Â鶹ÒùÔº

Powering spacecraft with solar energy may not seem like a challenge, given how intense the sun's light can feel on Earth. use large solar panels to harness the sun for the electricity needed to run their communications systems and science instruments.

However, the farther into space you go, the weaker the sun's light becomes and the less useful it is for powering systems with solar panels. Even in the inner solar system, spacecraft such as lunar or Mars rovers need alternative power sources.

As an , I teach a senior-level aerospace engineering course on the space environment. One of the key lessons I emphasize to my students is just how unforgiving space can be. In this extreme environment where spacecraft must withstand intense solar flares, radiation and temperature swings from hundreds of degrees below zero to hundreds of degrees above zero, engineers have developed innovative solutions to power some of the most remote and isolated space missions.

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

RTGs are essentially nuclear-powered batteries. But unlike the AAA batteries in your TV remote, RTGs can provide power for decades while hundreds of millions to billions of miles 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 sounds similar to that of a nuclear power plant, RTGs work on a different principle.

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

, or nuclear decay, happens 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, since the nucleus can lose protons.

When plutonium-238 decays, it emits , which consist 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 material surrounding the plutonium, which heats up that material. The radioactive decay of plutonium-238 releases enough energy that it can glow red from its own heat, and it is this powerful heat that is the energy source to power an RTG.

Heat as power

Radioisotope thermoelectric generators can turn heat into electricity using a principle called the Seebeck effect, discovered by . As an added benefit, the heat from can help keep electronics and the other components of a deep-space mission warm and working well.

In its basic form, 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 . These thermocouples allow RTGs to produce electricity from the difference in temperature created by the heat of plutonium-238 decay and the frigid 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 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 is kept hot—close to 1,000°F (538°C)—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 .

This strong temperature difference allows an RTG to turn the heat from radioactive decay into electricity. That electricity powers all kinds of spacecraft, from communications systems to science instruments to rovers on Mars, .

But don't get too excited about buying an RTG for your house. With the current technology, they can produce only a . That may be enough to power a standard laptop, but 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, in an RTG will have decayed away. An RTG requires no moving parts to generate electricity, which makes them much less likely to break down or stop working.

Additionally, they have an , 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 and the 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 and then journey beyond it.

Each craft was , 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, .

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 . 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.