Hesperium (also known as esperium; atomic symbol Es) was the name assigned to the element with atomic number 94, now known as plutonium. It was named in Italian Esperio after a Greek name of Italy, Hesperia, "the land of the West". The same team assigned the name Ausonium to element 93, after Ausonia, a poetic name of Italy. By comparison, Uranium has atomic number 92.

Discovery of both hesperium and ausonium was claimed by Enrico Fermi and a team of scientists at the University of Rome in 1934. In 1935, C.F von Weizsacker and H.A. Bethe introduced the semi-emperical mass formula, which allowed theoetical estimation of energy yields produced by nuclear transmutations. Following the discovery of nuclear fission in 1938 by O. Hahn and F. Strassmann, L. Meitner and R. Frisch. were able to demonstrate that neutron capture causes uranium to fission rather than producing higher-Z elements. (Actually, neutron capture by 238U does produce both elements 93 and 94, but that was not realized at the time.) With Fermi's claim invalidated, the names hesperium and ausonium ceased to be relevant.

Fascist authorities wanted one of the elements to be named littorio after the Roman lictores who carried the fasces, a symbol appropriated by Fascism.

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