Some pilot astronauts have waited as long as 12 years before flying in space. While waiting, astronauts continue to work on various engineering assignments. Some astronauts become experts in several support or operational areas. Such special knowledge helps astronauts get assigned to flights on which their specialty is necessary. Once assigned to a crew, astronauts spend most of their time training in simulators.
Simulators are devices that reproduce the conditions of space flight.
Crew members spend as many as eight hours a day in simulators rehearsing every part of their mission. Instructors continually give the crew problems to solve and correct to prepare them for all possible emergency situations. Astronauts spend more time in simulators than in space. They regard the simulators as valuable preparation for what they will later face on actual flights. For example, the Apollo 13 astronauts used the oxygen and power supply of their lunar module to return home safely after an explosion damaged their main spacecraft.