Cosmic ray is at once an umbrella term and a misnomer. It is not, as at one time believed, a form of electromagnetic radiation. It is a charged particle accelerated to a super-high energy level. Moreover, a ray may consist of any of a great number of particles, ranging from the simple electron to protons and all sorts of subatomic particles, as well as the nuclei of virtually all atoms that appear in the periodic table. So we see the defining consideration is not what it is but rather what it does.
Most cosmic rays are galactic, meaning that they begin life outside our solar system. Additionally, some cosmic rays originate in the sun. Others are generated when less energetic particles anywhere in the solar system are accelerated to higher energy levels.
Before the advent of high-energy particle colliders, our knowledge of many cosmic ray particles came from observing ionized trails made visible in a cloud chamber. (It is possible to devise a home-made cloud chamber. The details are here. Construction involves isopropyl alcohol, dry ice, neodymium magnets, and a few household items.)
What all cosmic rays have in common are high amounts of energy. For perspective, recall that one electron volt is the amount of energy imparted to an electron when it is accelerated from one point in space to another point in space. These two points have a potential difference of one volt.
The typical energy level of a galactic cosmic ray ranges from 100 MeV to 10 GeV. Some cosmic rays have vastly higher energy levels, but there are a lot fewer of them. That is because as the energy levels rise, cosmic ray particles — most of which have considerable mass for such small entities — approach close to the speed of light where relativistic phenomena apply.
Cosmic rays, being electrically charged, are deflected by magnetic fields. This behavior has some important effects. For one thing, the earth’s magnetic field is a powerful shield that protects terrestrial life forms from the harmful ionizing effects of these charged particles. In its distant past, Mars lost its magnetic field, and it has been suggested that the sudden blast of ionizing particles may have extinguished whatever life inhabited the Red Planet.
For the same reason, cosmic rays will pose a threat to space travelers on long interplanetary voyages. To be sure, spacecraft can be fitted out with sufficiently thick metal shielding, but this will boost the amount of fuel needed, and there is the related issue of cosmic ray protection at the destination.
Because they are prone to electromagnetic deflection, the direction of travel through space of cosmic rays has become randomized. For this reason it is not always obvious where some of them originated. And because of their diverse composition, much remains to be discovered regarding their behavior and effect on our habitat.
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