Cosmic rays are energetic particles originating from space that impinge on Earth's atmosphere. Almost 90% of all the incoming cosmic ray particles are protons, about 9% are helium nuclei (alpha particles) and about 1% are electrons (beta minus particles). The term "ray" is a misnomer, as cosmic particles arrive individually, not in the form of a ray or beam of particles.

The variety of particle energies reflects the wide variety of sources. The origins of these particles range from energetic processes on the Sun all the way to as yet unknown events in the farthest reaches of the visible universe. Cosmic rays can have energies of over 1020 eV, far higher than the 1012 to 1013 eV that man-made particle accelerators can produce. (See Ultra-high-energy cosmic rays for a description of the detection of a single particle with an energy of about 50 J, the same as a well-hit tennis ball at 42 m/s [about 94 mph].) There has been interest in investigating cosmic rays of even greater energies.

In high-energy physics, an ultra-high-energy cosmic ray (UHECR) or extreme-energy cosmic ray (EECR) is a cosmic ray (subatomic particle) which appears to have extreme kinetic energy, far beyond both its rest mass and energies typical of other cosmic rays. These particles are significant because they have energy comparable to (and sometimes exceeding) the Greisen-Zatsepin-Kuzmin limit.

Galactic cosmic rays (GCRs) consist of those cosmic rays that enter the solar system from the outside. They are high-energy charged particles composed of protons, electrons, and fully ionized nuclei of light elements.

Astronauts outside the magnetosphere of the Earth (such as during the Apollo program) perceived a visual phenomenon in which they saw spontaneous flashes of light. It is believed that cosmic rays were responsible for these flashes of light, which were the result of Cerenkov radiation created as the cosmic ray particles passed through the vitreous humor of the astronauts' eyes.

The health threat from cosmic rays is the danger posed by cosmic rays generated by the Sun and other stars to astronauts on interplanetary missions. Cosmic rays consists of high energy protons and other nuclei. They are one of the most important barriers standing in the way of plans for interplanetary travel by crewed spacecraft.

Like other ionizing radiation, high-energy cosmic rays can damage DNA, increasing the risk of cancer, cataracts, neurological disorders, and non-cancer mortality risks.

See also: Cosmic Rays

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