345 SYSTEMS: ENVIRONMENTAL CONTROL AND LIFE SUPPORT SYSTEM—SUPPORTING THE HUMAN ELEMENT OF THE ISS CHAPTER 19 lowered, which decreases the dew point on the station and, in turn, causes more water to condense out of the air. Another option would be to raise the temperature to reduce the amount of water collected via condensation. If the condensate tank is full but the WPA is not operating, the crew can drain the tank into a CWC, which later can be transferred back to the tank when it is empty. The OGA uses water to generate O 2 , which also has to be carefully balanced, thereby further complicating the process. The variables change often enough that water balance predictions are not accurate beyond 3 days in the future. Therefore, the ETHOS team must evaluate all these variables multiple times a day to ensure that the crews’ upcoming plan properly accommodates the water balance needs. The racks that make up the Water Recovery System and the OGA are shown in Figure 9. Emergencies In addition to maintaining life support, ETHOS must respond to emergencies. The three classes of emergencies on the ISS include fire, rapid depressurization (“depress”), or toxic chemical spill. Fire is a serious threat in space. In the event of a fire on Earth, one option involves quickly exiting a structure. By contrast, astronauts who live on the ISS can leave the station only as a last resort, given this results in the costly loss of vehicle utilization. Fire detection and suppression is part of the ECLSS. The first step in preventing fires is assuring the materials used on the ISS are fire- retardant. Strict rules are enforced to ensure the space station contains nothing that is highly flammable. The most significant way to mitigate fires in space is to make it difficult for one to start however, if a fire should occur, being in space helps. Fires in space are also difficult to maintain, due to weightlessness. On Earth, convection (the process of lighter, warmer air rising while cooler gas falls due to gravity) can replenish the consumed O 2 , thus allowing the fire to continue to burn. However, in the absence of forced airflow in space, convection is nonexistent once the O 2 around a fire is consumed, the fire will extinguish. Therefore, electrical parts of the ISS are usually located behind panels where airflow is not possible. If a fire should start for some reason, it will likely extinguish itself. Airflow Obscuration Photodiode Beam Dump Laser Diode Lens Hood Second Mirror First Mirror Mirror Assembly Slits Scatter Photo- diode Figure 10. Picture of a cabin smoke detector (left) and a schematic (right). Laser light bounces off of several mirrors into a photodiode detector. If particles of smoke are present, the beam will be obscured with a reduced brightness. Laser light will also scatter off the particles and into a second photodiode to ensure that a false alarm is not triggered by a single problem with the obscuration sensor. However, an electrical or chemical fire can still occur in a location such as a systems/payload rack or in the open cabin where there is regular airflow therefore, the fire won’t quickly extinguish. As a precaution, smoke detectors are placed throughout the ISS to alert the crew and flight controllers to a fire, much in the same way smoke detectors are used as a precaution on Earth. The detectors on the ISS are similar to many in terrestrial buildings. Laser light is monitored to see whether particles of smoke are blocking the light (Figure 10). Once smoke is detected, the software will automatically