In spacecraft design, the function of the thermal control system (TCS) is to keep all the spacecraft's component systems within acceptable temperature ranges during all mission phases. It must cope with the external environment, which can vary in a wide range as the spacecraft is exposed to the extreme coldness found in the shadows of deep space or to the intense heat found in the unfiltered direct sunlight of outer space. A TCS must also moderate the internal heat generated by the operation of the spacecraft it serves. A TCS can eject heat passively through the simple and natural infrared radiation of the spacecraft itself, or actively through an externally mounted infrared radiation coil. Thermal control is essential to guarantee the optimal performance and success of the mission because if a component is subjected to temperatures which are too high or too low, it could be damaged or its performance could be severely affected. Thermal control is also necessary to keep specific components (such as optical sensors, atomic clocks, etc.) within a specified temperature stability requirement, to ensure that they perform as efficiently as possible. Active or passive systems The thermal control subsystem can be composed of both passive and active items and works in two ways: Protects the equipment from overheating, either by thermal insulation from external heat fluxes (such as the Sun or the planetary infrared and albedo flux), or by proper heat removal from internal sources (such as the heat emitted by the internal electronic equipment). Protects the equipment from temperatures that are too low, by thermal insulation from external sinks, by enhanced heat absorption from external sources, or by heat release from internal sources. Passive thermal control system (PTCS) components include: Multi-layer insulation (MLI), which protects the spacecraft from excessive solar or planetary heating, as well as from excessive cooling when exposed to deep space. Coatings that change the thermo-optical properties of external surfaces. Thermal fillers to improve the thermal coupling at selected interfaces (for instance, on the thermal path between an electronic unit and its radiator). Thermal washers to reduce the thermal coupling at selected interfaces. Thermal doublers to spread on the radiator surface the heat dissipated by equipment. Mirrors (secondary surface mirrors, SSM, or optical solar reflectors, OSR) to improve the heat rejection capability of the external radiators and at the same time to reduce the absorption of external solar fluxes. Radioisotope heater units