Astronauts are specially trained individuals who travel and work in space. They undergo rigorous preparation to operate spacecraft, conduct scientific experiments, and perform spacewalks. Key historical astronauts include Yuri Gagarin, the first human in space; Neil Armstrong, the first person to walk on the Moon; and Alan Shepard, the first American in space. The term "astronaut" means "star sailor" from Greek, and astronauts come from various countries, with different terms such as cosmonauts (Russia) and taikonauts (China). Astronauts live and work in microgravity, which causes unique physical effects like temporary height increase, and