CHAPTER 11 SYSTEMS: THERMAL CONTROL—THE “CIRCULATORY SYSTEM” OF THE INTERNATIONAL SPACE STATION 192 A picture of the International Space Station (ISS) showing the largest and most visible portions of the Thermal Control System: the radiator panels. Four white radiators project down in the photo, and provide dedicated cooling to the ISS Electrical Power System. Two sets of three larger radiators project upward in the image, and provide cooling to all the other systems on the ISS. Fluid lines loop through ISS systems, and the coolant within collects heat from computers, electronics, air conditioners, and other mechanical systems around the ISS. That heated coolant then flows through the radiators where the heat is rejected into space, and the coolant, returned to its starting temperature, runs though the same cycle again. Other chapters have explored the “brains” of the International Space Station (ISS), the “lungs,” the ability to “see,” “hear,” and “speak,” and even how its energy is generated. Equally important is the “circulatory system” of the space station. As the ISS orbits the Earth, it spends roughly half of each orbit in daylight and half in darkness. Beyond the Earth’s atmosphere is a harsh thermal environment. When equipment in space is exposed to direct sunlight— without the protection of the Earth’s atmosphere—solar radiation can cause severe heating, thereby rapidly increasing temperatures up to 150°C (302°F). When equipment is exposed to the vast darkness of space, temperatures plunge sometimes as deeply as -130°C (-202°F). The Thermal Control Systems (TCSs) of the ISS perform two important, but seemingly opposite, functions. Fluid systems both inside and outside the vehicle act as a circulatory system, picking up excess heat from around the vehicle and then rejecting that heat overboard, thus maintaining equipment at proper working temperatures. These fluid systems are called Active TCSs (ATCSs), and there are several variations of active thermal systems on the ISS. Conversely, heaters installed throughout the vehicle protect equipment from freezing in the deeply cold periods of darkness during each orbit. These heater systems are known as Passive TCSs (PTCSs), and such systems are used on almost every segment of the ISS. For many years over the course of its history, the ISS could experience both of these extremes simultaneously. In the early years of assembly, the ISS flew what was known as a solar inertial attitude (X-Perpendicular Out of Plane). The orientation of the vehicle was such that one side faced the sun throughout the daytime periods while the opposite side faced deep space. Therefore, the opposing sides of the ISS were exposed