What happens when a Cosmic Ray Reaches Earth

Cosmic rays, or extremely high-energy particles originating from sources outside of the solar system, reach Earth on a regular basis. Typically, they collide with atoms in the atmosphere to produce dramatic sprays or “showers” of lower-energy particle debris. These showers can be detected by sensitive instruments, and secondary cosmic rays caused by collisions in the atmosphere regularly pass through human bodies without causing harm.

According to the Cosmic Ray Observatory Project at the University of Nebraska-Lincoln, cosmic rays “are very energetic charged particles that continually bombard the earth.” Their origins are unknown, but recent research indicates that one likely source are the gamma ray bursts emitted by supernovae, or exploding giant stars. The observatory explains that “weak” cosmic rays, with an energy of about  reach the Earth at a rate of about 10,000 per square yard per second. Higher-energy cosmic rays are less common. The most energetic ones, which are trillions of times more powerful than these “weak” rays, strike that same square yard only once every few years.

Cosmic rays can be pulled off course by gravity or deflected by magnetic fields, which dramatically reduces the number that actually reach the Earth. When they enter the atmosphere, however, many ultimately collide with the nuclei of atoms that are already in the atmosphere – free-floating oxygen, nitrogen and so on. When that happens, says the Pierre Auger Observatory, the effect is dramatic. A spectacular “shower of lower energy secondary particles” is emitted by the collision.

It is this lower-energy fallout from atmospheric collisions by the actual cosmic rays which is most easily observable on Earth’s surface. The cosmic rays detected by surface detectors are typically these so-called “secondary cosmic rays,” as opposed to primary cosmic rays which originated in deep space.

Of course, such showers are spectacular only on the subatomic scale. Physicists make use of equipment like ionization chambers, cloud chambers, atmospheric fluorescence monitoring and photomultiplier tubes to detect the effects of lower-energy cosmic rays and their collisions in Earth’s atmosphere. High-energy cosmic rays produce faint blue flashes as they collide with the atmosphere, which can be picked up by less sensitive telescopes.

Normally, cosmic rays which are not absorbed by the atmosphere are absorbed into the ground, although the higher the energy of the ray, the deeper into Earth it can penetrate. Researchers at Michigan State University state that various subterranean detectors are now being set up in abandoned mines to study high-energy particles as they penetrate the Earth, including cosmic rays.

Cosmic rays are a form of particle radiation. They are generally harmless to humans because of the combined shielding provided by the Sun’s solar wind, Earth’s magnetic field and the absorbing capacities of the atmosphere. However, when astronauts venture into outer space at times when there is a lull in the solar wind, they are at increased risk of exposure to potentially harmful cosmic rays. NASA and other space agencies are currently researching what health hazards can be caused by higher numbers cosmic rays striking astronauts once they leave the protection of Earth.