The
ISS Propulsion module was proposed as a backup to functions performed by the Zvezda Service Module and Progress spacecraft. Critical ISS functionality such as guidance, navigation, control and propulsion are provided only by Russian and the European (ATV) spacecraft. A Propulsion Module would have been needed for ISS altitude maintenance and reboost, debris avoidance maneuvers, attitude control and propellant supply in the event the Zvezda Service Module was not available to the International Space Station. If the Zvezda had not been available, the Interim Control Module would have been used at first. It only had a lifetime of three years; then the Propulsion Module would have been necessary.