How RTGs Power Spacecraft Billions of Miles From the Sun

The Nuclear Batteries That Explore the Cosmos

More than 15 billion miles from Earth, Voyager 1 continues to send data home from interstellar space — nearly five decades after its 1977 launch. No solar panel could work that far from the Sun. Instead, the probe relies on a radioisotope thermoelectric generator (RTG), a device often called a "nuclear battery" that converts the heat of radioactive decay into electricity. RTGs have powered more than two dozen U.S. space missions over the past half-century, and they remain the only proven technology for exploring the outer solar system.

How an RTG Works

The principle is elegantly simple. An RTG contains a core of plutonium-238, an isotope that generates intense heat as its atoms naturally decay. That heat flows outward through an array of thermocouples — pairs of different semiconductor materials joined at two junctions. When one junction is hot (near the plutonium) and the other is cold (facing the vacuum of space), the temperature difference drives an electric current, a phenomenon known as the Seebeck effect.

There are no moving parts, no turbines, no pumps. Nothing to lubricate, nothing to jam. That mechanical simplicity is the reason RTGs can operate for decades without maintenance — a crucial trait when the nearest repair shop is billions of miles away.

Why Plutonium-238?

Not just any radioactive material will do. Plutonium-238 has a half-life of 87.7 years, meaning it releases heat slowly enough to last for decades but intensely enough to be useful. It emits primarily alpha particles, which are easy to shield and pose minimal radiation risk once encapsulated. According to the U.S. Department of Energy, Pu-238 is purified and encapsulated at Los Alamos National Laboratory under strict safety protocols. RTGs have never caused a spacecraft accident.

A Track Record Spanning the Solar System

RTGs debuted in space in 1961 aboard the Transit 4A navigation satellite. Since then, they have powered an extraordinary roster of missions:

• Pioneer 10 and 11 — the first probes to Jupiter and Saturn (40 watts each)
• Viking 1 and 2 — the first successful Mars landers
• Voyager 1 and 2 — the only spacecraft in interstellar space (158 watts each at launch)
• Cassini — 13 years orbiting Saturn (292 watts)
• New Horizons — the Pluto flyby mission
• Curiosity and Perseverance — Mars rovers using the newer Multi-Mission RTG (MMRTG), which provides about 110 watts when freshly fueled

NASA's next RTG-powered mission is Dragonfly, a rotorcraft lander headed for Saturn's moon Titan.

The Slow Fade

RTGs are not immortal. Plutonium-238's decay steadily reduces heat output, and thermocouple materials degrade over time. Voyager 1's generators, which supplied about 470 watts at launch, now produce roughly two-thirds of that — losing approximately 4 watts per year. Engineers at NASA's Jet Propulsion Laboratory have responded by systematically shutting down instruments to keep the spacecraft alive. Only two science instruments remain active: a plasma wave sensor and a magnetometer.

"They are still working great, sending back data from a region of space no other human-made craft has ever explored," said Kareem Badaruddin, Voyager mission manager at JPL.

What Comes Next

Engineers are developing a plan called "the Big Bang" — a simultaneous swap of multiple powered components for lower-power alternatives — that could extend Voyager's science mission into the 2030s. Meanwhile, NASA is designing next-generation RTGs capable of producing over 250 watts, ensuring that future probes to the ice giants, ocean moons, and beyond will carry their own reliable power source into the dark.