Blankets, special paints, radiators, heat pipes and small heaters balance the Sun's heat against the cold of space so that every part stays within its safe range.
In orbit there is no air to carry heat away or bring it in. A satellite is heated by the Sun on one side, by its own electronics inside, and loses heat only by glowing it away as infrared light.
The side facing the Sun can get hot while the side in shadow gets very cold. Batteries, electronics and propellant all have limits, so engineers must manage every watt of heat.
NASA's small-spacecraft review defines passive thermal control as methods that need no electrical power. They are cheap, light and low-risk.
ISRO's GSAT-19 flew a miniaturised heat pipe as one of its new technologies. NASA adds that surface coatings are tougher than MLI and often better for the outside of a small spacecraft, and that louvres — shutters that open as they warm — change how much heat a surface radiates.
When passive means are not enough, satellites use powered devices. The commonest are thin film heaters, switched by a thermostat or computer. NASA notes they are often used to keep batteries warm. Other active devices include coolers for sensitive detectors and pumped fluid loops.
Suppose a satellite's electronics turn 1,000 W into heat. A good radiator at about room temperature sheds roughly 355 W per square metre to deep space (see the calculation below). So it needs about 2.8 m² of radiator facing away from the Sun.
Why away from the Sun? Even a white surface that absorbs only 20 percent of sunlight takes in about 0.2 × 1,361 ≈ 272 W per square metre in full Sun — most of what the radiator can shed.
Watch the ground track cross from day to night: every orbit, the station's thermal system swings from full sunlight to deep shadow and back. Open the ISS tracker
A radiator loses heat by thermal radiation, given by the Stefan–Boltzmann law:
P = ε × σ × A × T⁴
σ = 5.670 × 10⁻⁸ W m⁻² K⁻⁴ (NIST), ε is the surface's infrared emissivity, A the area and T the radiator temperature in kelvin. Deep space is close enough to 0 K to ignore.
Worked example: radiator at 293 K (20 °C), ε = 0.85 (illustrative for a white paint):
T⁴ = 293⁴ ≈ 7.37 × 10⁹ K⁴
σT⁴ ≈ 5.670 × 10⁻⁸ × 7.37 × 10⁹ ≈ 418 W/m²
P/A = 0.85 × 418 ≈ 355 W/m²
To reject 1,000 W: A ≈ 1,000 ÷ 355 ≈ 2.8 m².
Because of the T⁴ term, a colder radiator is much less effective. At 273 K (0 °C), σT⁴ ≈ 315 W/m² — about a quarter less.
Now add sunlight. A surface facing the Sun absorbs α × S, where S ≈ 1,361 W/m² (NSSDCA) and α is its solar absorptance. Even a white surface with α = 0.2 absorbs 0.2 × 1,361 ≈ 272 W/m² — most of what the radiator above can shed. That is why radiators should have low α and high ε, and why NASA tells designers to use end-of-life values: as coatings darken, α rises and the radiator runs hotter.
The full design is an energy balance for each part: heat in (Sun, Earth's reflected light and infrared, electronics) equals heat out (radiation), checked for a hot case (end of life, full Sun, full power) and a cold case (eclipse, low power).
Space has no air, so heat cannot be carried away by moving gas; the satellite can lose heat only by radiating it.
Passive control (insulation, coatings, radiators, heat pipes) needs no electrical power. Active control (heaters, coolers, fluid loops) uses power to hold tighter temperature limits.
A blanket of 10 to 20 thin reflective layers that cuts heat flow by radiation; it is the shiny gold or silver covering on many satellites.