They mean the same thing and differ by programme. Astronaut is used by NASA, ESA and most Western agencies; cosmonaut by the Russian programme and formerly the Soviet one. China uses taikonaut in English-language contexts, and India has adopted vyomanaut for its programme.
The core difference
One role, several national words.
- astronaut — NASA and most Western agencies. Greek: star sailor.
- cosmonaut — Russian programme. Greek: universe sailor.
- taikonaut — used in English for Chinese crew, from Mandarin taikong, space.
- vyomanaut — Indian programme, from Sanskrit vyoma, sky.
Where the boundary is drawn
Agencies also disagree on where space begins. The Fédération Aéronautique Internationale uses the Kármán line at 100 km; the US has often used 80 km for awarding astronaut wings. So whether a given flight makes someone an astronaut can depend on which definition applies.
The shared ending
All of these use the Greek nautes, sailor — the same element as in nautical and Argonaut. Space travel was named on the model of seafaring, which is why every version is a kind of sailor.
