It’s been exactly 44 years since Voyager 2 was launched into space. Its achievements are more than grand, and even scientists consider its progress spectacular. However, for it to function, it needs energy, obviously, and that is something the scientists at NASA had thought about long before they launched the probe. The method of powering a space probe we will discuss has been used for more than two dozen space probes over the years.

Now let’s get into it. If you think right now about one option for powering a spacecraft, the first thing you think about could be batteries. If this sounds too basic in your head when thinking about a spacecraft, the second answer could be solar panels. Why wouldn’t these two work? The fact is they do work, quite well in fact, but not for the sorts of missions Voyager was designed for.

Batteries, well, you would need lotta batteries to power a spacecraft until reaching interstellar space. Solar batteries could be an option if only our technology would be a bit more advanced. As you may know, solar panels are no longer efficient from a certain distance since the probe is so far away from the Sun.

You could try and solve the problem by extending the size of the panels, which would eventually get you more energy. Also, extending the capacity of energy storage in the cells would help too. Wonderful projects have been designed by scientists and enthusiasts alike, such as the one in the picture below: 

World Space Week - Origami and space research - Scientix blog Scientix blog
A solar panel made based on an origami; | Credits: Scientix blog.

Until now, the most distant uses of solar arrays are in the case of Juno (a probe designed to orbit Jupiter, launched in 2011), with a 45 m2 planar array area, Dawn (a probe launched 2007 to explore the protoplanets Vesta and Ceres, in the asteroid belt), with 36.4 m2 planar array area, and the world-known Rosetta (which visited a comet for the first time), with 64 m2 planar array area. Don’t get me wrong, the panel size is not the only thing that matters, but as cells are harder to improve, increasing the size was a good option for a shorter frame of time.

Radioisotope Thermoelectric Generator

More commonly called RTG. It is an innovative method of powering space probes for much longer trips. The RTG is a battery, in some way, but it works a bit differently. Much better to say it is a nuclear battery. It functions with the decay of some radioactive material that releases heat, which is further transformed into energy with the help of a thermocouple.

A thermocouple produces a temperature-dependent voltage. What is actually transformed into voltage is the difference between temperatures. When two different metals are connected, and there is a temperature difference between them, a magnetic field is produced, further transformed into the current. How is that difference created?

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The main component of this whole thing is a container of radioactive material, called the fuel. When the radioactive material decays, it produces heat, which heats one side of the thermocouple, one of the metals. On the other side, the cool temperature results from the ambient temperature and deep space, which is really cold, as you know already. This is an easy way of explaining since I simplified the design to imagine the device. However, in reality, these things are much tighter under control.

The thermocouple actually has heat distribution blocks that capture the heat generated by the decay and send it somewhere better localized and condensed. For the cool part, materials with very low thermal conductivity and high electrical conductivity are used. As a result, better and better RTGs are created each day. 

Radioisotope thermoelectric generator - Wikipedia
A model for an RTG; Credits: Wikipedia

Then.. how much do they last?

Nothing is forever. Each of the radioactive materials has a half-life. Since we are here, half-life, which I will explain in a bit, is not the only characteristic that one needs to have in mind when choosing a material: the ability to produce high-energy radiation, tendency to produce radiation decay heat, large heat power-to-mass ratio are also among the most important things. The original list of 1300 radioactive isotopes considered was reduced to 47 with the suitable characteristics. Now, what is half-life?

Basically, it tells us how long it takes for atoms to undergo radioactive decay. That helps us choose a material with a longer half-life since we know we can rely on that one for a longer period of time. So now we will reduce the number of 47 suitable materials to 3, most commonly used, Plutonium-238, Curium-244, and Strontium-90. 

How does Voyager 2 work?

Voyager 2 works with Plutonium-238. Collectively, the 3 RTGs it has supplied the spacecraft with 470 watts at launch, and those halve every 87.7 years. Why 87.7? You might have guessed it. It is the half-life of Plutonium-238. It comes down to NASA now how they manage the resources the spacecraft has. And they did that incredibly well until now, saving already a lot of power! How do they do that? They turn off systems when not needed (permanently) or broken with no chances of improvement. The PPS (Photopolarimeter System) was the first Voyager 2 instrument turned off because it was defective in 1991, saving 1.2 watts. 

44 Years & More Than 20 Billion km In Space: The Source That Powers The Two Voyager Spacecraft. 1
Voyager 2 is one of the farthest artificial objects in space

In time, other instruments were turned off too, but still, Voyager 2 is expected to be no longer able to power any instrument by 2025 or a bit later. It may be able to send feeble signals, but we can’t know that for sure. In any case, it worked for longer than most scientists expected, and its results are magnificent. Even after Voyager 2 will no longer send signals back to Earth, it will continue its travel through space until it possibly reaches another star if it doesn’t collide with anything along the way.

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