As with fusion, a great amount of energy can be released in fission because for heavy nuclei, the summed masses of the lighter product nuclei is less than the mass of the fissioning nucleus. Fission occurs because of the electrostatic repulsion created by the large number of positively charged protons contained in a heavy nucleus. Two smaller nuclei have less internal electrostatic repulsion than one larger nucleus. So, once the larger nucleus can overcome the strong nuclear force which holds it together, it can fission. In fission reactions, electrostatic repulsion wins.
Fission is a process that has been occurring in the universe for billions of years.
As mentioned above, we have not only used fission to produce energy for nuclear bombs, but we also use fission peacefully everyday to produce energy in nuclear power plants. Cosmic Rays High energy electrons, protons, and complex nuclei can be produced in a number of astronomical environments. Such particles travel throughout the universe and are called cosmic rays. Some of these particles reach our Earth. As these objects hit our atmosphere, other particles called pions and muons are produced.