The Technology of Nuclear Batteries: How RTGs Power Spacecraft

01HEAT FROM DECAY, NOT FISSION

A radioisotope thermoelectric generator, or RTG, is a nuclear battery. But it is not a nuclear reactor. The distinction matters: a reactor performs controlled nuclear fission in a chain reaction, with control rods to vary power output. An RTG produces heat through spontaneous radioactive decay at a non-adjustable, steadily decreasing rate that depends only on the amount of fuel isotope and its half-life.

The Andy Mcloone video documents the Soviet Union's deployment of over 1,000 RTGs to power uncrewed lighthouses and navigation beacons along the Arctic coast. By the late 1980s, these units were scattered across some of the most remote coastline on Earth — many with no fences, no warning signs, and no maintenance for decades after the Soviet collapse.

Wikipedia records the RTG's invention in 1954 by Mound Laboratories scientists Kenneth Jordan and John Birden. The first RTG launched into space was SNAP-3B in 1961, powered by 96 grams of plutonium-238, aboard the Navy Transit 4A spacecraft.

02THE SEEBECK EFFECT: HEAT INTO ELECTRICITY

The core mechanism is elegantly simple. Radioactive decay produces heat. Thermocouples placed in the walls of the fuel container convert that heat into electricity via the Seebeck effect: when two different metals or semiconductors are joined in a loop and the two junctions are at different temperatures, an electric current flows. A large number of thermocouples connected in series generate a useful voltage.

There are no moving parts. No turbines, no pumps, no fluids. This is why RTGs are ideal for decades-long space missions where maintenance is impossible — the Voyager probes have been running on their RTGs for over 45 years.

The trade-off is efficiency. A typical RTG converts only about 3-7% of the thermal energy into electricity. The rest radiates away as waste heat. For a spacecraft, that heat is not entirely wasted — it keeps electronics warm in deep space. But it means you need a lot of radioactive material to produce modest electrical power.

03WHY PLUTONIUM-238

The radioactive material in an RTG must meet several criteria. Its half-life must be long enough for steady energy output over the mission duration. The decay must produce easily absorbed radiation — alpha particles are ideal because they deposit all their energy in a short distance and require minimal shielding. And it must produce high power per unit mass.

Plutonium-238 is the gold standard. It has an 87.7-year half-life, decays almost exclusively by alpha emission, and produces about 0.57 watts per gram. A typical space RTG contains 4-8 kilograms of plutonium-238 dioxide. The Cassini spacecraft carried 32.8 kg across three GPHS-RTGs, producing 300 watts of electrical power and 4,400 watts of heat.

04THE VOYAGER POWER CURVE

Wikipedia's life span section provides a vivid illustration of RTG decay over time. The MHW-RTGs on the Voyager probes started with about 470 watts of electrical power. After 23 years (by 2000), the radioactive material had decreased to 83.4% of initial output — 392 watts. But the thermocouples also degraded, so the actual output was only 67% of the original: about 315 watts for Voyager 1 and 319 watts for Voyager 2.

By 2022, those numbers had dropped to around 220 watts. NASA has progressively shut down instruments to keep the spacecraft alive. Yet both Voyagers continue to transmit from interstellar space, over 24 billion kilometers away, powered by the same RTGs that were launched in 1977.

This is the fundamental promise of the RTG: predictable, steady, decades-long power with zero maintenance. The power declines, but it declines on a known curve. You can plan around it.

05THE SOVIET LIGHTHOUSE PROBLEM

The Andy Mcloone video highlights a darker chapter in RTG history. The Soviet Union deployed approximately 1,007 RTGs to power uncrewed lighthouses and navigation beacons along its Arctic coast. These used strontium-90, not plutonium-238, and were designed for a 10-year operational life.

When the Soviet Union collapsed in 1991, the lighthouses were abandoned. Records were lost. Some units disappeared — looted for their metal casings, or swept away by ice and storms. In one documented incident, three woodsmen in Georgia found two ceramic RTG sources stripped of their shielding. Two were hospitalized with severe radiation burns; the third died.

By 2021, a joint Russian and international effort had removed all the remaining RTGs. But the incident illustrates a fundamental tension: RTGs are safe when maintained and accounted for, but dangerous when forgotten.

06NOT A REACTOR: WHY IT MATTERS

The distinction between an RTG and a nuclear reactor is critical for public understanding. A spectacular failure like a nuclear meltdown or explosion is physically impossible with an RTG — there is no chain reaction to lose control. Heat generation cannot be varied, shut off, or saved for later.

However, there is still a risk of radioactive contamination if a rocket explodes, the device reenters the atmosphere and disintegrates, or a terrestrial RTG is damaged or vandalized. The Apollo 13 RTG, never used on the Moon, rests in the South Pacific near the Tonga Trench — intact, as designed, but monitored.

For the Curiosity and Perseverance Mars rovers, NASA chose RTGs over solar panels for practical reasons: more landing site flexibility, no dust accumulation concerns, and a longer operational lifespan. The MMRTG on Curiosity produces about 110 watts from 4.8 kg of plutonium-238 dioxide — enough to drive, sample, and run a full science payload on another planet.

07THE FUTURE: SKUTTERUDITE AND AMERICIUM

NASA has developed next-generation MMRTGs using skutterudite thermocouples — a cobalt arsenide compound that functions with a smaller temperature difference than current tellurium-based designs. This would generate 25% more power at mission start and at least 50% more after 17 years.

Meanwhile, the US faces a plutonium-238 shortage. Americium-241 has been proposed as an alternative for interstellar precursor missions. It decays more slowly (432-year half-life vs 87.7 years), enabling mission extensions up to 1,000 years — but at lower power density. A subcritical RTG design that uses alpha-neutron reactions with beryllium to boost output by up to 10% has also been proposed.