A collision between a high-energy cosmic ray particle and an atom in a photographic emulsion as viewed through the microscope. (Dr. David P. Stern, NASA Goddard Space Flight Center, January 2005)
The energy of cosmic ray particles are measured in electron volts and the first measurement of a cosmic ray particle that has energy exceeding 1e8 TeV is observed at the Volcano Ranch experiment in New Mexico, 1962.
Since then cosmic rays of even higher energy has been observed and the highest so far observed is on the evening of 15th October 1991 at Dugway Proving Grounds using the Fly’s Eye Cosmic Ray Detector which the particle has an energy of 3e8 TeV (50 joules) which is equivalent to the kinetic energy of a 142 g baseball travelling at 96 km/h.
The energy of this particle is 50 million times more than any particle accelerators on earth can produce and the effective energy of collision of this particle would be 750 TeV which is about 50 times the collision energy of the Large Hadron Collider operated by CERN in Geneva.
The sources of such highly energetic particles were a mystery and the discovery was a shock to astrophysicist. There were theories suggesting these particles could have originated from the active galactic core where they come from super-massive black holes.