Understanding Power Subsystem
Generation, storage and the eclipse cycle
A satellite's arrays are sized for the end of the mission, not the start: charged particles in the Van Allen belts slowly degrade the cells, so engineers oversize the array to still meet demand years later. Modern space cells are multi-junction gallium-arsenide devices around 30% efficient — the best exceed 34%, far better than typical rooftop silicon. Batteries are sized to carry the full load through eclipse and to survive thousands to tens of thousands of charge cycles. The cycling pattern depends on the orbit: a low Earth orbit satellite is eclipsed on almost every lap, around 16 times a day, whereas a geostationary satellite sees the Sun continuously except for two short eclipse seasons around the equinoxes.
| Orbit | Eclipse pattern | Battery cycles/yr | Depth of discharge |
|---|---|---|---|
| LEO | Up to ~36 min every orbit | Up to ~5,500 | Shallow (20–40%) |
| GEO | Only near the equinoxes; up to ~70 min/day | ~90 | Deep (up to 60–80%) |
The power budget and the bus
Because generation is finite, every spacecraft runs to a strict power budget that splits the available watts between the payload and 'housekeeping' loads — the computer, radios, thermal-control heaters and attitude actuators — with a margin held back for degradation and contingencies. Power is delivered over a common electrical bus. Small satellites often use an unregulated 28-volt bus, while larger platforms move to 50 or 100 volts to carry the same power at lower current, which saves harness mass. The biggest systems go higher still: the ISS distributes primary power at 160 volts. Missions that use electric propulsion are especially power-hungry, as their thrusters can draw several kilowatts on their own.
| Spacecraft | Typical EPS output |
|---|---|
| CubeSat (1–3U) | A few to ~20 W |
| Small satellite | Tens to a few hundred W |
| Large GEO comsat | ~5–25 kW |
| ISS | ~120 kW (up to ~215 kW with new arrays) |
When sunlight runs out: RTGs and nuclear power
Sunlight weakens with the square of distance from the Sun, so an array collecting about 1.36 kW per square metre near Earth gathers only roughly 4% of that at Jupiter. Far beyond the asteroid belt solar power becomes impractical for most missions, and probes switch to radioisotope thermoelectric generators (RTGs), which convert heat from the natural decay of plutonium-238 into electricity through solid-state thermocouples with no moving parts. Voyager, Cassini and New Horizons all fly on RTGs, as do the Mars rovers Curiosity and Perseverance. Output is modest — hundreds of watts — and fades gently as the fuel decays, the plutonium-238 half-life being about 88 years. Juno is the notable exception, reaching Jupiter on solar power alone with three enormous arrays.