Konstantin Eduardovich Tsiolkovsky (1857 – 1935) was a Russian and Soviet rocket scientist and pioneer of the astronautic theory.
He theorized many aspects of space travel and rocket propulsion. His most important work, published in 1903, was The Exploration of Cosmic Space by Means of Reaction 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). In 1903 he published an article in which for the first time it was proved that a rocket could perform space flight.
Robert Hutchings Goddard (1882 – 1945) was an American professor, physicist, and inventor.
He’s credited with creating and building the world’s first liquid-fueled rocket, which he successfully launched in 1926. Two inventions of Goddard’s 214 patented - a multi-stage rocket, and a liquid-fuel rocket - were important milestones toward spaceflight. He was first to fire a liquid-fuel rocket faster than the speed of sound. Goddard successfully applied three-axis control, gyroscopes and steerable thrust to rockets, to effectively control their flight.
Hermann Julius Oberth (1894 – 1989) was an Austro-Hungarian-born German physicist and engineer.
In 1929, Oberth conducted a static firing of his first liquid-fueled rocket motor. He became a mentor to a student by the name of Wernher von Braun, who would later become a giant in both German and American rocket engineering from the 1940s onward, culminating with the gigantic Saturn V rockets that made it possible for men to land on the Moon in 1969. Oberth wrote a book The Rocket into Planetary Space, which explained how rockets could escape the Earth’s gravitational pull. After receiving a patent for his rocket design, Oberth’s first rocket was launched in 1931.
These three men made possible to explore the Universe.