195 SYSTEMS: THERMAL CONTROL—THE “CIRCULATORY SYSTEM” OF THE INTERNATIONAL SPACE STATION CHAPTER 11 space station could become free- floating water, which could cause irreparable damage if it came into contact with station electronics. Additionally, condensation can cause corrosion of metal structures in the form of rust. Corrosion can lead to small holes in the critical shell of the ISS or make the ISS more susceptible to structural fatigue under dynamic stresses that occur due to normal thermal cycling as the space station orbits the Earth or during propulsive events such as vehicle reboosts (see Chapters 7 and 8). Multilayer Insulation Anodized Coating Heaters (bonded to the inside of the LAB pressure shell) Figure 1. Various examples of PTCS are seen in this photograph of the Z1 truss connection to the Laboratory Module. The silver-like anodized coating of the modules helps reflect sunlight. Heaters (not visible) are bonded to the interior side of the pressure shell to keep it from getting too cold. Fluid connectors are wrapped in multilayer insulation to keep the temperature of the fluids within operational ranges. Active Thermal Control Systems—General Passive systems are effective for keeping particular areas at the right temperature however, sometimes large quantities of heat need to be moved from one area to another or removed from the system altogether. Active systems are required when heat needs to be moved from its source to a different location where it can be expelled. The ISS has three types of active thermal systems: one internal and two external. A third external was available temporarily. This section describes the common characteristics of these active systems. Every ATCS on the ISS is a closed- loop system, with fluid lines connecting heat “loads” (i.e., the equipment that needs to be cooled) to heat rejection points. This is exactly the same way a building or automobile air conditioner works. In an air conditioner, air moves over a tube filled with cold liquid so the heat transfers to the coolant. The now- warm liquid passes through tubes on the outside of the unit so that the heat can be absorbed by the outside air. In the ITCS, the heat loads are individual pieces of equipment such as computers, air conditioners, water processors, and experiments. Fluid lines pass close by these loads to