Astronauts (and cosmonauts) are people specially trained to work in space, spending weeks or months carrying out mission tasks and research in science on a space station or spacecraft. The first astronaut was Yuri Gagarin, who was launched into space in April 1961 aboard Vostok 1. The first woman astronaut was Valentina Tereshkova, who was launched into space in June 1963 aboard Vostok 6.

The term "astronaut" derives from the Greek words meaning "space sailor," and refers to all who have been launched as crew members aboard NASA spacecraft bound for orbit and beyond. Since the inception of NASA's human space flight program, we have also maintained the term "astronaut" as the title for those selected to join the NASA corps of astronauts who make "space sailing" their career profession. The term "cosmonaut" refers to those space sailors who are members of the Russian space program.

A space station is a construction in space where astronauts can live and work. The Russian Mir has been orbiting the Earth since 1986, and the crew is replaced every 2-3 months. The International Space Station (ISS) represents a permanent human presence in space. It is located in orbit around the Earth at an altitude of approximately 386 km, a type of orbit usually termed 'low Earth orbit'. (The actual height varies over time by several kilometres due to atmospheric drag and reboosts.) It is serviced primarily by the Space Shuttle, and Soyuz and Progress spacecraft units. It is still being built, but is home to some experimentation already. At present, the station has a crew of three, mostly from Russia and the United States, but occasionally from some of the other partners in the project.

Manned space missions beyond Earth orbit have been carried out by the United States only: to the Moon. NASA's Apollo program landed twelve people on the Moon and returned them to Earth: Apollo 11-17, except 13, i.e. six missions, with each time three astronauts of which two landed on the Moon.

The farthest that astronauts have been is to the Moon, and their next likely destination is Mars. Great advances in technology will be needed before they can travel to any destinations beyond the Solar System; current propulsion systems are far too slow to cover the vast interstellar distances during a human lifetime.

Experiments are carried out in space to determine the conditions there, to discover their effects on materials and life, providing advances in science, physics, technology, medicine, etc. Work includes maintaining the equipment and launching and repairing satellites.

Spacecraft pilots are trained as military pilots. Pilot astronauts serve as both space commanders and pilots. During flight, the commander has onboard responsibility for the vehicle, crew, mission success, and safety of flight. The pilot assists the commander in controlling and operating the vehicle and may assist in the deployment and retrieval of satellites using the remote manipulator system (RMS), referred to as the robot arm or mechanical arm.

Mission specialists are highly qualified engineers or scientists. Mission specialist astronauts work with the commander and the pilot and have overall responsibility for coordinating operations in the following areas: systems, crew activity planning, consumables usage, and experiment/payload operations. Mission specialists are trained in the details of the onboard systems, as well as the operational characteristics, mission requirements/ objectives, and supporting equipment/systems for each of the experiments conducted on their assigned missions. Mission specialists perform extravehicular activities (EVAs), or space walks, operate the remote manipulator system, and are responsible for payloads and specific experiment operations.

Payload specialists are persons other than NASA astronauts (including foreign nationals) who have specialized onboard duties; they may be added to shuttle crews if activities that have unique requirements are involved and more than the minimum crew size of five is needed.

First consideration for additional crew members is given to qualified NASA mission specialists. When payload specialists are required they are nominated by NASA, the foreign sponsor, or the designated payload sponsor. In the case of NASA or NASA-related payloads, the nominations are based on the recommendations of the appropriate Investigator Working Group (IWG).

Although payload specialists are not part of the Astronaut Candidate Program, they must have the appropriate education and training related to the payload or experiment. All applicants must meet certain physical requirements and must pass NASA space physical examinations with varying standards depending on classification.

Training prepares astronauts for the unusual conditions in space and during take-off. They are launched by massive rockets (e.g. in the Space Shuttle) which creates a force of up to 6g (6 times gravity). To prepare for weightlesness, they train in giant water tanks and high-altitude aircraft.

Any adult man or woman in excellent physical condition who meets the basic qualifications can be selected to enter astronaut training. For mission specialists and pilot astronauts, the minimum requirements include a bachelor's degree in engineering, science or mathematics from an accredited institution. Three years of related experience must follow the degree, and an advanced degree is desirable. Pilot astronauts must have at least 1,000 hours of experience in jet aircraft, and they need better vision than mission specialists. Competition is extremely keen, with an average of over 4,000 applicants for about 20 openings every 2 years. Astronaut recruiting occurs periodically.

Over 40% of astronauts suffer from 'space sickness' for the first few days because their sense of balance depends on gravity. Lack of gravity also reduces their number of red blood cells carrying oxygen, causing tiredness.

Astronauts can grow 2" (5 cm) taller in space, and their heart, muscles, and bones weaken. These changes can be controlled with a special diet and by performing regular exercise in a gym.

Spacecraft are bombarded by radiation particles that would normally be absorbed by the Earth's atmosphere. Each astronaut carries an instrument to measure their exposure to this radiation, which limits the amount of time that astronauts can spend in orbit.