Mission specialist astronauts work with pilot astronauts to maintain the spacecraft and its equipment. They also conduct experiments, launch satellites, and perform spacewalks. Mission specialist astronauts in the military are paid according to their rank. Civilian mission specialist astronauts receive salaries based on an equivalent civil service rank. A third kind of astronaut is called a payload specialist.
Most payload specialists are scientists who work for the owner of the payload.
The CIS took over the program after the Soviet Union broke up in 1991. Before that time, astronauts were selected in groups of 7 to 35 people. All applicants must be U.S. citizens and must hold a bachelor s degree or higher in engineering, biological or physical science, or mathematics. There is no age limit, but all candidates must pass the NASA space flight physical during a week of medical evaluations and interviews. Pilot astronaut candidates require 1,000 hours as a command pilot in high-performance jet aircraft.