213 DAY IN THE LIFE: EMPTY HOUSE—DECREWING THE INTERNATIONAL SPACE STATION CHAPTER 12 System Changes Using the priorities established above, the decrewed ISS configuration was developed to ensure the station would continue to fly safely and perform scientific research. Although there would not be a crew on board to respond to emergency situations or repair broken equipment, many of the decrewing actions dealt with minimizing the potential for these events or increasing the ground operators’ ability to handle these situations. The already-docked Progress vehicle, 42P, would be configured so that it could be undocked remotely by the Russian flight control team, if needed, yet continue to provide the ability to perform debris avoidance maneuvers (see Chapter 8). Similar to the Soyuz vehicle, this Progress had limited lifetime and would need to be undocked before posing a risk to the ISS, or before it would be unable to complete its mission. Also, undocking the Progress once its resources were expended would provide an additional open docking port for either a new Progress full of supplies or a redundant docking port for a returning crew. The most critical step in this preparation would be to have the crew remove the clamps that helped hold the vehicles together. Prior to its undocking, the ISS would be reboosted to a higher altitude to make use of available Progress propellant and prolong the orbital lifetime of the ISS in the event of a major delay of fuel resupply. Although the ISS has been designed to minimize the risk of an on-board fire, there is still the low likelihood that one could occur (see also Chapter 19). Since the crew would not be available to fight a fire, the risk would be further minimized by powering down non-essential equipment (based on priorities). This non-essential equipment includes crew-tended payloads and crew- support equipment such as the toilet. The crew also responds to ISS rapid depressurization emergencies potentially caused by orbital debris impacts by closing hatches between modules, which will hopefully isolate the leak to a single module. Initially, the team believed the best decrew configuration would be to isolate each module. Therefore, if a module started to depressurize, only that module would be affected. However, analysis of the isolated configuration for longer durations showed that a potentially large variation in pressure and temperature would occur between the modules. To keep the atmosphere of the ISS within temperature and pressure limitations, it would be best to keep air moving between the modules. The decrew configuration would have the crew close hatches prior to leaving, but Intermodular Ventilation (IMV) (see Chapter 19) would be left enabled, meaning vent ports and fans between modules would be left open and blowing. In the event of a depress, automatic software would deactivate the fans and close the vent ports, thereby isolating all of the modules. Leaving IMV enabled would assist with maintaining a uniform atmosphere between modules and could assist in some emergency events (see below). Humidity levels in the ISS atmosphere were another concern. Normally, water is added to the ISS atmosphere as the crew exhales and perspires. Humidity is removed from the atmosphere by the ISS Thermal Control System (TCS) to keep the crew comfortable, prevent condensation from damaging the equipment, and add to recycled water stores. This water is sent to the Regenerative Environmental Control and Life Support System (ECLSS) (see Chapter 19) for recycling. However, little to no water would be added to the atmosphere without a crew on board, and there was no safe controllable way for ground controllers to release water to increase humidity. Low humidity is a concern to the ISS critical electronics as it can lead to the buildup of static electrical charges. These static charges could potentially cause electrical arcs that would damage critical systems. These arcs would be similar to the shocks that can be felt when touching a metal door handle on a cold, dry winter day on Earth. To prevent drying out the ISS atmosphere too much prior to the crew leaving, the internal cooling loop temperatures would be raised to stop condensing water out of the atmosphere. The Environmental and Thermal Operating Systems (ETHOS) officer would need to actively monitor the fine balance of maintaining enough humidity in the atmosphere while preventing condensation on cooler surfaces. It is important to note that the ISS did not have a humidity sensor thus, all of the information ETHOS would use for this monitoring would be based on temperature data and analysis. If the analysis was wrong and the atmosphere dried out, the ground team would not be able to recover the correct humidity levels.