Space stations are used to learn about how being in space for a long time effects the human body. They also are a place for science experiments and research. Space stations have been designed for more than one crew. Each crew member may stay aboard the station for weeks or months, and then is replaced by others. Some have spent more than one year at a time on a space station. This includes Vladimir Titov, Musa Manarov, Sergei Avdeyev and Valeriy Polyakov aboard "Mir".