Sputnik was only the beginning, but it was also the culmination of efforts to get into space that dated back to the start of the 20th century. Of the several engineering challenges that had to be addressed along the way, the first and foremost was building a rocket engine with enough thrust to overcome the pull of gravity and lift a vehicle into orbit. Rockets themselves had been around for centuries, almost exclusively as weapons of war, driven by the burning of solid fuels such as gunpowder. By the 19th century it was clear to experimenters that, although solid fuel could launch missiles along shallow trajectories, it couldn't create enough thrust to send a rocket straight up for more than a few hundred feet. You just couldn't pack enough gunpowder into a rocket to blast it beyond Earth's gravity.

Three men of the 20th century can justly lay claim to solving the problem and setting the stage for spaceflight. Working independently, Konstantin Tsiolkovsky in Russia, Robert Goddard in the United States, and Hermann Oberth in Germany designed and, in Goddard's and Oberth's cases, built rocket engines propelled by liquid fuel, typically a mixture of kerosene or liquid hydrogen and liquid oxygen. Tsiolkovsky took the first step, publishing a paper in 1903 that mathematically demonstrated how to create the needed thrust with liquid fuels. Among his many insights was the notion of using multistage rockets; as each rocket expended its fuel, it would be jettisoned to reduce the overall weight of the craft and maintain a fuel-to-weight ratio high enough to keep the flight going. He also proposed guidance systems using gyroscopes and movable vanes positioned in the exhaust stream and developed formulas still in use today for adjusting a spacecraft's direction and speed to place it in an orbit of virtually any given height.

Goddard was the first to launch a liquid-fuel rocket, in 1926, and further advanced the technology with tests of guidance and stabilization systems. He also built pumps to feed fuel more efficiently to the engine and developed the mechanics for keeping engines cool by circulating the cold liquid propellants around the engine through a network of pipes. In Germany, Oberth was garnering similar successes in the late 1920s and 1930s, his gaze fixed steadily on the future. One of the first members of the German Society for Space Travel, he postulated that rockets would someday carry people to the Moon and other planets.

One of Oberth's protégés was responsible for rocketry's next advance. Wernher von Braun was a rocket enthusiast from an early age, and when he was barely out of his teens, the German army tapped him to develop a ballistic missile. The 20-year-old von Braun saw the work as an opportunity to further his own interests in spaceflight, but in the short term his efforts led to the V- 2, or Vengeance Weapon 2, used to deadly effect against London in 1944. (His rocket design worked perfectly, von Braun told a friend, "except for landing on the wrong planet.") After the war, von Braun and more than a hundred of his rocket engineers were brought to the United States, where he became the leading figure in the nation's space program from its earliest days in the 1950s to its grand achievements of the 1960s.