Kvant / Mir modules
The add-on modules that turned Mir from a single core into a sprawling orbital complex
Background
Mir was designed to grow. Around its six-port core, module after module was launched and docked over a decade: Kvant-1 (astrophysics, 1987), Kvant-2 (an airlock and life support, 1989), Kristall (materials and a docking node for the Shuttle), Spektr and Priroda (research, and the American science modules of the international era).
Most were built on Chelomei's TKS functional-block heritage, the armed-station lineage repurposed for peaceful research. Each module added laboratories, greenhouses, instruments and living space, until Mir massed over 130 tonnes.
It was in these modules that the long-duration science matured: the greenhouses, the medical monitoring, the psychology of a crew sealed together for months, the real world the novel borrows for its secret experiment.