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The control of trace contaminants on the International Space Station (ISS) is carried out by a combination of activated carbon absorption and catalytic oxidation. The carbon bed absorbs most hydrocarbons, chloro and chlorofluorocarbons (CHCs and CFCs) while the catalytic oxidizer removes compounds such as methane, ethylene, ethane, and carbon monoxide that cannot be absorbed by the charcoal bed. Unfortunately, the Space Station catalyst of 0.5% palladium on alumina does not effectively oxidize CHCs and CFCs, and in fact is powerfully poisoned by them (Wright et al. 1996). Thus, even though the charcoal bed has little affinity for CFCs and CHCs, it must be sized to completely remove these compounds in order to protect the crew and prevent poisoning of the catalytic oxidizer. TDA Research Inc. (TDA), under contract to NASA-JSC, has designed, built, and tested an all-catalytic trace contaminant control system (TCCS) to be used in Phase III of the Early Human Testing Program.

The possibility and consequences of a fire on board a spacecraft and the subsequent effects of the resultant toxic gases and smoke on the crew, equipment and mission is an ever-present hazard for the National Aeronautics and Space Administration (NASA). The necessity to remove these contaminants in the presence of high levels of humidity and carbon dioxide has prompted the development of a new prototype atmospheric filter (smoke eater) that can scrub acid gases, basic gases, and carbon monoxide from a spacecraft atmosphere in a post-fire event to a concentration below one half the Spacecraft Maximum Allowable Concentration (SMAC) levels. TDA Research, Inc. (TDA) is developing an advanced smoke eater to remove combustion byproducts. The material makeup of the smoke eater will also be applicable to spacecraft evacuation masks and the shipboard atmospheric revitalization system.

The Trace Contaminant Control System (TCCS) removes most hazardous contaminants from the space station atmosphere using a carbon bed, but some must be destroyed in a high temperature catalytic oxidizer. While the oxidizer is protected from catalyst poisons by the carbon bed, if contaminant loads are greater than anticipated, the catalyst may be exposed to a variety of poisons. Thus, we studied the effect of halocarbons, sulfides and nitrogen compounds on the catalytic activity and the products produced. We found that even if poisoning occurs, the catalyst will recover, and will not produce toxic partial oxidation products.

The current Extravehicular Mobility Unit (EMU) system provides thermal control using a sublimator to reject both the heat produced by the astronaut's metabolic activity as well as the heat produced by the Portable Life Support Unit (PLSS). This sublimator vents up to eight pounds of water each Extravehicular Activity (EVA). If this load could be radiated to space, the amount of water that would need to be sublimated could be greatly reduced. There is enough surface area on the EMU that almost all of the heat can be rejected by radiation. Radiators, however, reject heat at a relatively constant rate, while the astronaut generates heat at a variable rate. To accommodate this variable heat load, NASA is developing a new freeze tolerant radiator where the tubes can selectively freeze to “turn down” the radiator and adjust to the heat rejection requirement. This radiator design significantly reduces the amount of expendable water needed for the sublimator.

Paradoxically, trace contaminant control systems that suffer unexpected upsets and malfunctions can release hazardous gaseous contaminants into a spacecraft cabin atmosphere causing potentially serious toxicological problems. Trace contaminant control systems designed for spaceflight typically employ a combination of adsorption beds and catalytic oxidation reactors to remove organic and inorganic trace contaminants from the cabin atmosphere. Interestingly, the same design features and attributes which make these systems so effective for purifying a spacecraft’s atmosphere can also make them susceptible to system upsets. Cabin conditions can be contributing causes of phenomena such as adsorbent “rollover” and catalyst poisoning can alter a system’s performance and in some instances release contamination into the cabin. Evidence of these phenomena has been observed both in flight and during ground-based tests.

The development of a portable diode laser based gas sensor and its application to sensitive, selective, on-line monitoring of formaldehyde concentrations present in a catalytic Trace Contaminant Control System (TCCS) in a 5-day period in August 1999 is reported. The TCCS was originally developed for the Lunar-Mars Life Support Test program in 1996-1997 at NASA-JSC. The motivation for monitoring H2CO levels in a sealed human rated environment is that its presence can cause headaches, throat and ear irritation at low concentrations (>100 ppb), and more serious adverse effects at higher concentration levels. Consequently, NASA has established a spacecraft maximum allowable H2CO concentration of 40 ppb for crew exposure for a 7 to 180 days period [1].