Harnessing solar energy for spacecraft might appear straightforward, especially considering how powerful the Sun’s rays feel on Earth. Spacecraft operating within Earth’s proximity utilize extensive solar panels to capture sunlight, generating the necessary electricity to operate their communication systems and scientific instruments.

However, as missions venture deeper into space, the intensity of the Sun’s light diminishes significantly, making solar panels less effective for energy generation. Even in the inner solar system, spacecraft like lunar and Mars rovers often require supplementary power sources to maintain their functionality.

As an astrophysicist and educator specializing in physics, I lead a senior-level aerospace engineering course focusing on the challenges presented by the space environment. One crucial lesson I impart to my students is the harsh reality of space, where spacecraft must endure extreme conditions, including intense solar flares and drastic temperature fluctuations ranging from hundreds of degrees below zero to soaring temperatures above zero. Engineers have ingeniously devised solutions to power some of the most isolated and challenging space missions.

So, what innovative methods do engineers employ to power missions located in the farthest reaches of our solar system and beyond? The answer lies in technology pioneered in the 1960s, rooted in scientific principles established two centuries earlier: radioisotope thermoelectric generators (RTGs).

RTGs function as nuclear-powered batteries, yet they differ significantly from the AAA batteries you might find in your remote control. Unlike conventional batteries, RTGs can provide reliable power for decades, even while being hundreds of millions to billions of miles away from Earth.

Understanding Nuclear Power in Spacecraft

Radioisotope thermoelectric generators operate on a principle distinct from the chemical reactions that power everyday batteries, such as those in mobile devices. Instead, RTGs utilize the radioactive decay of specific elements to generate heat, which is then converted into electricity. While the underlying concept may appear similar to that of a conventional nuclear power plant, the mechanisms at play in RTGs are fundamentally different.

Most RTGs rely on plutonium-238 as their energy source. This particular isotope is not suitable for use in nuclear power plants, as it does not sustain fission reactions. Rather, plutonium-238 is an inherently unstable element that undergoes radioactive decay, releasing energy in the process.

Radioactive decay, or nuclear decay, occurs when an unstable atomic nucleus spontaneously emits particles and energy, transitioning to a more stable state. This decay process frequently results in the transformation of the element itself, as the nucleus may lose protons during this emission.

During the decay of plutonium-238, alpha particles are released, which are composed of two protons and two neutrons. As plutonium-238, which contains 94 protons, releases an alpha particle, it loses two protons and converts into uranium-234, which has 92 protons.

These alpha particles interact with the surrounding material, transferring energy and generating heat. The radioactive decay of plutonium-238 is so energetic that it can emit a red glow from its own heat. This intense heat serves as the fundamental energy source for powering an RTG.

Transforming Heat into Electricity: The Seebeck Effect

Radioisotope thermoelectric generators leverage the Seebeck effect to convert heat into electricity, a phenomenon first identified by German scientist Thomas Seebeck in 1821. Furthermore, the heat produced by certain RTGs can maintain the operational temperature of electronic components and other critical systems in deep-space missions, ensuring they function optimally in harsh conditions.

At its core, the Seebeck effect illustrates how two wires made from different conductive materials, when joined in a loop, generate a current when subjected to a temperature differential.

Devices that utilize this principle are referred to as thermoelectric couples or thermocouples. These thermocouples enable RTGs to generate electricity by harnessing the temperature difference created by the heat emitted from plutonium-238 decay and the extremely cold conditions of space.

Designing Efficient Radioisotope Thermoelectric Generators

A basic radioisotope thermoelectric generator consists of a container housing plutonium-238, typically in the form of plutonium dioxide, often configured as a solid ceramic to enhance safety in case of an accident. This plutonium material is enveloped by a protective layer of foil insulation, to which a comprehensive array of thermocouples is affixed. The entire assembly is encased within a protective aluminum shell.

Inside the RTG, one side of the thermocouples is maintained at a high temperature—nearly 1,000 degrees Fahrenheit (538 degrees Celsius)—while the other side faces the frigid void of space, which can plummet to several hundred degrees Fahrenheit below zero. This substantial temperature gradient empowers the RTG to convert heat generated from radioactive decay into usable electricity. This electricity is crucial for powering various spacecraft systems, including communication tools, scientific instruments, and even Martian rovers, encompassing five active NASA missions.

However, prospective buyers should temper their excitement about acquiring an RTG for personal use. Current technology limits their power output to a few hundred watts. While this may suffice for running a standard laptop, it falls short for more demanding applications such as gaming with high-performance graphics cards.

Nonetheless, for deep-space missions, a couple of hundred watts prove more than adequate.

The true advantage of RTGs lies in their capacity to deliver stable and consistent power. The rate of radioactive decay of plutonium is unwavering—occurring every second of every day for decades. Over a span of approximately 90 years, only half the plutonium in an RTG will have decayed. Furthermore, RTGs do not rely on moving parts for electricity generation, significantly reducing the likelihood of mechanical failure or operational interruptions.

Additionally, RTGs boast an impressive safety record, having been meticulously designed to withstand regular operational conditions and to remain safe even in the event of an accident.

Exploring the Impact of RTGs in Space Missions

Radioisotope thermoelectric generators have played a pivotal role in the success of numerous NASA missions targeting our solar system and beyond. Notable examples include the Mars Curiosity and Perseverance rovers, as well as the New Horizons spacecraft, which conducted a historic flyby of Pluto in 2015. New Horizons is currently journeying beyond the solar system, where its RTGs continue to supply power in regions where solar panels become ineffective.

However, no missions exemplify the significance of RTGs quite like the Voyager missions. NASA launched the twin spacecraft, Voyager 1 and Voyager 2, in 1977, embarking on a mission to explore the outer solar system and subsequently venture into interstellar space.

Each spacecraft was outfitted with three RTGs, which provided a total of 470 watts of power at launch. Nearly 50 years have passed since the Voyager probes were launched, and both are still operational, conducting scientific investigations and transmitting data back to Earth.

Voyager 1 and Voyager 2 are currently located approximately 15.5 billion miles and 13 billion miles (nearly 25 billion kilometers and 21 billion kilometers) from Earth, respectively, making them the most distant human-made objects known to exist. Remarkably, even at these extreme distances, their RTGs continue to provide reliable power.

These spacecraft stand as a testament to the ingenuity and foresight of the engineers who designed RTGs in the early 1960s, ensuring their functionality and reliability for decades.

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