(RHU), used by some planetary and exploratory missions to produce heat for TCS purposes. Active thermal control system (ATCS) components include: Thermostatically controlled resistive electric heaters to keep the equipment temperature above its lower limit during the mission's cold phases. Fluid loops to transfer the heat emitted by equipment to the radiators. They can be: single-phase loops, controlled by a pump; two-phase loops, composed of heat pipes (HP), loop heat pipes (LHP) or capillary pumped loops (CPL). Louvers (which change the heat rejection capability to space as a function of temperature). Thermoelectric coolers. Thermal control systems Environment interaction Includes the interaction of the external surfaces of the spacecraft with the environment. Either the surfaces need to be protected from the environment, or there has to be improved interaction. Two main goals of environment interaction are the reduction or increase of absorbed environmental fluxes and reduction or increase of heat losses to the environment. Heat collection Includes the removal of dissipated heat from the equipment in which it is created to avoid unwanted increases in the spacecraft's temperature. Heat transport Is taking the heat from where it is created to a radiating device. Heat rejection The heat collected and transported has to be rejected at an appropriate temperature to a heat sink, which is usually the surrounding space environment. The rejection temperature depends on the amount of heat involved, the temperature to be controlled and the temperature of the environment into which the device radiates the heat. Heat provision and storage. Is to maintain a desired temperature level where heat has to be provided and suitable heat storage capability has to be foreseen. Environment For a spacecraft the main environmental interactions are the energy coming from the Sun and the heat radiated to deep space. Other parameters also influence the thermal control system design such as the spacecraft's altitude, orbit, attitude stabilization, and spacecraft shape. Different types of orbit, such as low Earth orbit and geostationary orbit, also affect the design of the thermal control system. Low Earth orbit (LEO) This orbit is frequently used by spacecraft that monitor or measure the characteristics of the Earth and its surrounding environment and by uncrewed and crewed space laboratories, such as EURECA and the International Space Station. The orbit's proximity to the Earth has a great influence on the thermal control system needs, with the Earth's infrared emission and albedo playing a very