Thermal control keeps every part of a spacecraft within its temperature limits despite +120°C sunlight on one face and −150°C space on the other. Passive tools — coatings, insulation blankets, radiators — do most of the work; heaters, heat pipes and louvres handle the rest.
Understanding Thermal Control
The α/ε game
Two numbers rule passive design: solar absorptance α (how much sunlight a surface soaks up) and infrared emittance ε (how well it radiates). White paint (low α, high ε) runs cool in sunlight; black paint absorbs and emits everything; polished metal (low both) isolates; optical solar reflectors — quartz mirrors — achieve the radiator ideal of rejecting sunlight while radiating hard. Surfaces degrade with ultraviolet and atomic oxygen exposure, so end-of-life properties, not launch-day ones, size the design.
Moving heat around
Heat pipes — sealed tubes whose working fluid evaporates at the hot end and condenses at the cold — move hundreds of watts with no pumps or power, and lace virtually every satellite panel. Larger platforms add pumped loops (the ISS circulates ammonia through deployable radiators); infrared telescopes stack cryocoolers and sunshields to reach detector temperatures where their own warmth would otherwise blind them. Thermal architecture, invisible in renderings, is often what actually shapes a spacecraft.