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Powering spacecraft in the vastness of space presents unique challenges, despite the Sun’s powerful energy felt on Earth. For missions close to home, such as those operating within the inner solar system, large solar panels efficiently capture sunlight to supply the necessary electricity for communication and scientific instruments.
As spacecraft travel deeper into space, however, the sunlight becomes increasingly faint. This drop in solar energy significantly reduces the viability of solar panels, making alternative energy solutions essential for operations on planetary surfaces, such as those found in lunar and Martian exploration.
In an aerospace engineering course I conduct, emphasizing the harsh realities of space is fundamental. Space presents extreme conditions that spacecraft must endure, including intense radiation, severe temperature fluctuations ranging from frigid cold to extreme heat, and powerful solar flares. To tackle these conditions, engineering ingenuity has led to innovative power solutions for exploration missions in the furthest reaches of our solar system.
One key technology developed in the 1960s, rooted in scientific principles known for over two hundred years, is the radioisotope thermoelectric generator (RTG). These sophisticated devices serve as nuclear-powered batteries capable of functioning for decades, even when hundreds of millions to billions of miles away from Earth.
Understanding Nuclear Power
RTGs operate differently from conventional batteries, such as those found in consumer electronics. Rather than relying on chemical reactions, RTGs harness the energy produced through the radioactive decay of specific elements, primarily plutonium-238. This element is unsuitable for nuclear reactors as it does not undergo fission, but its decay process is invaluable for generating long-lasting power.
Radioactive decay involves an unstable atomic nucleus releasing energy and particles to stabilize itself. This decay often results in the transformation of elements as particles—specifically alpha particles—are emitted from the nucleus. For plutonium-238, which initially has 94 protons, this decay results in the creation of uranium-234, reducing the proton count to 92.
The emitted alpha particles transfer energy to surrounding materials, effectively converting heat into a usable power source. In fact, the radioactivity in plutonium-238 generates enough energy to radiate noticeable heat, which RTGs harness for electricity generation.
Harnessing Heat for Power
The conversion of heat to electricity in RTGs is achieved through a phenomenon known as the Seebeck effect, discovered by Thomas Seebeck in 1821. The principle illustrates that when two different conductive materials are joined and exposed to a temperature gradient, a current is generated.
RTGs incorporate thermocouples that facilitate this process, generating electricity from the temperature differential between the heat emitted from the decay of plutonium-238 and the extreme cold of the surrounding space.
Design of Radioisotope Thermoelectric Generators
The structure of an RTG consists of a container filled with plutonium-238 dioxide, which is often sealed in a ceramic form for safety. A protective layer of insulating foil encompasses this core, which is linked to numerous thermocouples. The entire assembly is encased within a protective aluminum shell.
In operation, one side of the RTG maintains extremely high temperatures—close to 1,000 degrees Fahrenheit (538 degrees Celsius)—while the opposite side, exposed to the coldness of space, can plummet to several hundred degrees below freezing. This substantial gradient enables efficient conversion of thermal energy into electrical power.
RTGs reliably supply power for diverse spacecraft systems, from communications to scientific instruments and rovers on Mars, with five active NASA missions currently utilizing this technology.
However, the power output of RTGs remains limited to a few hundred watts, adequate for deep-space applications, but not for conventional home use.
What makes RTGs particularly advantageous is their ability to deliver continuous, dependable power across decades. The consistent nature of radioactive decay means that plutonium within an RTG will only decay by half after approximately 90 years, enabling RTGs to function for extended periods without mechanical failures.
Furthermore, RTGs boast an impressive safety record, designed to endure operational demands as well as any potential accidents.
The Legacy of RTGs in Space Exploration
RTGs have been instrumental in numerous successful missions conducted by NASA. Notable examples include the Mars Curiosity and Perseverance rovers along with the New Horizons spacecraft that visited Pluto in 2015. This last mission is particularly significant, as New Horizons continues its journey outside the solar system, relying entirely on RTGs for power where solar energy is insufficient.
The Voyager missions, launched in 1977, exemplify the capabilities of RTGs. Eager to explore the outer solar system, Voyager 1 and Voyager 2 were fitted with three RTGs, delivering a total initial power output of 470 watts. Nearly five decades later, both spacecraft remain operational, transmitting valuable scientific data back to Earth.
As the most distant human-made objects—traveling approximately 15.5 billion miles and 13 billion miles (nearly 25 billion kilometers and 21 billion kilometers) from Earth—Voyager 1 and Voyager 2 showcase the remarkable reliability of RTGs, continuing to power their long-term scientific endeavors.
These missions are a testament to the brilliant engineering and innovative design that has propelled human understanding of the cosmos.
Source
www.astronomy.com