The use of atomic energy for rocket propulsion was proposed long before nuclear fission was discovered in 1939. As early as 1906, Robert Goddard published papers describing the energy inherent in a unit mass of radium. Scientists and engineers were neither able to efficiently direct the energy released to produce thrust nor produce more energy by spontaneous disintegrations in radium during that time period. Gaetano Arturo Crocco, in 1923, suggested directing radium's alpha particles using a magnetic field to produce thrust. In 1924, Soviet scientist K. E. Tsiolkowski, decided that it was impractical to use radium for rocket propulsion for the same reasons Goddard had deduced 18 years earlier, i.e., the energy release is low and slow and the cost is high.