Skylab was a 70s Earth orbiter, launched by the United States in 1973. It was the first space station to be placed in orbit and was visited by three crews of astronauts during its operational life. Skylab was designed to study the effects of long-term exposure to zero gravity on humans, as well as to conduct experiments in astronomy, solar physics, and Earth observation. Skylab remained in orbit until 1979, when it re-entered the atmosphere and broke apart over the Indian Ocean and Western Australia.