Atom detail
The realistic proposal of space travel goes back to Konstantin Tsiolkovsky. His most famous work, “???? ??? ??? ??? ?????” (The Exploration of Cosmic Space by Means of Reaction Devices), was published in 1903, but this theoretical work was not widely influential outside of Russia.
Spaceflight became an engineering possibility with the work of Robert H. Goddard’s publication in 1919 of his paper ‘A Method of Reaching Extreme Altitudes’; where his application of the de Laval nozzle to liquid fuel rockets gave sufficient power that interplanetary travel became possible. This paper was highly influential on Hermann Oberth and Wernher Von Braun, later key players in spaceflight.
The first rocket to reach space was a prototype of the German V-2, on a test flight on October 3, 1942. On October 4, 1957, the Soviet Union launched Sputnik 1, which became the first artificial satellite to orbit the Earth. The first human spaceflight was Vostok 1 on April 12, 1961, aboard which Soviet cosmonaut Yuri Gagarin made one orbit around the Earth.
Rockets remain the only currently practical means of reaching space.