Crew Space Transportation System (CSTS), or Advanced Crew Transportation System (ACTS), was a proposed design for a crewed spacecraft for low Earth orbit operations such as servicing the International Space Station, but also capable of exploration of the Moon and beyond. It was originally a joint project between the European Space Agency (ESA) and the Roscosmos, but later became solely an ESA project. This study was conceived as a basic strategic plan to keep a viable European human spaceflight program alive.