The current modern spacesuits have allowed people to facilitate space explorations and existence in space outside spacecraft. They are specifically designed to ensure that an astronaut is safe and enjoys the right temperature, pressure, and receives oxygen as well as water. The occupant of the spacesuit is exposed to various sources of heat. However, the main source of heat comes from the metabolic reactions which tend to grow with the increase in extravehicular activities. The mechanical and electrical processes that take place in the portable life support system also generate a significant amount of heat. The spacesuit is also designed to act as a heat sink so that it eliminates excess heat to keep the astronaut cool. The major components that deal with heat include the heat exchanger and pressure garment which is fitted with water tubes. Spacesuit technology uses sublimation to eject heat into space. The main concerns with the current technology include overdependence on the water for cooling. This tends to limit the time an astronaut can stay in the suit because it requires constant maintenance.
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Keywords: Spacesuits, extravehicular activities, heat sink, heat exchanger, sublimation, water tubes
Astronauts on a Space-Walk
The conditions in space are significantly different from the ones on Earth. This explains why humans cannot exist in space without special spacesuits. They are designed to ensure that the conditions the astronauts are exposed to are maintained within some given range that do not differ substantially from the normal atmospheric conditions on Earth. The astronauts’ success in their performance of extravehicular activity relies on the functionality of spacesuit they are wearing. In particular, the life-supporting technologies incorporated into the spacesuits are essential for surviving in harsh space conditions. Spacewalking astronauts encounter various heat challenges as temperatures can drop or rise to the levels that threaten life. Consequently, for astronauts’ survival, the spacesuit contains environmental and garments system designed to ensure that astronauts stay alive and shielded from extreme temperatures.
Various sources of heat in space exist. However, Sun appears to be the one with the most intense heat when it is faced directly. This implies that the external temperature of the spacesuit can significantly rise when it directly encounters Sun. The radiation from the Sun is capable of heating the spacesuit to temperatures above 300oC (May, 2014). This means that the spacesuit have to be fitted with technologies that protect the astronauts from very high temperatures. For instance, the external layers of the suit consist of insulating materials such as Dacron, neoprene, and Gore-Tex. In addition, the outermost layer is made of best-reflecting materials like Mylar and is always white in color to reflect maximum radiation from the Sun. The gold film is used to cover the visor. The visor prevents solar radiation from reaching the astronauts’ face as and eyes. In sum, solar radiation is one of the heat sources that astronauts are exposed to, and from which spacesuits offer protection in order to prevent harm.
Besides the Sun’s solar radiation which appears to act from outside of the spacesuit, there are several other heat sources emanating from different processes (including mechanical and electrical) within the spacesuit. The astronaut’s body generates heat due to his or her own metabolism, particularly when he or she is engaged in an extravehicular activity (EVA). The portable life support system (PLSS) also generate a substantial amount of heat during EVA. The PLSS generate heat from various processes like the metabolic carbon dioxide absorption heat, the heat that emanates from the process of water condensation, and the heat eliminated from the body by the system of cooling garment. From this, it is evident that the astronaut is potentially exposed to intense heat. Such heat must be eliminated to the space to maintain a favorable condition. As a consequence, cooling the astronauts is one of the main concerns that are given significant technological consideration while designing the spacesuit.
Concerns about the Heat Sources
The heat sources generate a considerable amount of heat energy which poses a significant health risk to the astronauts if left unchecked. The mechanism of heat loss in space is different from that of the Earth. For instance, on Earth, the process of heat loss is efficient as it depends on several processes including convection which uses air. In space, the only mechanism through which heat escapes the body is radiation. Compared to convection, radiation tends to be slow and is significantly inefficient. This explains why astronauts have internal garments fitted with tubes through which water flows to ensure the temperature is regulated (Izenso et al., 2015). Without these measures, the heat load can cause significant harm to their bodies due to metabolic reactions.
The presence of various sources of heat in the space, as outlined above requires special modification of the spacesuit. Currently, the spacesuit technology is designed to maintain air inside using five inner layers. The outer stratums made up of almost thirteen layers are also designed to protect the inner layers. They represent a group of layers coated with various materials, such as rubber. Furthermore, given that convection and conduction do not work in the space, astronauts depend on the heat exchanger. This system functions by directing heat to frozen liquid which then evaporates into space. Therefore, cooling in the spacesuit is possible only due to the heat exchanger. As a result, it remains a big concern due to the possibility of malfunction of the system of the heat exchanger given the numerous heat sources that the astronaut is exposed to.
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The spacesuit is functioning as a heat sink, insulator, and heat source. This can be seen from the fact that it has several layers. The spacesuit is designed to work as a heat sink to countercheck the numerous heat sources the astronaut is exposed to. However, at the same time, it is greatly insulated to prevent heat loss. The first case of the heat sink can be seen from the initial layer that the astronauts are wearing, known as Liquid Cooling and Ventilation Garment (LCVG) (May, 2014). This cloth suit specially designed with tubes throughout its structure to enable water to freely flow, and it covers the whole astronaut’s body except the feet, hands and head. In this way, the one wearing the spacesuit is cooled. This process is very critical as temperatures in the space can be extremely high or low depending on the position in relation to the Sun. The design of the astronaut’s cloth suit complements the entire system of spacesuit’s heat sink. They work together to achieve the ultimate goal of cooling. This is because the fluid that flows through the cloth suit’s tubes which primarily cools the astronaut is the same one that flows in the heat exchanger that is constructed in the rear end of the spacesuit. The heat is transferred to vaporize some manufactured ice into space. The suit apparently achieves its efficient insulation properties from the action mechanism of the heat sink. Without this system of built into the spacesuit, it would be difficult to efficiently regulate temperatures.
Concerns about the Sinks
Presently, the thermal-control technology built in the spacesuit pressure garments was designed for missions that do not last for longer duration in low Earth orbit. Technology primarily relies on water venting and regular suit maintenance. This design presents a significant obstacle to success space exploration as safety concerns remain high, especially astronaut’s cooling mechanism. This is because the specialists must schedule maintenance frequently. Therefore, the need to improve the current heat sink system is necessary. The enhancement will not only improve the safety concerns related to the use of water venting, but also enhance spacewalking as astronauts will be able to stay for longer durations in the spacesuit while performing various assignments in the space outside spacecraft or international space station. This reasoning implies that in the future, exploration missions will need to shift away from water venting in order to solve the issue of condensation of perspiration in the suit (Izenso et al., 2015). This will be obligatory, as such condensations interfere with the comfort and hygiene as well as cause frequent maintenance.
Astronauts’ metabolisms tend to produce a considerable amount of water vapor when they are engaged in high work rates. Figure 1 reveals that the rate of perspiration can rise as high as 1000 g/hr. during peak EVA (Izenso et al., 2015). As it was noted before, generated water vapor is released into the pressure garment. As a result, it tends to escalate the partial pressure and increase the dew point. Almost simultaneously, the coolant temperature is expected to decrease in order to absorb the additional metabolic heat load. The result is condensation of water vapor that emanates from the ventilation gas. This implies that the process leads to accumulation of water in the pressure garment. The LCVG can absorb the same liquid water which potentially might cause some risks to the health of the astronaut, since overcooling takes place as a result.
The present insulation technology relies on aluminized Mylar with several layers of materials in between which create multilayered insulation. This system is meant to shield astronauts from temperatures that go as low as -275oC and as high as 300oC (May, 2014). The system functions like the thermos bottle in which a vacuum layer is created within the cases. Existence of the vacuum layer is the best technology to prevent heat loss as there is no medium to conduct the cold or heat to the inner layer. However, it tends to conflict heat sink system which is based on water venting. Although it ensures that the astronaut in the spacesuit is not affected by the extreme cold or heat, its presence means that the only way through which heat can be eliminated is sublimation.
The final removal of the heat from the extravehicular mobility unit depends on water vapor. This design seems to work well currently. However, it has serious challenges and shortcoming that must be addressed for the future prosperity in space missions. For instance, it costs approximately one liter (one kg) of water for every four hours of extravehicular activity (Izenso et al., 2015). This loss rate looks unsustainable for an extended duration of space exploration as it points to the fact that for successful long duration exploration activities a lot of water would be required. In addition, the cooling method is dependent on sensible cooling. This method is not the best alternative for human metabolism. Maintenance of sensible cooling with the rise in the rates of metabolism need lower coolant and skin temperatures (Izenso et al., 2015). However, since skin and coolant temperatures tend to increase nonlinearly as revealed in Figure 2, the approach is not the most appropriate for humans.
The hermetically sealed spacesuits in which astronauts stay during spacewalk experience a significant amount of heat which must be moved to ensure the comfort of the spaceman. As mentioned earlier, the heat load is caused by several sources, but a significant part of the heat comes from the spacesuit occupant’s metabolism. The liquid cooling garment is primarily intended to eliminate metabolic heat energy. It is an efficient system of heat movement as it is worn by the astronaut. The heat is absorbed and transported by the cooling medium to other devices which then transfer the heat away. Presently, most of the spacesuits eliminate the heat generated within it through the process of sublimation (Izenso et al., 2015). Sublimation is the process by which a solid vaporizes without turning into liquid first. Water contained in a separate tank is exposed to the vacuum of space to sublimate.
The exposure to the vacuum of space is achieved through the use of the porous plate. The porous plate is an element of the heat exchanger. It traps the water and then freezes it. In the meantime, the cooling medium flows continuously through the different passages in the heat exchanger. The layer of ice is sublimated by the cooling medium producing a cloud of water vapor in the process. Thus, the heat energy in the cooling medium is efficiently removed from the spacesuit (Izenso et al., 2015). The resultant water vapor is emitted. Although the process of heat removal through sublimation eliminates the heating load in the spacesuit, it also has a set of issues. For instance, water transportation cost into space is exorbitant. In addition, the vapor emitted from the system into space may condense on critical instruments of the suit technology and hamper their performance. It can also stick to mirrors or absorb infrared radiation.
Preventing Heat from Moving
Prevention of heat from moving is primarily achieved by the insulation properties of the extravehicular mobility units. The design of the current spacesuits incorporates several layers some of which have vacuum space. These layers play different functions and achieve various safety goals in order to protect the astronaut. These functions include preventing loss of heat, particularly, when the spacesuit is in a cold place away from the radiation of the solar. According to Tate (2013), the spacesuit has fourteen layers organized in three assemblies. The first three layers compose the LCVG. The bladder layer is at the top of the LCVG layer designed to maintain appropriate body pressure. In addition, the bladder layer contains the oxygen for the astronaut. The next layer is responsible for holding the bladder layer in correct position as well as for the shape for the spacesuit occupant. Its material is equivalent to the one used to manufacture camping tents. The tear-resistant layer is also present and comes after the one that holds bladder layer in shape. The other seven layers are the most important ones as far as maintenance of constant temperature is concerned. They are the ones that give spacesuits their insulation properties which also prevent heat movement (Gon & Paul, 2011). The layers behave like thermos which is known to significantly minimize movement of heat. This is due to the fact that the vacuum space does not allow heat to move due to lack of thermal convection or conduction. Therefore, the only way of heat movement is through radiation which is poor in vacuum space. In sum, heat in the spacesuit is prevented from escaping due to the vacuum areas created by various layers of the suit.
The development of spacesuits made space missions a reality to humans. Given the uninhabitable conditions in the space, human beings must be wearing spacesuits whenever they want to undertake some explorations in the space. The spacesuits are designed to protect the explorers from lack of oxygen, absence of the gravity, and radiation. Given this, it is evident that astronauts potentially face significant health consequences if the technologies fail. Space is characterized by both extremely low and high temperatures. The sources of heat in the space include the solar radiation from the Sun. It is known that temperatures can go as high as 300oC if one faces the Sun directly, and it is equally low in the absence of the solar radiations. However, for an astronaut, the main source of heat is his or her own, generated as a result of metabolic reactions. Other sources of heat the astronaut is exposed to include the heat emanating from the portable life support system (PLSS) processes. The spacesuit also acts as a heat source as it is designed to remove excess heat to prevent overheating and thus making the suit comfortable. The pressure garment is fitted with tubes in which water flows to cool the astronaut body. Heat elimination primarily relies on the process of sublimation. Currently, it is the best technology. However, there might be a need to invent other efficient ways of cooling that do not depend on water venting.