Utilizing solar energy to power spacecraft may seem straightforward, given the intense sunlight on Earth. However, spacecraft near Earth require large solar panels to generate the necessary electricity for their communication systems and scientific instruments.

As you venture deeper into space, the Sun’s light becomes weaker, making it less effective for powering systems with solar panels. Even in the inner solar system, spacecraft like lunar or Mars rovers need alternative power sources.

As an astrophysicist and physics professor, I teach a senior-level aerospace engineering course on the space environment. One crucial lesson I emphasize to my students is the unforgiving nature of space, where spacecraft must withstand intense solar flares, radiation, and extreme temperature fluctuations. In this environment, engineers have developed innovative solutions to power remote and isolated space missions.

So, how do engineers power missions in the outer reaches of our solar system and beyond? The solution lies in technology developed in the 1960s, based on scientific principles discovered two centuries ago: radioisotope thermoelectric generators, or RTGs.

RTGs are essentially nuclear-powered batteries that can provide power for decades, even when hundreds of millions to billions of miles away from Earth.

Nuclear Power

Radioisotope thermoelectric generators do not rely on chemical reactions like the batteries in your phone. Instead, they rely on the radioactive decay of elements to produce heat and eventually electricity. While this concept is similar to that of a nuclear power plant, RTGs work on a different principle.

Most RTGs are built using plutonium-238 as their energy source, which is not usable for nuclear power plants since it does not sustain fission reactions. Instead, plutonium-238 is an unstable element that undergoes radioactive decay.

Radioactive decay, or nuclear decay, occurs when an unstable atomic nucleus spontaneously and randomly emits particles and energy to reach a more stable configuration. This process often causes the element to change into another element, as the nucleus can lose protons.

When plutonium-238 decays, it emits alpha particles, consisting of two protons and two neutrons. When the plutonium-238, which starts with 94 protons, releases an alpha particle, it loses two protons and turns into uranium-234, which has 92 protons.

These alpha particles interact with and transfer energy into the surrounding material, heating it up. The radioactive decay of plutonium-238 releases enough energy that it can glow red from its own heat, and this powerful heat is the energy source that powers an RTG.

Heat as Power

Radioisotope thermoelectric generators can convert heat into electricity using the principle of the Seebeck effect, discovered by German scientist Thomas Seebeck in 1821. Additionally, the heat from some types of RTGs can help keep electronics and other components of a deep-space mission warm and functioning properly.

The Seebeck effect describes how two wires of different conducting materials joined in a loop produce a current in that loop when exposed to a temperature difference.

Devices that use this principle are called thermoelectric couples, or thermocouples. These thermocouples allow RTGs to produce electricity from the temperature difference created by the heat of plutonium-238 decay and the cold of space.

Radioisotope Thermoelectric Generator Design

In a basic radioisotope thermoelectric generator, you have a container of plutonium-238, stored in the form of plutonium-dioxide, often in a solid ceramic state that provides extra safety in the event of an accident. The plutonium material is surrounded by a protective layer of foil insulation to which a large array of thermocouples is attached. The whole assembly is inside a protective aluminum casing.

The interior of the RTG and one side of the thermocouples are kept hot – close to 1,000 degrees Fahrenheit (538 degrees Celsius) – while the outside of the RTG and the other side of the thermocouples are exposed to space. This outside, space-facing layer can be as cold as a few hundred degrees Fahrenheit below zero.

This strong temperature difference allows an RTG to convert the heat from radioactive decay into electricity. That electricity powers all kinds of spacecraft, from communication systems to science instruments to rovers on Mars, including five current NASA missions.

However, don’t expect to buy an RTG for your house, as they can produce only a few hundred watts of power. That may be enough to power a standard laptop, but not enough to play video games with a powerful GPU.

For deep-space missions, however, those couple hundred watts are more than enough.

The real benefit of RTGs is their ability to provide predictable, consistent power. The radioactive decay of plutonium is constant – every second of every day for decades. Over the course of about 90 years, only half the plutonium in an RTG will have decayed away. An RTG requires no moving parts to generate electricity, making them much less likely to break down or stop working.

Additionally, they have an excellent safety record, and they’re designed to survive their normal use and also be safe in the event of an accident.

RTGs in Action

RTGs have been key to the success of many of NASA’s solar system and deep-space missions. The Mars Curiosity and Perseverance rovers and the New Horizons spacecraft that visited Pluto in 2015 have all used RTGs. New Horizons is traveling out of the solar system, where its RTGs will provide power where solar panels could not.

However, no missions capture the power of RTGs quite like the Voyager missions. NASA launched the twin spacecraft Voyager 1 and Voyager 2 in 1977 to take a tour of the outer solar system and then journey beyond it.

Each craft was equipped with three RTGs, providing a total of 470 watts of power at launch. It has been almost 50 years since the launch of the Voyager probes, and both are still active science missions, collecting and sending data back to Earth.

Voyager 1 and Voyager 2 are about 15.5 billion miles and 13 billion miles (nearly 25 billion kilometers and 21 billion kilometers) from the Earth, respectively, making them the most distant human-made objects ever. Even at these extreme distances, their RTGs are still providing them consistent power.

These spacecraft are a testament to the ingenuity of the engineers who first designed RTGs in the early 1960s.

Benjamin Roulston, Assistant Professor of Physics, Clarkson University. This article is republished from The Conversation under a Creative Commons license. Read the original article.