Pilot Astronauts
Early in the U.S. manned space program, jet aircraft and engineering training were prerequisites for selection as an astronaut. Today, scientific education and experience are equally important prerequisites in selecting both pilots and mission specialists.
Pilot astronauts play a key role in Shuttle flights, serving as either commanders or pilots. During flights, commanders are responsible for the vehicle, the crew, mission success and safety -- duties analogous to those of the captain of a ship. Shuttle commanders are assisted by pilot astronauts who are second in command and whose primary responsibilities involve controlling and operating the Shuttle. During flights, commanders and pilots usually assist in spacecraft deployment and retrieval operations using the RMS arm or other payload-unique equipment on board the Shuttle.
Mission Specialist Astronauts
Mission specialist astronauts, working closely with the commander and pilot, are responsible for coordinating on board operations involving crew activity planning, use and monitoring of the Shuttle's consumables (fuel, water, food, etc.), and conducting experiment and payload activities. They are required to have a detailed knowledge of Shuttle systems and the "operational characteristics, mission requirements and objectives and supporting systems for each of the experiments to be conducted on the assigned missions." Mission specialists perform on-board experiments, spacewalks (called extravehicular activity (EVA) and payload handling functions involving the RMS arm.
Payload Specialists
This newest category of Shuttle crew member, the payload specialist, is a professional in the physical or life sciences or a technician skilled in operating Shuttle-unique equipment. Selection of a payload specialist for a particular mission is made by the payload sponsor or customer. For NASA-sponsored spacecraft or experiments requiring a payload specialist, the specialist is nominated by an investigator working group and approved by NASA.