In 1912 the Austrian physicist Victor Hess discovered the cosmic radiation. Under this term we understand today the high energy portion of the extraterrestrial radiation that impinges on and penetrates into the earth's atmosphere. This radiation does not originate from the sun. It consists chiefly of atomic nuclei ranging in mass from hydrogen (protons) to iron, gamma rays, a few electrons and positrons, and an unknown number of neutrinos of different kind.

The outstanding feature of the particle radiation is that its energy spectrum extends to gigantic values. The most energetic particles recorded so far exceed energies of 10^20 eV (electron volts). This is more than a million times the energy the protons will get in the large hadron collider (LHC), the world's largest atom smasher, now under construction at the Laboratory for Particle Physics (CERN) in Geneva, Switzerland. In order to produce this energy in a nuclear reactor, the fission of 10^12 uranium nuclei is required.

Neither the origin of these particles nor the mechanisms that accelerate them to these fantastic energies are known, though there are plenty of speculative theories. The direction of arrival can be determined to within a degree of arc. However, in spite of the fact that they travel at nearly the speed of light, their trajectories cannot be extrapolated to reveal the location of their sources since randomly oriented magnetic fields in space deflect the charged cosmic ray particles constantly.

Consequently, in order to locate the sources of the cosmic radiation a different, electrically neutral (uncharged) radiation must be used that is not affected by magnetic fields nor significantly disturbed on its long journey through space. However, this radiation must be intimately related to the dominating electrically charged component, such as being co-produced during the acceleration of the cosmic ray particles, or as a by-product with an evident energy signature, to yield the desired information.