Cosmic ray

Cosmic rays are energetic charged subatomic particles, originating in outer space. They may produce secondary particles that penetrate the Earth's atmosphere and surface. The term ray is historical as cosmic rays were thought to be electromagnetic radiation. Most primary cosmic rays (those that enter the atmosphere from deep space) are composed of familiar stable subatomic particles that normally occur on Earth, such as protons, atomic nuclei, or electrons. However, a very small fraction are stable particles of antimatter, such as positrons or antiprotons, and the precise nature of this remaining fraction is an area of active research.
About 89% of cosmic rays are simple protons or hydrogen nuclei, 10% are helium nuclei or alpha particles, and 1% are the nuclei of heavier elements. These nuclei constitute 99% of the cosmic rays. Solitary electrons (much like beta particles, although their ultimate source is unknown) constitute much of the remaining 1%.
The variety of particle energies reflects the wide variety of sources. The origins range from processes on the Sun (and presumably other stars as well), to as yet unknown physical mechanisms in the farthest reaches of the observable universe. There is evidence that very high energy cosmic rays are produced over far longer periods than the explosion of a single star or sudden galactic event, suggesting multiple accelerating processes that cover very long distances with regard to the size of stars. The obscure mechanism of cosmic ray production at galactic distances is partly a result of the fact that (unlike other radiations) magnetic fields in our galaxy and other galaxies bend cosmic ray direction severely, so that they arrive nearly randomly from all directions, hiding any clue of the direction of their initial sources. Cosmic rays can have energies of over 1020 eV, far higher than the 1012 to 1013 eV that Terrestrial particle accelerators can produce. There has been interest in investigating cosmic rays of even greater energies.
Cosmic rays are enriched in lithium, beryllium, and boron with regard to the relative abundance of these elements in the universe compared to hydrogen and helium, and thus are thought to have a primary role in the synthesis of these three elements through the process of "cosmic ray nucleosynthesis". They also produce some so-called cosmogenic stable isotopes and radioisotopes on Earth, such as carbon-14. In the history of particle physics, cosmic rays were the source of the discovery of the positron, muon, and pi meson.

Cosmic rays compose a part of natural background radiation on Earth, averaging about 10-15% of it. However, persons living at higher altitude can obtain several times more cosmic radiation than at sea level, and long distance airline crews can double their yearly ionizing radiation exposure due to this source. Since the intensity of cosmic rays is much larger outside the Earth's atmosphere and magnetic field, it is expected to have a major impact on the design of spacecraft that can safely transport humans in interplanetary space.