The Orbital Piloted Assembly and Experiment Complex (, Orbital'nyj Pilotirujemyj Sborochno-Eksperimental'nyj Kompleks; ОПСЭК, OPSEK) was a 2009–2017 proposed third-generation Russian modular space station for low Earth orbit. The concept was to use OPSEK to assemble components of crewed interplanetary spacecraft destined for the Moon, Mars, and possibly Saturn. The returning crew could also recover on the station before landing on Earth. Thus, OPSEK could form part of a future network of stations supporting crewed exploration of the Solar System.
In early plans, the station was to consist initially of several modules from the Russian Orbital Segment (ROS) of the International Space Station (ISS). However, after studying the feasibility of this, the head of Roscosmos stated in September 2017 the intention to continue working together on the ISS. In April 2021, Roscosmos officials announced plans to exit from the ISS programme after 2024, stating concerns about the condition of its aging modules. The OPSEK concept had by then evolved into plans for the Russian Orbital Service Station (ROSS), which would be built without modules from the ISS, and was anticipated to be launched starting in the mid-2020s.[1][2]