Astronauts, which are also known as cosmonauts, are trained to serve in space on spacecraft. The term is also often used for scientists and others who travel in space, but are not professional astronauts. NASA prefers to use the term spaceflight participant to distinguish astronauts from space tourists.
Traditionally, Russian astronauts have been referred to as cosmonauts. The terms astronaut and cosmonaut are considered synonymous. The first group of astronauts for NASA was selected in 1959. These first astronauts were selected from the military and most of them were test pilots. Throughout the years, both NASA and other governments’ space organizations have developed prerequisites for selecting astronauts. These requirements include height restrictions, education, and experience. There are different requirements for different positions, which include commander, pilot, payload specialist, and mission specialist. The commander and pilot both must have pilot training and they must have 20/20 vision. They also undergo extensive astronaut training. The training involves experiencing weightlessness as well as time in planes and simulators for flying an actual Space Shuttle.
As of September 2009, there are 505 astronauts from 38 different countries who have reached space. The definition of reaching space is attaining at least 100 km or more in the atmosphere, although the U.S. only requires astronauts to reach 80 km. Only 24 people have actually traveled beyond a low Earth orbit, which is defined as extending to 2,000 km in altitude. All of the Space Shuttle and space station missions were within a low Earth orbit. The missions to the moon and the suborbital Mercury Program flights were beyond low Earth orbits.
The first astronaut into space was the Russian Yuri Gagarin who went up on April 12, 1961 and orbited Earth. The youngest astronaut, or cosmonaut, to ever go up was Gherman Titov who flew in the Vostok 2. He was also a Russian and went up in space when he was only 25. The oldest astronaut to fly in space was John Glenn, a distinguished astronaut who had gone on many missions. His last flight was at 77. Throughout the history of spaceflight, 18 astronauts have died on missions as of 2008. They were from various countries although America lost the most astronauts. Some of the most famous astronauts are Neil Armstrong, Alan Shepard, and James Lovell. Neil Armstrong was the first man to walk on the Moon while Alan Shepard was the first American in space. James Lovell was the commander on the Apollo 13 mission that almost ended in tragedy.
Universe Today has articles on Apollo astronauts and how to become an astronaut.
Astronomy Cast has an episode on the US Space Shuttle.