CSTS (Crew Space Transportation System) or ACTS (Advanced Crew Transportation System) was a human spaceflight system proposal.
It was originally a joint project between the European Space Agency (ESA) and the Russian Space Agency (FKA), but is now solely an ESA project. It aims to design a spacecraft for low earth orbit operations such as servicing the International Space Station but also capable of exploration of the Moon and beyond. This study was conceived as a basic strategic plan to keep a viable European human space program alive when NASA officials announced the NASA's Vision for Space Exploration Orion spacecraft.
CSTS had completed an initial study phase, which lasted for 18 months from September 2006 to spring 2008, before the project was shut down before an ESA member state conference in November 2008.