ISS Propulsion Module

The ISS Propulsion Module was proposed as a backup for functions normally carried out by the Zvezda Service Module and by Progress spacecraft. Because guidance, navigation, control, and propulsion for the International Space Station are provided only by Russian systems and by the European ATV cargo spacecraft, a Propulsion Module would have been needed to maintain the station's altitude, perform reboosts and debris avoidance maneuvers, and provide attitude control and propellant if the Zvezda module were ever unavailable. In that scenario, the Interim Control Module would have covered the gap at first, since it only had a three year working lifetime before a Propulsion Module would have become necessary. This description is adapted from Wikipedia contributors under CC BY-SA 4.0; changes were made. https://creativecommons.org/licenses/by-sa/4.0/