What is nuclear fission? How and by whom was it discovered?

Imagine the center of an atom as a tight cluster of particles called protons and neutrons. This is the atomic nucleus. Nuclear fission is like a tiny universe's version of a significant event - a colossal release of energy caused by this nucleus splitting apart. When a nucleus undergoes fission, it divides into two smaller nuclei, along with a few neutrons and photons, which are packets of pure energy often in the form of gamma rays. The process is impressive because it unleashes an amount of energy that’s enormous relative to the size involved - and this is the principle behind nuclear power and atomic bombs.
The controlled splitting of the atomic nucleus in nuclear power plants generates heat, which is then used to produce steam. This steam spins turbines to generate electricity, showcasing a peaceful application of fission. In contrast, uncontrolled fission in atomic bombs releases this energy violently, resulting in an explosion.

The path to the monumental discovery of nuclear fission began in the laboratory, with scientists probing deeply into the nature of atoms. In the 1930s, the quest to understand transmutation - the conversion of one chemical element to another - led to a breakthrough. Researchers bombarded uranium with neutrons, expecting to find heavier elements, but instead, they uncovered something entirely unexpected. The products of the reaction were lighter than uranium, baffling scientists. The odd results indicated that the uranium nucleus was not merely absorbing neutrons and transforming; it was actually breaking apart, or fissioning. This revelation paved the way for a new understanding of atomic interactions and signified the beginning of a new era in science and technology.
To grasp the significance of this, consider that prior to this discovery, it was believed that atoms were the smallest indivisible units of matter. The realization that an atomic nucleus could split and release so much energy expanded the horizons of physics and further demystified the powerful forces within the atom.

Otto Hahn and Fritz Strassmann, two German chemists, stand tall in the annals of science for their role in the discovery of nuclear fission. Their collaboration began in the early 1930s, when they were studying the products of neutron-irradiated uranium. By December 1938, they had obtained results that defied the prevailing wisdom in nuclear physics.
Strassmann's expertise in analytical chemistry allowed them to meticulously isolate the elements formed during bombardment, leading to the clear identification of barium, a much lighter element than uranium, after neutron irradiation. This was the evidence needed to prove that something unprecedented was happening - the nucleus was splitting.

While Hahn and Strassmann were the experimentalists behind the discovery of fission, it was Lise Meitner, an Austrian-Swedish physicist, and her nephew Otto Frisch, a physicist as well, who provided the theoretical explanation. After fleeing Nazi Germany, Meitner continued her research in Sweden. It was during the Christmas holiday of 1938 that Meitner and Frisch, while enjoying a snowy walk, outlined the theoretical interpretation of Hahn and Strassmann's experiments.
They described the nucleus as a liquid drop that, when bombarded by neutrons, could elongate and eventually split apart, an elegant explanation invoking both the simplicity and complexity of physical laws. Their insights, later published in the famous paper where they coined the term 'fission', emphasized the mass-to-energy relationship described by Einstein's E=mc^2 and explained the extraordinary energy release observed by Hahn and Strassmann.