the pioneering days of spaceflight, most astronauts were recruited
from the ranks of test pilots. Later a few scientists were added into
the astronaut corps to conduct research in space.
the multipurpose Space Shuttle, the roles of astronauts became more
varied. The astronaut corps expanded to include crew members called
mission specialists, who are responsible for spacecraft systems
and operations other than piloting. Scientists, engineers, physicians,
and pilots can become mission specialists.
many missions, guest astronauts-- called payload specialists--who
are not part of the NASA astronaut corps train with the crew. Payload
specialists come from universities, research centers, government
agencies, businesses, and other nations that use the Shuttle. They
are responsible for science and engineering payloads.