The human motivation for exploration has always been clear: hope for national and individual profit or acclaim, the simple gratification of geographical curiosity, and the discovery and identification of the unknown. While these motives were still present in the twentieth century, fundamental developments in technology began to change the character of exploration.

Two significant discoveries propelled twentieth-century exploration to new heights—literally. In 1903 the historic flight of the Wright brothers ushered in a new era of technology, and with it new possibilities in exploration. Around the same time, American inventor Robert Goddard (1882-1945) began experimenting with rocket propulsion. In a 1920 technical report for the Smithsonian, Goddard outlined how a rocket might reach the moon. The scientific community labeled him a crackpot, but his report became the foundation for the early rocket program of the Nazi military, which made further advancements...