How do spacecraft get power far from the Sun? Solar panels, batteries and plutonium

Why sunlight runs out

Sunlight spreads out as it travels, so the same panel collects less and less light the farther it is from the Sun. NASA puts it simply for Jupiter: the planet is five times farther from the Sun than Earth, and the sunlight that reaches it packs 25 times less punch. For the Europa Clipper mission, NASA gives the same picture another way: around Jupiter, the spacecraft receives only 3 to 4 per cent of the sunlight Earth gets.

That does not make solar power impossible in the outer Solar System, but it makes it expensive in size and mass. Each extra unit of distance has to be paid for with a larger panel, and at Saturn or beyond the panels would have to be larger still. How NASA’s robotic missions are planned and flown is described on Missions and How a Mission Flies.

How far solar panels can go

Juno shows where the limit lies today. On 13 January 2016, on its way to Jupiter, it became the most distant solar-powered spacecraft, at about 793 million kilometres from the Sun. The previous record belonged to ESA’s Rosetta, whose orbit peaked at about 792 million kilometres in October 2012. Juno carries three solar arrays, each about 9 metres long, with 18,698 individual solar cells, and at Jupiter they generate only about 500 watts.

NASA says this was possible thanks to better solar cells, energy-efficient instruments and spacecraft systems, and a mission design that keeps Juno out of Jupiter’s shadow; its polar orbit also limits the total radiation dose. All eight spacecraft that had reached Jupiter before Juno used nuclear power.

Europa Clipper took the same route on a larger scale. Each of its two solar arrays is about 14.2 metres long and 4.1 metres high, and with them deployed the spacecraft spans more than 30.5 metres. Together they are expected to provide roughly 700 watts at Jupiter. The arrays were built by the Johns Hopkins Applied Physics Laboratory and by Airbus in Leiden, in the Netherlands.

On Mars, the problem is dust

Mars is closer to the Sun than Jupiter, but solar power there has a different enemy: dust, and the changing light of day, night and the seasons. The rover Opportunity, solar-powered, was designed to work for 90 Martian days and to travel about 1,000 metres. It lasted almost 15 years and drove more than 45 kilometres. Its last transmission came on 10 June 2018, when a severe dust storm covering the whole planet blanketed its location, and NASA declared the mission over on 13 February 2019.

For its two large rovers, Curiosity and Perseverance, NASA chose a radioisotope generator instead. JPL explains why for Perseverance: it frees the rover from the limitations of solar panels, whose output would vary with daily and seasonal sunlight and with dust, and lets the team run communications, driving and science at any hour and at different latitudes. More about the rovers is on Mars.

The “nuclear battery”: how an RTG works

A radioisotope thermoelectric generator, or RTG, turns heat into electricity with no moving parts. Its fuel, mostly plutonium-238, warms up as it naturally decays. That heat warms one side of a set of thermocouples, the cold of space or of the Martian surface keeps the other side cool, and the temperature difference produces a voltage. The principle, NASA notes, goes back to Thomas Johann Seebeck, who found that a voltage forms when two different conductors are joined in a circuit whose junctions are at different temperatures.

The model used on Curiosity and Perseverance, the Multi-Mission RTG, holds 4.8 kilograms of plutonium dioxide fuel and has 768 thermocouples. The fuel gives off about 2,000 watts of heat at first, from which the generator makes about 110 watts of electricity; by design, after 17 years, three of them in storage before launch, it should still provide 72 watts. The whole unit weighs about 45 kilograms and measures about 64 by 66 centimetres.

A hundred-odd watts sounds small, and Perseverance can at times need up to 900 watts. The generator therefore charges two lithium-ion batteries, which cover the peaks. The heat is used too: excess heat is collected in a fluid loop and carried around the rover to keep its electronics warm through the cold Martian nights. JPL expects the generator to work for at least 14 years.

Is it a reactor, and is it safe?

An RTG is not a nuclear reactor: nothing in it splits atoms in a chain reaction. NASA stresses that plutonium-238 is not the type of plutonium used for nuclear weapons and would not work well as fuel in a reactor. Its heat comes from natural radioactive decay, with a half-life of 88 years, and the alpha particles it emits can be stopped by material as thin as a sheet of paper.

The main concern is an accident during launch. NASA acknowledges that in such an unlikely event fuel could be released into the environment, and says that several layers of protection are designed to minimise that risk. The fuel is a ceramic, which NASA compares to the material of a coffee mug: it tends to break into large pieces rather than turn into fine dust. The fuel is clad in iridium and protected by graphite sleeves and a carbon-fibre aeroshell. According to NASA, radioisotope power systems have flown on 31 US space missions since 1961, and in the three failed missions that carried them, the power systems performed as designed.

The fuel is made in the United States by the Department of Energy. Plutonium oxide is produced at Oak Ridge National Laboratory, pressed into pellets at Los Alamos and stored at Idaho National Laboratory. US production stopped in the late 1980s, and Congress has funded its restart since 2011.

Why the Voyagers are switching off, one instrument at a time

An RTG never stops suddenly; its output falls a little every year as the plutonium decays and the thermocouples age. NASA’s early missions show how long that can last: Pioneer 10 and 11 were designed for five years but kept communicating with Earth for 30 and 22 years.

The two Voyagers, launched in 1977 and still working, live with that decline every day. Each loses about 4 watts a year. To save power, NASA switched off Voyager 1’s cosmic ray subsystem in February 2025 and Voyager 2’s low-energy charged particle instrument in March 2025, and on 17 April 2026 it also switched off Voyager 1’s low-energy charged particle experiment. Each probe carried ten instruments, and on Voyager 1 most of them are now off. In 2025 engineers expected both spacecraft to keep at least one science instrument working into the 2030s (status as of 7 November 2026).

Every watt the Voyagers still produce also has to carry a signal across billions of kilometres, which is the job of the antennas described on Deep Space Network; how data come back from Mars is explained in How do Mars rovers send their data to Earth?, and the story of a Mars lander with a Polish-built instrument in What did Poland build for InSight?