193 SYSTEMS: THERMAL CONTROL—THE “CIRCULATORY SYSTEM” OF THE INTERNATIONAL SPACE STATION CHAPTER 11 to opposing thermal extremes throughout the daytime portion of each orbit (see Chapter 7). This attitude was necessary for dynamic stability and power generation but caused thermal stress on the vehicle. Portions of the ISS facing the Earth experienced a middle ground in this realm of extremes. The Earth radiates heat better than deep space while providing only a fraction of the heat of the sun, thus the portion of the ISS that faces Earth achieves a goldilocks middle-ground temperature of not too hot and not too cold. The thermal systems on the ISS were designed to protect it from these extreme thermal environments while keeping the astronauts in comfortable, shirt- sleeve surroundings. Convection—Gravity’s Cooling Mechanism Of the three primary mechanisms of heat transfer—conduction, convection, and radiation—convection is intuitively understood by most people based on their experiences on Earth. Hot air rises through convection, taking heat with it. For example, when placing a hand above a hot cup of coffee, a person can feel the rising warmth as the coffee cools. Convective heat transfer occurs when a fluid (air would be a fluid in this instance) is heated and becomes less dense. In a gravity environment, a warmer, less-dense fluid rises through the cooler fluid above it because the less-dense fluid is “lighter” than the colder, denser fluid. This free convection is responsible for the weather patterns on Earth, the flight of hot air balloons, and the usefulness of old-fashioned radiators. Without gravity, however, air will get hot and expand but will stay exactly where it is. This occurs because things are not “light” or “heavy” when there is no gravity. Without gravity, hot air will not rise. On Earth, if a computer is generating a lot of heat, it can be placed on a table, uncovered, and free convection will allow the heat to float up and away, thus keeping the computer from overheating. Inside the ISS, that same computer left floating in the middle of a module would simply heat the air around it, creating a bubble of heat surrounding the computer. On the ISS, a little more effort is required to keep that computer from baking itself. A fan can be used to blow the hot air away from the computer (which is known as forced convection), but the heat is simply being moved around to some other pocket of air. The heat will need to be removed altogether or all the air in the ISS will eventually get too hot. Computers outside the ISS pose yet another challenge since, beyond gravity, the other important part of convection is the air that carries the heat away. Computers outside the ISS are not surrounded by air. They are in the vacuum of space therefore, a fan cannot help with cooling. Convection is not possible without some type of gas such as air or a liquid. Conduction is the process in which heat is transferred from something that is warm to something that is cool through direct contact. The ISS systems use conduction to carry unwanted heat from objects such as computers into fluid systems, which then carry the heat away from the heat-generating equipment and send that warm fluid flow through radiators, using radiation to release that heat into space. Beyond the extremes of space itself, the ISS also requires a cooling process to maintain its many systems at operational temperatures. As on Earth, electronics and machines generate heat. In the gravity environment on Earth, system designers frequently use convective heat transfer in designing cooling systems, where warm air rises and moves away from the equipment to remove heat from machines. Thermal systems on the ISS rely on conduction, where heat is transferred from one substance to another through direct contact to provide cooling to heat-generating equipment, followed by radiation of that heat into space. (See sidebar: Convection— Gravity’s Cooling Mechanism.) A particularly critical symbiosis exists among the Command and Data Handling, the Electrical Power System (EPS), and the TCS. The TCS is powered by the EPS, controlled by the software in Command and Data Handling and, in turn, cools both to keep them functioning. Every other system on the ISS is equally reliant on these three core systems in one way or another, creating an interconnected web of dependencies that have to be carefully managed in both normal operations and failure scenarios. The TCSs on the ISS are comprised of a number of subsystems, all working together to maintain the various structures and components of the space station at the temperatures required for operation and survival. Most of those systems arrived in orbit as part of modules or structures installed during ISS assembly missions of the Space Shuttle (see Introduction). One by one, each