Spacecraft Thermal Balance in Orbit

Orbit & Surface

Orbital altitude 400 km

Absorptivity α 0.30

Emissivity ε 0.85

Thermal mass 40 kJ/K

Live Readouts

☀ In sunlight

Surface temperature

288.0 K

Temperature (°C)

14.9 °C

Absorbed power

0 W

Radiated power

0 W

Orbital period

92.5 min

Eclipse fraction

36%

How it works

A satellite in low orbit swings between direct sunlight and the planet's shadow every orbit. Its surface temperature is set by an energy balance between absorbed solar power and radiated thermal power:

P_abs = α · S · A_cross   (0 while eclipsed)
P_rad = ε · σ · A_total · T⁴
C · dT/dt = P_abs − P_rad

where S = 1361 W/m² is the solar constant, σ = 5.67×10⁻⁸ W/m²K⁴ is the Stefan–Boltzmann constant, α is absorptivity (how much sunlight the surface absorbs), ε is emissivity (how efficiently it radiates heat away), A_cross and A_total are the sunlit cross-section and total radiating area of a 1 m spherical satellite, and C is the object's thermal mass (heat capacity).

Orbital altitude — sets orbital period via Kepler's third law and the fraction of each orbit spent in the planet's cylindrical shadow (eclipse).
Absorptivity α / Emissivity ε — surface coatings change these independently; a low α/ε ratio (white paint) stays cooler, a high ratio (bare metal) runs hotter — this is exactly why spacecraft use special thermal-control coatings.
Thermal mass — a heavier or higher-heat-capacity structure smooths out the temperature swing between eclipse and sunlight; a light structure swings almost instantly.

Real spacecraft thermal engineers size coatings, radiators and heaters against exactly this balance so electronics stay within their operating range through every eclipse cycle.

Satellite Sun direction Orbit path

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