As commonly used, the term "communication" refers to the exchange of information between one person and another or between one social system and another, for the purpose of reaching a shared understanding. Conrath (1972) and Katz and Kahn (1978), among others, place the management of the communication process at the essence of organization. Through the control of communication, managers seek to provide individuals with access to the information they need for the competent performance of their jobs, while limiting their access to inessential communications in order to reduce the overall level of distraction.
Planning a space mission involves all the complex communication issues found in any large scale endeavor. It also includes those issues that are special to isolated and confined individuals and to groups separated by large distances. Future flights, involving crews away for long periods of time, will further complicate communication questions. Because of the importance of such issues to the success of future space missions, this chapter focuses specifically on the process and effects of communication in extended spaceflight.
Space missions involve communication at several levels, the precise number of which depends on such factors as the size and complexity of the mission and the availability of communication media. The three simplest levels include (1) communication within  the confines of the space capsule; (2) communication between a space capsule and another capsule, satellite, or space habitat; and (3) communication between the mission and Earth. Communication which takes place at these different levels may differ in the rate and quality of information flow; for instance, discontinuities may be introduced when communication must depend on electronic mediating systems.
Communication in space as elsewhere serves both task and socioemotional functions. In terms of task functions, both hierarchical and lateral communication are necessary to organize and coordinate crewmembers in pursuit of mission goals. Information must flow to managers to ensure that informed decisions are reached; information must flow from managers to subordinates to ensure that decisions are understood and implemented. Lateral communication is necessary for the general coordination and implementation of activity. Communication is essential also for maintaining the psychological and social well being of the crewmember; it provides updated knowledge of other people's attitudes and views, which, as noted elsewhere in this book, is necessary for social comparison processes and for conflict management. In addition to its use for casual enjoyment, communication is a prerequisite for social recognition and censure, both of which have been shown to be potent controllers of human behavior. Finally, miscommunication (i.e., a failure in the communication process) can contribute to interpersonal friction and conflict within the crew or between the crew and those on the ground.