 The concept of the American frontier is also related to the coming-of-age theme.Young protagonists across Heinlein's novels attain manhood by confronting a hostile "wilderness" in space; coming-of-age in a military, alien context is a common theme in Heinlein's earlier works as well.Rico's coming-of-age has also been described as being related to his relationship with his father; the journey "outward" through the novel also contains a search for Rico's childhood and a reunion with his estranged parent.