Spacecraft Thermal Calculator

Temperature range from solar flux, albedo, and surface properties.

Orbital altitude (km)

Solar absorptance (alpha)

Thermal emittance (epsilon)

Solar projected area (m²)

Radiator area (m²)

Internal heat dissipation (W)

Eclipse fraction (0-0.5)

Tab to move between fields

Equilibrium temp (°C)

-19

Hot case temp (°C)

-1.9

Cold case temp (°C)

-75.5

Equilibrium temp (K)254.2

Total heat input (W)1,207.1

Solar heat (W)530.8

Albedo heat (W)134.9

Earth IR heat (W)341.4

Alpha/epsilon ratio0.35

Earth view factor0.85

T_fahrenheit-2.14

How to Use This Calculator

Enter orbital altitude (km) and the spacecraft's solar absorptance (α) and thermal emittance (ε) surface properties.
Set the projected solar area (m²), radiator area (m²), and internal heat dissipation (W).
Review equilibrium temperature (°C and K), hot case temperature, and cold case temperature.
Ensure all temperatures fall within the allowable operating ranges for electronics and propellant.
Adjust surface coatings (α/ε ratio) or radiator area to bring temperatures within acceptable limits.

How the result changes with Radiator area (m²)

Radiator area (m²)	Equilibrium temp (°C)	Hot case temp (°C)	Cold case temp (°C)
100	-147.3	-138.9	-175.3
350	-181.2	-175	-201.6
650	-194.4	-189.1	-211.9
900	-200.5	-195.6	-216.7

Orbital altitude (km): Orbit altitude affects Earth view factor (albedo and IR heating). Higher altitude = less Earth heating.
Solar absorptance (alpha): Fraction of solar radiation absorbed. White paint: 0.2, bare aluminum: 0.15, solar cells: 0.75, black paint: 0.95.
Thermal emittance (epsilon): IR emissivity. White paint: 0.85, polished aluminum: 0.04, black paint: 0.90, OSR: 0.80.
Solar projected area (m²): Cross-sectional area facing the sun (projected area of body + solar panels if body-mounted).
Radiator area (m²): Total surface area that radiates heat to space. Typically total external surface area.
Internal heat dissipation (W): Waste heat from electronics, batteries, and instruments that must be radiated away.
Eclipse fraction (0-0.5): Fraction of orbit spent in Earth's shadow. LEO ~0.35, GEO ~0-0.04, sun-sync dawn/dusk ~0.

Equilibrium temp (°C): Orbit-average equilibrium temperature.
Equilibrium temp (K): Same temperature in Kelvin.
Hot case temp (°C): Maximum temperature (no eclipse, full solar illumination).
Cold case temp (°C): Minimum temperature (full eclipse, reduced internal heat).
Total heat input (W): Sum of all heat sources.
Solar heat (W): Absorbed direct solar radiation.
Albedo heat (W): Absorbed reflected sunlight from Earth.
Earth IR heat (W): Absorbed infrared radiation from Earth.
Alpha/epsilon ratio: Key thermal design parameter. Low ratio = cooler spacecraft.
Earth view factor: Geometric view factor to Earth from the spacecraft.

Identify Input Parameters
4 parameters
Orbital altitude (km) = 550, Solar absorptance (alpha) = 0.3, Thermal emittance (epsilon) = 0.85, Solar projected area (m²) = 2 = 7 input(s) provided
Calculate Equilibrium temp
Equilibrium temp = T_kelvin - 273.15
-19 = -19
Calculate Hot case temp
Hot case temp = T_hot_K - 273.15
-1.9 = -1.9
Calculate Cold case temp
Cold case temp = T_cold_K - 273.15
-75.5 = -75.5
Calculate Equilibrium temp
254.2 = 254.2
Calculate Total heat input
Total heat input = qSolar + qAlbedo + qEarthIR + internalHeatW
1207.1 = 1207.1