Thermal Control Subsystem

The thermal control subsystem maintains a spacecraft and its onboard equipment within allowable temperature limits throughout all mission phases. Unlike on Earth, where heat may be lost through convection, conduction, and radiation, a satellite in space can reject heat only through radiation, although conduction and convection may still occur within closed internal fluid systems.

Thermal inputs to a spacecraft are dominated by solar radiation, together with reflected sunlight from the Earth and infrared radiation emitted by the Earth, while internal heat is generated by onboard electronics and payloads. As a result, spacecraft surfaces may experience very large temperature gradients, with one side exposed to intense solar heating and the opposite side facing deep space. These variations are further amplified during eclipse periods when the spacecraft passes into the Earth’s shadow. The thermal control subsystem must therefore manage large and rapidly changing thermal loads to maintain thermal equilibrium.

Thermal control techniques are generally classified as passive or active. Passive methods include careful equipment layout, selection of structural and surface materials, radiators, thermal blankets, coatings, reflectors, insulation, heat sinks, and louvres. Active methods include electrical heaters, heat pipes, and pumped fluid loops with heat exchangers. Most uncrewed spacecraft rely primarily on passive thermal control, with active techniques used to manage transient conditions such as eclipse entry and exit.