Cosmic rays come from outer space, and about thirty of them zip through your body every second. They pose a great danger for manned missions to Mars, can damage electronics, and made Apollo astronauts see flashes in the dark, even with their eyes closed. Some aren't cosmic, none are rays, and a few seem to be impossible. What are they and where do they come from?

Why cosmic rays?
Cosmic rays were discovered early in the twentieth century, and for quite some time scientists thought that they were a kind of electromagnetic radiation like visible light or X-rays. The Sun was an obvious possible source, but the rays came from all directions. They were therefore dubbed cosmic because they seemed to have come from beyond the Solar System.

However the “rays” turned out to be invisible, highly energetic charged particles – parts of atoms. There are small quantities of electrons, but most cosmic rays (89%) are protons, about 10% are the nuclei of helium atoms, and 1% are the nuclei of heavier atoms, even including uranium. Since they're charged particles, magnetic fields in space affect them, so we can't find their origins by tracing their paths backwards.

Some of the particles do come from the Sun, but there are plenty from outside the Solar System. There are also cosmic rays created when the more energetic ones enter the Earth's atmosphere and collide with air molecules. These collisions produce subatomic particles, which in turn have further collisions, producing an air shower of secondary cosmic rays.

Electron volts (eV)
Scientists measure the energy of atomic particles in electron volts (eV). An electron volt is the energy an electron would get from a 1-volt battery. That's not much. Even though cosmic rays are just pieces of atoms, they're moving at very high speeds, so they have a lot more energy than you'd think from the tiny mass. Therefore we use bigger units such as mega electron volts (MeV), which is a million electron volts, and giga electron volts (GeV), which is a billion electron volts.

Types of cosmic ray
There's a lot that we still don't understand about cosmic rays, so classifying them is a bit rough-and-ready. Here are four common categories:

Solar cosmic rays
Solar cosmic rays are particles from the Sun that are accelerated by solar events that produce coronal mass ejections. In a coronal mass ejection charged particles are flung out from the Sun at high speed. Solar cosmic rays are less energetic than those from outside the Solar System, yet they can damage the electronics of satellites and endanger astronauts. Some are funneled down the Earth's magnetic field lines at the poles and trigger auroral displays.

Galactic cosmic rays
The solar wind is a plasma – a gas which is a mixture of charged particles – blowing from the Sun into the depths of the Solar System. Its outward push reduces the number of cosmic rays getting into the inner Solar System. However the ones that arrive typically have energies between 100 MeV and 10 GeV. They're traveling at speeds between 45% and 99.6% of the speed of light.

Most galactic cosmic rays come from elsewhere in the Milky Way. They have twisted and turned their way drunkenly through the galactic magnetic field. There is strong evidence that they're accelerated by shock waves from supernova explosions.

Ultra High Energy (UHE) cosmic rays
The last type is the rarest and most mysterious. They have what seem to be impossibly high energies, and the Oh-My-God particle is the most astonishing of all. It was detected in Utah in 1991, traveling at what was within a whisper of the speed of light. Its energy was calculated at around thirty million trillion electron volts.

What in the Galactic neighborhood could accelerate a particle to such a speed? Merging black holes? Colliding galaxies? No one knows, but they do know that a supernova doesn't have nearly enough energy to do the job, even though it releases as much energy as an entire galaxy.

So far astronomers haven't found anything in nearby galaxies that seem likely candidates. But what about a galaxy far, far away? We don't think so. It shouldn't be possible to come from over 30 million light years away and still have so much energy. The particle would interact with the cosmic background radiation and lose energy before it got to us. Background radiation is the remnant of the energy of the Big Bang that fills the Universe.

Hazards of cosmic rays
The Earth's atmosphere and magnetic field protect us from most low energy cosmic rays. And although there are thousands of them passing through our bodies every minute, at sea level cosmic radiation is only a few percent of the natural background radiation. Since there's less protection at high altitudes, flight crews are exposed to somewhat more radiation.

In space, both astronauts and electronics are at risk from this radiation if the Sun is active. There wasn't any major solar activity for the Apollo missions. Yet Apollo 11 crew members were the first people to see random light flashes, even when their eyes were closed. These were cosmic rays. And think about astronauts on a manned Mars mission. They would be in deep space for a long time, but shielding humans and electronics against cosmic rays and high-energy radiation is a problem that hasn't yet been solved.