Human spaceflight is conducted as part of space exploration, the endeavour to reach, explore, and exploit the space outside the Earth's atmosphere, and also in commercial activities, such as space tourism. The first human spaceflight, Vostok 1, was conducted in 1961. Since then, more than 500 people have travelled past the Kármán line, the official edge of space, in support of various Earth orbital missions, space station expeditions, spacewalks and missions to the Moon.
Though the mission included both joint and separate scientific experiments (including an engineered eclipse of the Sun by Apollo for Soyuz to take photographs of the solar corona) and provided useful engineering experience for future joint US/Russian space flights such as the Shuttle-Mir Program and the International Space Station, its primary purpose was symbolic. ASTP was a symbol of détente that the two superpowers were pursuing at the time, and it ended the tension of the Space Race.
O'Neill began researching high-energy particle physics at Princeton in 1954 after he received his doctorate from Cornell University. Two years later, he published his theory for a particle storage ring. This invention allowed particle physics experiments at much higher energies than had previously been possible. In 1965 at Stanford University, he performed the first colliding beam physics experiment.
While teaching physics at Princeton, O'Neill became interested in the possibility that humans could live in outer space. He researched and proposed a futuristic idea for human settlement in space, the O'Neill cylinder, in "The Colonization of Space", his first paper on the subject. He held a conference on space manufacturing at Princeton in 1975. Many who became post-Apollo-era space activists attended. O'Neill built his first mass driver prototype with professor Henry Kolm in 1976. He considered mass drivers critical for extracting the mineral resources of the Moon and asteroids. His award-winning book The High Frontier: Human Colonies in Space inspired a generation of space exploration advocates. He died of leukemia in 1992.