Tight Control of the Space Shuttle/Spacelab Environment

The shuttle/Spacelab facilities have given scientists increased opportunities to explore these and many other questions. Investigators are studying diverse life forms from cells to whole organisms, including the human body with its many complex systems. The Spacelab module offers enough room and power to bring some major pieces of laboratory equipment into space in an environment with regulated temperatures and pressure. The space laboratory equipment includes standard medical tools, multipurpose reusable minilabs, and plant and animal habitats.

Most importantly, the Spacelab module can support a staff of trained crew members and scientists. Life sciences research in space demands heavy crew involvement as expert investigators, test subjects, and laboratory technicians. Crew members draw and process blood samples, record their own physiological symptoms, set up and participate in a variety of experiments, tend plant and animal experiments, and carry on their work much as they do in laboratories on the ground.

The shuttle/Spacelab combination has provided humans with a valuable laboratory in space that is comfortable and suitable for a wide range of physiological studies. This is made possible, first and foremost, because of a very sophisticated and technologically stable environmental control system. This system controls the gas composition, pressure, and temperature so that the environment inside the Spacelab comes as close as possible to matching the Earth's environment that our bodies are used to functioning in. If you'll remember from the Introduction, atmospheric pressure is a measure of the force that is exerted by the gas molecules in the air on any surface area in contact with them. You will often hear atmospheric pressure expressed in units of torr (which is the same as millimeters of mercury) or else in units of pounds per square inch (p.s.i.), or you'll even hear it expressed as 1 atmosphere (1 atm). Atmospheric pressure decreases as you move to higher altitudes such as the mountains, but at sea level here on Earth, atmospheric pressure is

1 atm = 760 torr = 14.7 p.s.i.

The shuttle was the first human spacecraft to use a 1 4.7-p.s.i. atmospheric pressure. The gas composition that is maintained in the shuttle and Spacelab is 80% nitrogen (N₂) and 20% oxygen (O₂), essentially the same as on Earth (except for additional trace gases that are found in our atmosphere).

Other gases are controlled for in the shuttle environment, including carbon dioxide (CO₂), which is a product of our respiration (breathing). On Earth, the concentration of CO₂ increases in crowded indoor environments where there are a lot of people breathing and exhaling CO₂. As the astronauts breathe, CO₂ is produced and exhaled into the shuttle atmosphere. On Earth, there is usually adequate ventilation inside to allow the CO₂ to escape an enclosed room. In contrast, the shuttle is a completely closed environment and the CO₂ cannot escape, so it must be actively removed. If it were not removed, the CO₂ levels would rise and cause the astronauts to experience increases in their heart rate and respiration rate, as well as problems with the acid-base balance of their bodies. These problems could become very dangerous. Therefore, special filters (disposable lithium hydroxide cartridges) are used to absorb and remove CO₂ from the spacecraft atmosphere. These filters are part of a larger air filter system that also uses activated charcoal to remove odors from the air.

Water vapor is also a normal constituent of Earth's atmosphere. The concentration and pressure of water vapor in the air will determine the humidity levels. The humidity is a very important aspect of the environment, must be controlled. High humidity can promote the rapid growth of microbes or fungus. Low humidity can cause drying of the eyes and skin and the mucous membranes of the nose and throat, as well as chapping of the lips. Also, in low humidity the cilia (hairlike structures) of the respiratory tract can become dry, thus providing less protection against respiratory infections. Water vapor pressure of 10 torr (0.19 p.s.i.) is optimum for habitability. The shuttle environmental control system controls water vapor pressure between 6 torr and 14 torr (0.12-0.27 p.s.i.), which is a comfortable range.

The temperature of the atmosphere is an important aspect of the heat balance that must be maintained in the bodies of the astronauts. The shuttle provides temperature control within a range of 1 8°C to 27°C (64°-81 °F). Since there are differences in opinion over what temperature is the most comfortable to work in (just think of how some people you know feel really cold when others are warm), the astronauts onboard the shuttle can modify their own heat balance by selecting lighter or heavier clothing or by using individual air motion controls (fans). Thermal control in the shuttle is accomplished using radiators on the insides of the cargo bay doors that radiate the heat to space. The cargo bay doors are opened or closed, more or less, depending on the need to contain heat or release heat.