Spacecraft thermal control is the engineering system that keeps a satellite, probe, or crew vehicle within safe temperature limits. In space, sunlight can strongly heat one side while the shaded side radiates heat away into deep space. There is almost no air, so convection cannot carry heat around the spacecraft.
Without thermal control, electronics, batteries, fuels, instruments, and crew cabins can quickly become too hot or too cold to function.
Key Facts
- In space, heat transfer occurs mainly by radiation and conduction, not convection.
- Radiated power is P = εσAT^4, where ε is emissivity, σ is the Stefan-Boltzmann constant, A is area, and T is temperature in kelvin.
- Absorbed solar power is P = αSA, where α is absorptivity, S is the solar constant, and A is the sunlit area.
- Near Earth, the solar constant is about S = 1361 W/m^2.
- Multilayer insulation reduces radiative heat transfer by using many thin reflective layers separated by low-conductivity spacers.
- A simple steady thermal balance is heat in + internal heat = heat radiated out.
Vocabulary
- Thermal control system
- A set of spacecraft materials, devices, and design choices used to keep components within their allowed temperature range.
- Multilayer insulation
- A blanket made of many reflective layers that slows radiative heat gain and heat loss.
- Radiator
- A surface designed to reject unwanted heat to space by infrared radiation.
- Emissivity
- A measure from 0 to 1 of how effectively a surface emits thermal radiation compared with an ideal blackbody.
- Absorptivity
- A measure from 0 to 1 of how much incoming radiation a surface absorbs rather than reflects.
Common Mistakes to Avoid
- Assuming space is always cold, which is wrong because direct sunlight can heat spacecraft surfaces intensely even when the surroundings are near vacuum.
- Using Celsius in P = εσAT^4, which is wrong because the radiation law requires absolute temperature in kelvin.
- Treating multilayer insulation like ordinary foam insulation, which is wrong because MLI mainly reduces radiation rather than air-based convection.
- Thinking a radiator cools by blowing heat into space, which is wrong because there is no air flow for convection and the radiator loses heat by emitting infrared radiation.
Practice Questions
- 1 A flat radiator has area 2.0 m^2, emissivity 0.85, and temperature 300 K. Using σ = 5.67 x 10^-8 W/(m^2 K^4), calculate the power it radiates.
- 2 A spacecraft panel with absorptivity 0.30 faces the Sun. If the solar constant is 1361 W/m^2 and the sunlit area is 1.5 m^2, calculate the absorbed solar power.
- 3 A satellite battery must stay warm during a 40 minute eclipse. Explain why heaters may be needed even if the satellite also has radiators for cooling.