Current Composition
The segment currently consists of five modules, which together essentially comprise the base configuration of the cancelled Russian space station Mir-2. The segment is controlled directly from Roskosmos's Mission Control Center in Moscow.
The first module, Zarya, otherwise known as the Functional Cargo Block or FGB, was the first component of the ISS to be launched, and provided the early station configuration with electrical power, storage, propulsion, and navigation guidance, until a short time after the Russian service module Zvezda docked and was transferred control. Zvezda contains the ESA built DMS-R Data Management System. Now primarily used for storage, Zarya provides ports for Soyuz spacecraft, Progress spacecraft and the European ATV to dock to the station. Ships boosting the station's orbit dock to the aft port (the rear port according to the station's normal orientation and direction of travel). The FGB is a descendant of the TKS spacecraft designed for the Russian Salyut program. 5.4 tons of propellant fuel can be stored and transferred automatically to and from ships docked. Zarya was originally intended as a module for the Russian Mir space station, but was not flown as of the end of the Mir-1 program. Development costs for Zarya were paid for by Russia (and the former Soviet Union), spread across previous space station programs, and construction costs were partly paid for by the United States.
The second module, Zvezda, is the station's Service Module - it provides a living environment for the crew, contains the ISS's main engine system, and provides a docking port for Soyuz, Progress and Automated Transfer Vehicle spacecraft.
The third module, Pirs, functions as the ROS's airlock, storing EVA spacesuits and providing the equipment necessary for cosmonauts to exit the space station. It also serves as a docking compartment for Soyuz and Progress spacecraft.
The fourth module, Poisk, is similar to Pirs. Redundancy in airlocks allows one airlock to be repaired internally and externally whilst crew use the other airlock to exit and re-enter the station.
The fifth module, Rassvet, is primarily used for cargo storage and as a docking port for visiting spacecraft.
Read more about this topic: Russian Orbital Segment
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