Contamination Control Onboard Space Station Alpha
By SUSAN ENGLISH
Marshall Space Flight Center, AL–The cabin of a spacecraft represents the worst case in “tight building” design. Air quality control aboard a spacecraft, or space station, has its own unique challenges, analogous, in many ways, to those found in today`s “tight” buildings–made even “tighter” by the fact that the inhabitants of this space “building” will not be allowed to go home at the end of the day to breathe the relatively fresh air of planet Earth! This is pretty much what life onboard.
NASA`s 16-nation International Space Station Alpha (ISSA) will be like: Six scientists will conduct scientific research and also live in a permanently manned space station consisting of 36,226 ft.3 of pressurized air, a space roughly equivalent to two Boeing 747 passenger cabins.
The newest station partner–the Russian Space Agency–will be participating in a series of cooperative U.S./Russia missions in preparation for Phase II–assembly of the space station itself–that include cosmonaut flights in the U.S. Shuttle Atlantis and reciprocal flights of American astronauts aboard Russia`s Mir space station, which has been in earth orbit since 1986. A major part of Phase 1 of the program involves joint sampling and monitoring of air inside both the U.S. Shuttle and Mir. The Russians bring to the ISSA project their extensive knowledge of air systems, a wealth of toxicological data, as well as practical knowledge of long-duration space flights and their effects on humans. (The Mir space station has been in orbit roughly nine years, and the U.S. shuttle Atlantis is scheduled to dock with it this June.)
Scheduled for completion by the year 2002, the space station will operate at an altitude of 240 nautical miles above the earth. Crews will serve six-month stints inside the habitation and laboratory modules. The presence of a human laboratory staff will provide long-awaited opportunities to perform research investigations that can be carried out in no other way–in a microgravity environment that will allow the flexibility of long-term experiments with real-time changes–investigating the most basic mechanics of life and matter. Such research is expected to provide a wide spectrum of practical applications, including the improved treatment of diseases, major advances in computer technology and robotics, improved resources management, and improved air and water quality.
Life in a small air volume and a large crew-to-volume ratio means “continuous exposure” to a pressurized atmosphere replete with chemical contaminants, where even a very small amount of contamination can cause problems which may threaten the physical and emotional well-being of the crew members and even jeopardize the mission. Just as in today`s building market, where the emphasis is on energy efficiency and very little fresh air is brought in from the outside, so too, exposure on a spacecraft to such substances as formaldehyde, alcohols, and ammonia can produce irritation of the eyes, throat, nasal passages, and mucous membranes, as well as chronic headaches, nausea, and fatigue. For example, methyl alcohol is used in wipes for cleaning surfaces, personal hygiene items, and as a solvent in cleaning.
Environmental Control and Life Support System
NASA has developed a combination of passive and active contamination control concepts to be implemented during all three phases of the Alpha program. Called ECLSS (environmental control and life support system), they include the evaluation and selection of materials to be used onboard the spacecraft, the establishment of air quality standards, and the use of active control means to minimize cabin atmosphere pollution.
The purpose is to provide both crew and scientists with an environment that is as free of pollutants as possible, with target concentrations up to 100 times lower than those specified for terrestrial indoor environments by OSHA and the American Conference of Governmental and Industrial Hygiene (ACGIA). According to the latter, the exposure level for ammonia is 18 milligrams per cubic meter for spacecraft personnel over a 180-day period of continuous exposure, while the ACGIA figure is at 7, based on a time-weighted average (TWA) of an 8-hour, 5-day work week on earth; methyl alcohol is 13 ml/m3, ACGIA 260; benzene 0.2 ml/m3, ACGIA 30.
Because it is not practical to reduce large amounts of the station`s atmosphere, much of it must be recirculated through an atmospheric revitalization system (ARS), which provides active control of trace chemical contaminants and carbon dioxide. In order to minimize the weight, power, and volume resources required by ARS, another passive control approach, employing strict material selection and control guidelines, together with a rigorous on-orbit monitoring program, is used to achieve the most effective system from both a performance and life cycle economics standpoint. To control contamination originating from the human metabolism, atmospheric revitalization hardware is used, providing for physical and chemical adsorption and catalytic oxidation of trace chemical contaminants.
Passive Control
During the spacecraft cabin design phase, materials are selected according to criteria established by NASA to minimize contamination from outgassing and particulates. Particulate contamination is also a concern. Since most of it originates from the crew members themselves, every effort is made prior to flight to select clothing and food to minimize or eliminate particulate production. Onboard filtration is also provided to remove the particulates that are generated. Because the spacecraft must utilize lightweight components, a high percentage of nonmetallic materials are used. However, while nonmetallic materials are more cost effective, they generate increased trace contaminants from outgassing.
A material control plan defines the specifications for both metallic and nonmetallic materials to be used onboard. Test data for those materials are documented in a database and used to determine overall generation rates for each trace chemical contaminant. These rates are then used to analyze the ability of the onboard active contamination control systems to maintain concentrations below specified concentrations. A final, system-level outgassing test is performed as a verification of the overall process.
In-orbit Verification
Final verification of cabin atmosphere occurs during the mission. On Shuttle and Spacelab module missions, samples of the cabin atmosphere are taken periodically during the mission, using evacuated sample bottles and adsorbent traps, and then returned to NASA for analysis at the conclusion of the mission.
In April 1994, a NASA delegation traveled to Russia to negotiate air and water quality standards with scientists at the Institute of Medical/Biological Problems, part of Moscow`s Ministry of Public Health, to determine maximum allowable concentrations of air contaminants and address toxocology concerns. Air samples from the orbiting Mir were taken in March of this year, and will be analyzed by both sides in the coming months. In fact, a modification of the EPA`s TO-14 (Toxic Organic) Method, a standard method for detecting toxic organic pollutants in the air, will be used specifically to analyze the air samples for traces of benzene in response to the reported incidence of disturbingly high concentrations found in the Mir`s atmosphere, the source of which remains elusive. (Benzene is a suspected human carcinogen and a proven animal carcinogen.) However, according to J. L. Perry, an Engineer at NASA`s Life Support Systems branch, responsible for trace contaminant control system design for Spacelab and ISSA, amounts of benzene in the Mir atmosphere may have been exaggerated because of limitations in the analytical techniques used by the Russian scientists.
“Without detailed understanding of their analytical methods, it`s difficult to make any judgments. Now, we think it (the report) may be exceptionally biased on the high end. It`s an area of risk mitigation for Space Station Alpha and the Shuttle/Mir program (Phase 1). There is a risk. Our medical people would like to know when the Shuttle Atlantis actually docks with Mir in June if there could be problems with air quality.” The same question will be asked when a NASA astronaut is launched onboard a Russian Soyuz spacecraft to join Mir for a three-month stay in space. n