Turning radioactive warmth into electricity
A deep-space probe’s plutonium power source is usually a radioisotope thermoelectric generator, or RTG. It does not produce electricity by splitting atoms in a controlled chain reaction. Instead, plutonium-238 atoms decay naturally, releasing energy as heat inside a sealed fuel unit.
That heat creates a temperature difference across thermoelectric materials. These materials produce an electric voltage when one side is warmer than the other, much like a tiny solid-state heat engine. The conversion has no pistons or turbines to wear out, an important advantage when a spacecraft is millions of miles from a repair shop.
Why not use solar panels?
Sunlight spreads out as it travels from the Sun, so the power available to a panel drops sharply with distance. Panels that work well near Earth would need to become very large—or deliver much less electricity—farther out. They can also be a poor fit when a mission must operate in shadow or in places with little sunlight.
An RTG carries its energy source with it. That makes it useful for missions where steady power matters more than getting the highest possible electrical output from a compact system. The tradeoff is that the generator supplies limited power, and its output gradually declines as the radioactive fuel decays and the conversion materials age.
Heat that serves two jobs
Not all the generator’s warmth has to be converted into electricity. Spacecraft can use some of it to keep sensitive equipment from becoming dangerously cold. So the same slow decay that powers instruments can also help maintain the conditions they need to function—letting a probe keep working where sunlight is too faint to do the job.
