|Crew||2 or more|
|Launch||Abandoned in 2017|
|Launch pad||Baikonur Cosmodrome|
|Mass||Over 100,000 kg when complete|
|Atmospheric pressure||1 atm|
|Periapsis altitude||370 to 450 km (planned)|
|Apoapsis altitude||370 to 450 km|
|Typical orbit altitude||370 to 450 km|
The Orbital Piloted Assembly and Experiment Complex (Russian: Орбитальный Пилотируемый Сборочно-Экспериментальный Комплекс, Orbital'nyj Pilotirujemyj Sborochno-Eksperimental'nyj Kompleks) (ОПСЭК, OPSEK) was a 2009–2017 Russian proposed third-generation 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, in the concept, 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 of the International Space Station (ISS). However, in September 2017, the head of Roscosmos Igor Komarov said that the technical feasibility of separating the station to form OPSEK had been studied and there were now "no plans to separate the Russian segment from the ISS... We keep the same position, that we should work on the ISS together with our partners".
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 Russian Orbital Service Station, operated entirely by Roscosmos, would be launched starting in the mid-2020s.
Around the predicted decommissioning of the International Space Station in the late 2020s, the Russian Federal Space Agency (Roscosmos) developed a concept in 2009 to construct a successor station in low Earth orbit.
The 2009 concept considered re-using several ISS modules to form the initial parts of a new station, which were to be subsequently replaced by new modules. On 17 June 2009, Roscosmos officially informed its ISS partner NASA about its intention to "build and prepare for operation the first elements of the orbital assembly and experimental piloted space complex by the end of the ISS life cycle". As of 2017,[update] those plans had been abandoned, and the new station was to be composed entirely of new purpose-built modules.
According to the Russian crewed spaceflight contractor RKK Energia, the proposed station would have needed to perform the following tasks:
OPSEK was to have followed the Salyut and Almaz series, Kosmos 557, and Mir as the 12th Russian space station launched. OPSEK is a third generation  modular space station.
Examples of other modular stations include the former Soviet/Russian Mir, the International Space Station, and the Chinese Tiangong space station. The first space station, Salyut 1, and other one-piece or "monolithic" first generation space stations, such as Salyut 2, 3, 4, 5, DOS-2, Kosmos 557, Almaz, and NASA's Skylab station, were not designed for re-supply. Generally, each crew had to depart the station to free the only docking port for the next crew to arrive. Skylab had more than one docking port but was not designed for resupply. Salyut 6 and 7 had more than one docking port and were designed to be resupplied routinely during crewed operation. Modular stations can allow the mission to be changed over time and new modules can be added or removed from the existing structure, allowing greater flexibility.
Expected Russian Orbital Segment modules around the time of OPSEK separation (2020 or later) arranged by launch dates:
Russian Orbital Segment modules scheduled for de-orbiting:
Russian Orbital Segment modules that were neither scheduled for de-orbit nor included in the OPSEK proposals:
Komarov: We have no plans to separate the Russian segment from the ISS... We keep the same position, that we should work on the ISS together with our partners.