Payload specialists are persons who have been designated by the National Aeronautics and Space Administration (NASA) or its commercial or international partners to serve as crewmembers in association with a specific payload and/or to accomplish a specific mission objective. Payload specialists include persons other than NASA astronauts who have specialized onboard duties; they may be added to the crew manifest if there are unique requirements or activities and more than the minimum crew size is needed. Individuals selected for crew assignment under the Space Flight Participant Program or similar programs also are referred to as payload specialists.

The payload specialist category represents an evolution in astronaut specialization. The first astronauts were required to have jet aircraft flight experience and engineering training. They conducted all operational and scientific activities aboard the spacecraft. Later, the emphasis shifted away from flight experience and toward superior academic qualifications. Some...