Cosmic rays are energetic particles originating from outer space that impinge on Earth's atmosphere. Almost 90% of all the incoming cosmic ray particles are protons, almost 10% are helium nuclei (alpha particles), and slightly under 1% are heavier elements and electrons (beta minus particles).

The term [cosmic "ray"] is a misnomer, as cosmic particles arrive individually, not in the form of a ray or beam of particles. [A better term might be "cosmic particles," but that itself is a great debate...]

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. ^[1] 

Photo: Micrograph of a Nuclear Track Emulsion previously flown on a LDB flight in Antarctica. The thick track in the center is a high charge and high energy cosmic ray recorded in the emulsion. The other tracks seen in the picture are records of other primary cosmic rays and secondary particles from cosmic ray interactions in the atmosphere. (Source: NASA’s Deep Space Test Bed for Radiation Shielding Studies.)