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 interplanetaryspacecraft 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:
Large spacecraft assembly
Flight tests and launches
Creating, servicing and completing inter-orbital tugs
Providing medical and biological conditions required for the rehabilitation of interplanetary expedition crews after their return to Earth orbit.
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.
Rassvet, left of center with the Russian science airlock temporarily stored on its side.
2000: Zvezda (DOS-8) - potential part of OPSEK.
2009: Poisk (MRM-2) - potential part of OPSEK - Poisk (Russian: По́иск; lit. Search), also known as the Mini-Research Module 2 (MRM-2), Малый исследовательский модуль 2, or МИМ 2. Poisk is a Russian airlock module with two identical hatches. Its predecessor, Pirs, was used to store, service, and refurbish Russian Orlan suits. The outermost docking ports on both airlocks allow docking of Soyuz and Progress spacecraft, and the automatic transfer of propellants to and from storage on the station.
2021: Nauka (FGB-2) - initially planned to form part of OPSEK - Nauka (Russian: Нау́ка; lit. "Science"), also known as the Multipurpose Laboratory Module (MLM) or FGB-2, (Russian: Многофункциональный лабораторный модуль, or МЛМ), is the major Russian laboratory module. This module was planned to be separated from the ISS before de-orbit, with support modules, to become the OPSEK space station. It contains an additional set of life support systems and orientation control. Nauka's mission has changed over time; during the mid-1990s it was intended as a backup for the first FGB, and later as a Universal Docking Module (UDM). Its docking ports will be able to support automatic docking of both spacecraft, additional modules and fuel transfer. Prior to the arrival of the MLM in July 2021, a Progress robotic spacecraft docked with the International Space StationPirs module, departed with that module, and both were discarded. Nauka then used its own engines to attach itself to the ROS. The European Robotic Arm, which will service the Russian Orbital Segment, was launched alongside the MLM.
2021: Prichal - also known as the Node Module (UM)/(NM). This 4-ton ball shaped module will support the docking of two scientific and power modules during the final stage of the station assembly and provide the Russian segment additional docking ports to receive Soyuz and Progress spacecraft. Prichal is to be incorporated into the ISS in 2021. It will be integrated with a special version of the Progress cargo ship and launched by a standard Soyuz rocket. The Progress would use its own propulsion and flight control system to deliver and dock Prichal to the nadir (Earth-facing) docking port of the Nauka MLM/FGB-2 module. One port is equipped with an active hybrid docking port, which enables docking with the MLM module. The remaining five ports are passive hybrids, enabling docking of Soyuz and Progress vehicles, as well as heavier modules and future spacecraft with modified docking systems. More importantly, the node module was conceived to serve as the only permanent element of OPSEK. Equipped with six docking ports, the Node Module would serve as a single permanent core of the future station with all other modules coming and going as their life span and mission required. Uzlovoy will be launched after Nauka MLM; launch dates will be 2022 or later.
2010, Rassvet (MRM-1) - currently docked at Zarya; if Zarya is not utilized in OPSEK, it would have to be moved to another ROS/OPSEK docking location.
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OPSEK information page
Presentation.pdf[dead link], by Head of Russian Federal Space Agency, 17 June 2009
Roscosmos YouTube channel
RSC Energia: Science Research on ISS Russian Segment