The human race has been enthralled by the exploration of space, but performing major long distance flights proved to be a major challenge since technology in the early stages of space exploration was limited. This frontier of space exploration was finally broken with the use and advancement of nuclear energy. There are several means of creating enough energy to power a spacecraft; however, none are as reliable and as favored as nuclear power when it comes to exploring the solar system past the moon. Innately, space provides for extreme temperatures, dark environments, exposure to radiation, etc. These punishing conditions are not conducive to conventional energy sources. While regular means of power prove to be unsustainable in deep space, nuclear energy provides spacecraft with sufficient power to complete deep space flights. This report will explore the various power sources that could be used in space travel focusing on the propulsion choice and illustrates a brief historical background of nuclear reactors and radioisotopes for space. Read the article