Powering Deep Space: The Ingenious Radioisotope Thermoelectric Generator

The allure of space exploration fuels humanity’s relentless pursuit to reach farther and uncover the mysteries of the cosmos. While the radiant energy of the sun generously bathes our planet, powering spacecraft with solar energy becomes increasingly challenging as we venture into the vast expanse of space.

Near Earth, spacecraft thrive on the sun’s abundant energy, utilizing large solar panels to power their vital communication systems and scientific instruments. However, as a spacecraft journeys deeper into the solar system, the sun’s intensity diminishes drastically, rendering solar panels less effective. Even missions within the inner solar system, such as lunar or Mars rovers, necessitate alternative power sources to sustain their operations.

As an astrophysicist and professor of physics, I impart a crucial lesson to my aerospace engineering students: the unforgiving nature of the space environment. Spacecraft must endure extreme conditions, including intense solar flares, radiation exposure, and dramatic temperature swings ranging from hundreds of degrees below zero to hundreds of degrees above zero. To overcome these challenges, engineers have developed innovative solutions to power the most remote and isolated space missions.

The key to powering missions in the outer reaches of our solar system and beyond lies in a technology developed in the 1960s, based on scientific principles discovered two centuries ago: radioisotope thermoelectric generators, or RTGs.

RTGs: Nuclear Batteries for Deep Space

RTGs are essentially nuclear-powered batteries, but unlike the common AAA batteries found in everyday devices, RTGs can provide power for decades, even at distances of hundreds of millions to billions of miles from Earth. Unlike chemical batteries, RTGs harness the radioactive decay of elements to generate heat and, subsequently, electricity.

While the concept may evoke comparisons to nuclear power plants, RTGs operate on a distinct principle. Most RTGs utilize plutonium-238 as their energy source, which is unsuitable for nuclear power plants because it cannot 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 emits particles and energy to achieve a more stable configuration. This process often transforms the element into another, as the nucleus may lose protons.

Plutonium-238 decays by emitting alpha particles, each consisting of two protons and two neutrons. When plutonium-238, with its 94 protons, releases an alpha particle, it loses two protons and transforms into uranium-234, which has 92 protons.

These alpha particles interact with the surrounding material, transferring their energy and causing it to heat up. The radioactive decay of plutonium-238 releases substantial energy, causing it to glow red from its own heat. This potent heat serves as the energy source for RTGs.

Radioisotope thermoelectric generators convert heat into electricity through the Seebeck effect, discovered by German scientist Thomas Seebeck in 1821. As an added benefit, the heat generated by some RTGs helps maintain the warmth of electronics and other components of deep-space missions, ensuring their proper functioning.

The Seebeck Effect: Converting Heat to Electricity

The Seebeck effect describes how two wires composed of different conducting materials, when joined in a loop and exposed to a temperature difference, produce an electric current in the loop. Devices utilizing this principle are called thermoelectric couples, or thermocouples. Thermocouples enable RTGs to generate electricity from the temperature difference created by the heat of plutonium-238 decay and the frigid cold of space.

A basic radioisotope thermoelectric generator consists of a container housing plutonium-238, typically in the form of plutonium-dioxide, often in a solid ceramic state to enhance safety in case of an accident. The plutonium material is encased in a protective layer of foil insulation, to which a large array of thermocouples is attached. The entire assembly is housed within a protective aluminum casing.

The interior of the RTG and one side of the thermocouples are maintained at a high temperature, close to 1,000 degrees Fahrenheit (538 degrees Celsius), while the exterior of the RTG and the other side of the thermocouples are exposed to the extreme cold of space, reaching temperatures as low as a few hundred degrees Fahrenheit below zero.

This substantial temperature difference empowers RTGs to convert the heat from radioactive decay into electricity, which powers various spacecraft systems, including communication systems, scientific instruments, and Mars rovers, supporting five current NASA missions.

RTGs: Not for Home Use

Despite their impressive capabilities, RTGs are not suitable for powering homes. With current technology, they can only generate a few hundred watts of power, sufficient for a standard laptop but insufficient for power-intensive activities like video gaming with a powerful GPU.

However, for deep-space missions, a few hundred watts are more than adequate. The primary advantage of RTGs lies in their ability to provide predictable, consistent power. The radioactive decay of plutonium is constant, occurring every second of every day for decades. Over approximately 90 years, only half of the plutonium in an RTG will decay. RTGs require no moving parts to generate electricity, making them highly reliable and less prone to failure.

Additionally, RTGs boast an excellent safety record and are designed to withstand normal use and remain safe in the event of an accident.

RTGs: Enabling Deep-Space Exploration

RTGs have been instrumental in the success of numerous NASA solar system and deep-space missions. The Mars Curiosity and Perseverance rovers and the New Horizons spacecraft, which visited Pluto in 2015, all rely on RTGs for power. As New Horizons ventures beyond the solar system, its RTGs will continue to provide power where solar panels would be ineffective.

However, the Voyager missions exemplify the remarkable capabilities of RTGs. Launched in 1977, the twin spacecraft Voyager 1 and Voyager 2 embarked on a tour of the outer solar system and then journeyed beyond.

Each Voyager spacecraft was equipped with three RTGs, providing a total of 470 watts of power at launch. Nearly 50 years later, both probes remain active science missions, collecting 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 ever. Even at these immense distances, their RTGs continue to provide consistent power.

These spacecraft stand as a testament to the ingenuity of the engineers who first conceived of RTGs in the early 1960s. Their innovation has enabled humanity to explore the farthest reaches of our solar system and beyond, expanding our understanding of the universe.