COSMIC RAYS are high-energy particles that travel throughout the Milky Way Galaxy. Some of them originate from the Sun, but the majority comes from sources outside the solar system. Cosmic-ray particles that arrive at the top of the Earth's atmosphere are termed primaries; their collisions with atmospheric nuclei create secondaries.

Before the 1950s, cosmic rays were the only source of high-energy particles. This is why they played a crucial role in scientific studies of the atomic nuclei and their components. With the advent of powerful particle accelerators in the 1950s, investigations of cosmic rays were continued, though on a more limited scale, because they contain particles with energies far beyond those attainable under laboratory conditions (>10¹⁵eV).

Victor Franz Hess (1883 - 1964), the Nobel Prize Winner for Physics in 1936 "for his discovery of cosmic radiation". Hess was engaged also in scientific research of radioactivity and atmospheric electricity. He discovered the "ionic wind".