His most important work, published in May 1903, was "Exploration of Outer Space by Means of Rocket Devices" (). Tsiolkovsky calculated, using the Tsiolkovsky equation, that the horizontal speed required for a minimal orbit around the Earth is 8,000 m/s (5 miles per second) and that this could be achieved by means of a multistage rocket fueled by liquid oxygen and liquid hydrogen. In the article "Exploration of Outer Space by Means of Rocket Devices", it was proved for the first time that a rocket could perform space flight. In this article and its subsequent sequels (1911 and 1914), he developed some ideas of missiles and considered the use of liquid rocket engines.