Backstory
Through the course of the novel, Gardner provides a framework, background, and conceptual structure for his future narrative. In this back-story, humanity attains a technology of "spacetime distortion" to create an effective "star drive", thus leaving the solar system to explore and colonize planets orbiting other stars. Through this exploration and colonization effort, humans come into contact with many different species of intelligent life, sometimes vastly different in nature, sometimes of a much higher state of technological and evolutionary development. The more advanced beings have forms that stretch the humans' definition of life: they can appear as "a cloud of red smoke, a glowing cube"—or as nothing at all.
These species have an organization for the galaxy, to control interstellar travel and contact; it is called the "League of Peoples". The League's primary rule is simple: no killing of other sentient beings. No war, no fatal violence; the League even prohibits lethal weapons in interstellar space. This is the guideline that determines sentience in the League's definition. Species that fail to obey this rule are restricted to their native solar systems, within which they can live as they please. If "dangerous non-sentients" attempt star travel, they are punished with death—delivered instantaneously, method unknown. (The actual implementation of this rule in practice, in specific and varying cases, is of course more complex and ambiguous than vague generalities can capture.)
For species that do obey the league's primary commandment, however, the rewards can be great: advanced technologies are doled out to co-operative societies. Much of humanity (though not all; a remnant still exists on "Old Earth") has accepted this bargain: from a genetically-engineered New Earth, a unified human culture calling itself the Technocracy sends out a fleet of ships to explore and colonize new planets. The fleet is run under a quasi-military and naval structure, under the command of a High Council of admirals; the spaceships' crews function much as traditional navies did on Old Earth—except for engaging in combat.
Exploring new planets, however, is dangerous work; in a society with no war, with little crime or violence, and with excellent advanced health care, the deaths of fit and intelligent young men and women—occasionally in the most extreme and elaborate ways—are a source of significant psychological trauma and social stress. The Technocracy has dealt with this problem by organizing a special Explorer Corps. These are individuals fit enough and smart enough to do the job, but afflicted with physical disadvantages (handicaps, diseases, of sometimes simple ugliness) that place them outside the norm of society. Candidates are identified in childhood, and conscripted into the Explorers; even if their deficiencies are easily correctible, they are left untreated. When these Explorers die in the course of their work—as they often do—the citizens of the Technocracy manage to cope with the shock.
Read more about this topic: Expendable