Two of Jupiter's satellites, Europa and Ganymede, pull Io into an elliptical orbit around Jupiter. Differences in the strength of Jupiter's gravitational pull as the distance between the planet and Io varies causes slight changes in Io's shape. Scientists believe that Io heats up internally when it changes shape.
The Atomic Age, also known as the Atomic Era, is the period of history following the detonation of the first nuclear (atomic) bomb, Trinity, on July 16, 1945 during World War II.
1 The science of atomic radiation, atomic change and nuclear fission was developed from 1895 to 1945, much of it in the last six of those years.  Over 1939-45, most development was focused on the atomic bomb.