285 SYSTEMS: EXTRAVEHICULAR ACTIVITIES—BUILDING A SPACE STATION CHAPTER 17 217 mm Hg (4.3 psi), working in the suit can be fatiguing, especially for a crew member’s hands. At pressures this low, air does not supply the necessary quantity of breathing oxygen, hence the pure oxygen atmosphere. The lowest allowable oxygen pressure in which humans can operate safely is approximately 160 mm Hg (3.1 psi). However, a pressure that low does not allow any margin for suit leaks or normal variation in system components. Very low suit pressure also increases the risk that the crew member will develop decompression sickness (DCS)—i.e., “the bends” that scuba divers work to avoid. Therefore, the EMU pressure of 217 mm Hg (4.3 psi) was selected in a balance of these factors. Oxygen is supplied by the primary oxygen supply tanks (~44000 mm Hg [850 psi]). Or, if needed, oxygen can be supplied by the Secondary Oxygen Package (SOP) tanks (~258,000- 310,000 mm Hg [5000-6000 psi]). The pressure regulation system uses the primary oxygen exclusively on an EVA unless the pressure in the suit drops to approximately 201 mm Hg (3.9 psi) due to primary oxygen depletion or a leak in the suit. At that point, the SOP will “come on line” to keep the suit pressure around 201 mm Hg (3.9 psi). The SOP tanks are sized to provide enough oxygen to keep the suit pressurized while the crew member quickly translates back to the airlock in the case of a small leak, and also for potential failures that would require the crew member to “purge” the suit by intentionally opening one of two small holes to allow gas to flow out. Purging might be needed if, for example, the fan shuts down and dangerous levels of exhaled carbon dioxide start to accumulate in the helmet. In these cases, the primary oxygen tanks will become depleted faster, requiring the SOP to provide the necessary make- up oxygen to allow the crew member enough time to reenter the airlock. During an EVA, a fan circulates the oxygen, and the crew generates carbon dioxide, heat, humidity, and other contaminants. The flow is forced through a Contaminant Control Cartridge (CCC), which is a replaceable container that “scrubs” (i.e., removes) carbon dioxide and other contaminants from the gas environment. The CCC contains filters and charcoal to remove contaminants and odor, and either heritage Space Shuttle-based lithium hydroxide (LiOH) or ISS-based metal oxide (Metox). The LiOH or Metox removes carbon dioxide through a chemical reaction, introducing heat and water vapor into the oxygen flow. After the flow is cooled and humidity is condensed out, the newly scrubbed oxygen combines with fresh oxygen from the primary system and is introduced to the crew member’s helmet, blowing over the face. The CCC is changed out prior to each EVA. LiOH cartridges are used only one time, whereas Metox is a regenerable cartridge that was created for ISS use and can be reused on a subsequent EVA after the carbon dioxide is removed using a specialized heating system in the airlock (Figure 4). Gas Trap PLSS Structure PLSS Shear Panel MMU Mount Isolation Valve Assembly Battery Optical Connection CCC Fan/Separator/ Pump/Motor Assy Primary 02 Tanks 02 Subsystem C02 Sensor and Bracket Primary 02 Regulator Assy and 02 Actuator H20 Tanks Hut Mtg Interface Sublimator Caution and Warning System Valve Module EVC Antenna Vent Flow Sensor Muffler Credit: “Suited for Spacewalking: An Activity Guide for Technology Education, Mathematics, and Science, EG-1998-03-112-HQ.” Figure 4. Some key life support system components in the backpack of the EMU. Temperatures in low-Earth orbit reach extremes of approximately -93ºC to +149ºC (-200ºF to +300ºF). The EMU keeps the crew member comfortable, although he or she can get too hot or cold depending on a variety of situations. Factors include