The term space station is usually taken to denote a permanent inhabitable base that orbits the Earth or perhaps, in the future,
another planet. More broadly, it can be defined as any platform for humans and instruments in outer space. Space stations have been envisioned as bases for scientific research, as facilities for manufacturing, and as locales for large human colonies. Hermann Oberth, the father of modern astronautics, was the first seriously to explore the problems of building such a station. In 1929, Hermann Noordung (a pseudonym for an Austrian army captain named Potocnik) wrote The Problems of Space Flight, in which he described a wheel-shaped station that would rotate to provide artificial gravity. This was popularized by Wernher von Braun and others. Notable designs also include those of American physicist Gerard O'Neill, who proposed space colonies able to house thousands of persons in an Earth-like environment. The first true space station, Salyut 1, was launched by the Soviet Union on Apr. 19, 1971. It was followed by a number of other Salyut stations and by the more versatile Mir, which remained in orbit in the 1990s following the Soviet Union's breakup, with plans to have it burn up in the atmosphere by the end of that decade. The only true U.S. space station yet launched by the National Aeronautics and Space Administration (NASA) was Skylab, a preassembled workshop orbited on May 14, 1973. It housed three crews in 1973Ð74, and a few years later it reentered the Earth's orbit and disintegrated. The United States began planning a larger station, but studies suggested that the project was impractical without a reusable transportation system to supply it. Priority was thus switched to the Space Shuttle program as a first step. In 1984, President Ronald Reagan further announced a multimillion-dollar project to establish a permanent station, named Freedom, within a period of ten years. The European Space Agency (ESA), Japan, and Canada agreed to take part in the effort. The U.S. program thereafter was subjected to further budget cuts and downgrading and to a rethinking of its basic purpose. In 1994, after Russia expressed its willingness to take part, the multinational effort took the name of the International Space Station project. Russian space hardware is planned to be at the center of the project, while modules from NASA, ESA, and Japan and equipment and payloads from Brazil, aided by a robot arm from Canada, will make up the facility. Plans were for construction of the station to be concluded by the early 21st century. The International Space Station got under way with Russia's successful launch of a core module named Zarya ("Sunrise") on Nov. 20, 1998, using the three-stage Proton booster rocket. Zarya entered an initial elliptical orbit from which it is to be boosted into a circular orbit about 390 km (242 mi) above the Earth. Because of the severe problems facing Russia's economy, NASA contributed $240 million to the funding of the module. The first assembly sequence took place in December 1998 when the crew of the Space Shuttle Endeavour successfully linked the U.S. module Unity to Zarya.