What Is a Solar Array? Spacecraft Power Explained

📘 Definition

A solar array is a spacecraft's primary power source — a panel of photovoltaic (solar) cells that converts sunlight directly into electricity. Space-grade arrays use multi-junction gallium-arsenide (GaAs) cells that reach around 30% efficiency, well above the ~20% of a typical terrestrial silicon panel, because each stacked layer captures a different band of the solar spectrum. Most arrays launch folded and deploy in orbit as articulated 'wings' that rotate to track the Sun, feeding the power subsystem that runs the payload and recharges the batteries. During eclipse — when the satellite crosses Earth's shadow — those batteries carry the load. Sunlight weakens with the square of distance from the Sun, so probes beyond Jupiter swap arrays for radioisotope generators. Engineers deliberately oversize an array to offset the few per cent of output that radiation strips away each year.

Triple-junction GaAs (~30%)

Cell type

≈2,500 m² · up to 120 kW

ISS arrays

1,361 W/m² at 1 AU

Solar constant

Li-ion batteries

Eclipse power

Understanding Solar Array

How a solar array generates power

Each cell is a semiconductor sandwich that frees electrons when photons strike it — the photovoltaic effect. Space arrays use multi-junction cells, stacking layers of gallium arsenide (GaAs) and related III-V materials so each junction harvests a different slice of the solar spectrum. Today's triple-junction cells convert about 30% of incoming sunlight (the best qualified designs reach ~32%), against roughly 20% for a good terrestrial silicon panel. In Earth orbit the Sun delivers about 1,361 watts per square metre — the 'solar constant' — so array area, not just efficiency, sets how much power a satellite can draw.

Cell technology	Typical space (AM0) efficiency	Use
Silicon (Si)	~14-16%	Early satellites; low cost
Single-junction GaAs	~19%	1990s workhorses
Triple-junction GaAs/Ge	~28-32%	Industry standard today
Four-/five-junction (advanced)	~32-34%	Highest-performance, emerging

Deployment, Sun-tracking and eclipse

Because arrays are large and fragile, they launch folded against the satellite and deploy once in orbit — rigid honeycomb panels that unfold like a fan, or flexible 'roll-out' blankets (ROSA) that extend like a tape measure. A Solar Array Drive Assembly then rotates the wings so they stay square-on to the Sun. What happens in shadow depends on orbit: a satellite in low Earth orbit is eclipsed on almost every ~90-minute lap, for up to ~35 minutes at a time, whereas a geostationary satellite basks in continuous sunlight except for two short 'eclipse seasons' around the equinoxes. Whenever sunlight is lost, lithium-ion batteries carry the load and recharge once the array is re-illuminated.

Why arrays are oversized: degradation

A solar array is at its strongest the day it deploys and weakens from then on. High-energy protons and electrons — intense inside the Van Allen belts and during solar storms — knock atoms out of the crystal lattice, while thousands of hot-cold thermal cycles, ultraviolet light and micrometeoroid strikes add further wear. Typical losses run a few per cent per year, so engineers deliberately oversize an array: a spacecraft built for a 15-year life in geostationary orbit must still meet its power budget at 'end of life', which means a generous surplus at 'beginning of life'.

Solar vs RTG: the deep-space limit

Sunlight thins out with the square of distance from the Sun, so an array that yields plenty of power at Earth (1 AU) gets only ~4% as much at Jupiter (~5.2 AU). Solar power stays practical out to roughly Jupiter's orbit — NASA's Juno probe flew enormous wings to manage it — but missions to Saturn and beyond, or into permanently shadowed craters, instead carry radioisotope thermoelectric generators (RTGs) that make electricity from the heat of decaying plutonium. Closer to home, the same abundant sunlight lets big arrays drive electric propulsion, powering ion thrusters for efficient orbit-raising and station-keeping.

Frequently Asked Questions

How much power does a satellite's solar array produce? ▼

It ranges from a few watts to over 100 kilowatts, depending on the spacecraft. A CubeSat might get 5-20 W from body-mounted cells; a large communications satellite draws 15-25 kW; and the International Space Station's arrays are the biggest ever flown, generating up to about 120 kW from its roughly 2,500 square metres of cells. Output falls gradually across a mission as the cells degrade.

What is the difference between a solar panel and a solar array? ▼

A solar panel is a single rigid substrate carrying solar cells, whereas a solar array is the complete power-generating assembly — often several panels joined into a deployable 'wing', plus the hinges, booms and drive mechanism that point it at the Sun. In everyday use the terms are swapped freely, but in spacecraft engineering 'array' means the whole structure and 'panel' one section of it.

Why do spacecraft use gallium arsenide instead of silicon? ▼

Gallium-arsenide (GaAs) multi-junction cells are used because they are far more efficient and far more radiation-tolerant than silicon. A triple-junction GaAs cell converts about 30% of sunlight versus roughly 20% for silicon, so it delivers the same power from a smaller, lighter array and degrades more slowly in the harsh radiation of space. The trade-off is cost: space-grade GaAs cells are expensive, so cheaper silicon still appears on some low-budget satellites.

What powers a satellite when it is in Earth's shadow? ▼

Rechargeable batteries — almost always lithium-ion today — power a satellite during eclipse, when Earth blocks the Sun. The solar array charges these batteries whenever it is illuminated, storing enough energy to run every system through the dark portion of each orbit. In low Earth orbit that shadow arrives on almost every ~90-minute lap; in geostationary orbit it only occurs during two brief eclipse seasons a year.

Why don't deep-space probes beyond Jupiter use solar panels? ▼

Because sunlight becomes too weak. Its intensity falls with the square of distance from the Sun, so at Jupiter a panel receives only about 4% of the sunlight it would at Earth, and far less further out. Missions to Saturn, Uranus, Neptune and the outer Solar System therefore use radioisotope thermoelectric generators (RTGs), which turn heat from decaying plutonium into electricity. Solar power remains workable as far as Jupiter, as NASA's large-winged Juno probe showed.

How long do spacecraft solar arrays last? ▼

Modern spacecraft solar arrays are designed to work for 15 years or more. They rarely fail outright but lose a few per cent of their output each year to radiation damage and thermal cycling, so engineers size them with a power surplus at launch that still meets the mission's needs at 'end of life'. Many geostationary communications satellites operate well past their 15-year design life on the margin built into their arrays.