder towards our planet's sparsely populated polar regions. Most particles that make it past Earth's magnetic shields collide with gases in our atmosphere. This is both good and bad for those of us on the planet's surface. When cosmic rays collide with atmospheric gases, the collisions generate new and different particles. In general, these collisions dilute the energy of the particles, producing larger numbers of less energetic particles. A single high-energy cosmic ray can produce a shower of thousands or millions of secondary particles, as secondary particles produced by collisions crash into other gas atoms and molecules, producing yet more particles, and so on. Though this particle shower or cascade (technically known as "spallation") generally produces lower energy, and thus less lethal, particles, some of the secondary byproducts are actually more harmful to living creatures.

How dangerous is cosmic ray radiation? Typically, normal Earthlings are exposed to roughly 2.3 millisieverts (a measure of radiation dosage, abbreviated mSv) of radiation per year. About 0.2 mSv of this dose, or about 9%, is due to cosmic rays. In other words, if you stay on Earth, you normally have little to fear from cosmic rays. However, if you decide to travel into space, and especially if you venture beyond the protective bubble of Earth's magnetosphere, cosmic rays could become a major problem. Scientists estimate that unshielded humans in interplanetary space might receive 400 to 900 mSv of radiation per year, mostly from cosmic rays. Astronauts on a 30 month long Mars mission could be exposed to 1,000 mSv or more of radiation. This is comparable to the recommended career limit of 1 to 4 Sieverts (1,000 to 4,000 mSv) of radiation exposure for astronauts in low Earth orbit. Because cosmic rays have such high energies, they can penetrate deeply into tissue and cause extensive damage to cellular DNA, producing cancers and similar maladies.

The number of cosmic ray particles reaching Earth varies over time. Solar activity varies dramatically over the course of an 11-year cycle. Oddly, the danger from cosmic rays is least when the Sun is most active and producing solar flares and other dramatic "space weather storms". The Sun's activity causes the heliosphere, the vast region in space dominated by the Sun's magnetic field, further outward. The heliosphere acts as another layer of magnetic shielding from galactic and extragalactic cosmic rays; so when the Sun is active this shield expands and fewer external cosmic rays reach Earth. When the Sun is active, we are exposed to larger doses of solar cosmic rays; however, we receive lesser doses of the higher energy, and thus more dangerous, galactic and extragalactic cosmic rays. The net effect is less total danger from radiation exposure. For this reason, when Earthlings do eventually venture beyond our planet on missions to Mars and other interplanetary voyages, they will probably do so when the Sun is at its most active phase!

Cosmic rays help produce some interesting natural phenomena as they zip through our atmosphere. Some cosmic ray collisions with nitrogen (the most abundant gas in Earth's atmosphere) transform the nitrogen atoms into atoms of carbon-14 (or ¹⁴C for short). This radioactive isotope of carbon is absorbed by living creatures, and is the basis for carbon-14 dating; you've probably heard of archeological artifacts and similar ancient remains being dated using the carbon-14 method. Cosmic rays in the atmosphere may also help to trigger lightning; the ionized trails the particles leave behind in the air as they pass through may be what's needed to allow atmospheric voltage differences to set off lightning strikes. Finally, cosmic rays might play a role in cloud formation; the radiation may help produce some cloud condensation nuclei, the tiny particles that act as "seeds" that start water droplet formation in clouds. These last two effects (triggering lightning and helping to seed clouds) are somewhat speculative; scientists are still studying both phenomena.

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). Click on image for full size (43 Kb) Windows to the Universe original artwork by Randy Russell using a photo courtesy UCAR (Nicole Gordon).

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.

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). Click on image for full size (43 Kb) Windows to the Universe original artwork by Randy Russell using a photo courtesy UCAR (Nicole Gordon).

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.