Astronauts must be extremely fit and also have very good eyesight.
The American space agency NASA trains its astronauts at the Johnson Space
Center near Houston, Texas.
The US space shuttle carries three kinds of astronaut – pilots, mission
specialists and payload specialists.
The pilot or commander’s job is to head the mission and control the spacecraft.
Mission specialists are crew members who carry out specific jobs, such as running experiments or going on space walks.
Payload specialists are not NASA astronauts, but scientists and other on-board guests.
Astronauts learn scuba diving to help them deal with space walks.
To cope with the demands of space missions and to help them deal with weightlessness, astronauts undergo tough physical training.
They also spend long hours in simulators and jet aircraft.
During training, astronauts experience simulated (imitation) weightlessness – first in a plunging jet aircraft, and then in a water tank. They are also exposed to very high and very low atmospheric pressure.
Weightlessness can make astronauts grow several centimeters in height during a long mission.