The Facts about Cosmic Rays

Cosmic rays are high-energy particles which originate outside the earth’s atmosphere. Almost 90 percent of all cosmic rays consist of single protons (ionised hydrogen atoms), 9 percent consist of helium nuclei (alpha rays), and the remaining one percent consist entirely of electrons (beta minus rays). Thus all cosmic rays carry a magnetic charge. It is possible that sufficiently sophisticated detection devices set outside our atmosphere may also someday identify cosmic rays consisting of the antimatter equivalents to these particles, although most of these would have been eliminated before entering our solar system.

Some cosmic rays come from the sun, while others come from the far reaches of the universe. Except for cosmic rays known to be of solar origin, determining the exact origin of specific cosmic rays is complicated by the effect of intervening magetic fields, which deflect their paths. Instead, their sources are inferred from the energies which could be produced by specific events. Most seem to originate from within our own galaxy, resulting from such events as supernova explosions and black holes. The most powerful cosmic rays are believed to originate from active galactic nuclei. In general, the further away the source, the more powerful seem to be the cosmic rays which reach us from that source. Their energies range from a single lowly gigaelectronvolt (GeV = 10^9 eV) to over a quadrillion gigaelectronvolts (10^21 eV).

Most cosmic rays decay long before reaching the earth’s surface. No cosmic ray with an energy less than 1 GeV can make it through the atmosphere at all. The magnetic field of the earth, and to some extent even that of the sun, acts to deflect the charged cosmic rays, an effect known as geomagnetic shielding. A few of the products of secondary radiation, such as muons, pions, and neutrinos, can make it to the surface and even penetrate some distance below its surface.

At the surface of the earth cosmic rays account for approximately 1/8 of normal background radiation, varying by altitude and location. Like all radiation, they can have a significant effect on electronics, even at ground level but especially in satellite components. The levels of energy involved mean that shielding does not have much effect, and in some cases may even increase the amount of net exposure (by increasing the amount of secondary radiation). One devised protection is for cosmic ray sensors to automatically trigger duplication of the last command before the cosmic rays struck. At ground level the health threat from cosmic rays is minimal, but the risk increases dramatically outside the earth’s protective magnetic field and atmosphere. Radiation of the type common to cosmic rays has been linked with DNA damage and cancer. Most of the Apollo astronauts have been diagnosed with early onset cataracts, which may have been induced jointly by passage through the Van Allen belts and by increased exposure to cosmic rays.

Thus the effect of cosmic rays is a significant concern in planning long-term space voyages, such as to Mars. However, because of the effect of the sun’s magnetic field as well as the direct effect of the generally heavier and denser particles, cosmic rays decrease during coronal mass emissions (CMEs). Ironically, since CMEs are much easier to shield against than cosmic rays, this means that space travel might best avoid cosmic radiation by trying to catch windows of increased solar activity.

Until the early 1900s, it was believed that because all known radioactivity originated within the earth, the resulting air ionisation must decrease with height: and indeed experiments within altitudes normally accessible supported this conclusion. It was not until 1912, when Domenico Pacini measured air ionisation over large bodies of water, that it began to be suspected that there must be other sources contributing to ionisation. Later that year, Victor Hess, ascending over five kilometres in a balloon, discovered that an electroscope discharged four times as rapidly at this altitudes than on the ground. He concluded that there must be “a radiation of very great penetrating power [which] enters our atmosphere from above.” Hess had discovered the existence of cosmic rays.

Hess, Victor F. (1928). The Electrical Conductivity of the Atmosphere and Its Causes. Constable & Company.