Orbital Piloted Assembly and Experiment Complex

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 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.

Source: Wikipedia — Orbital Piloted Assembly and Experiment Complex (CC BY-SA 4.0)