I said previously I wanted to do a post about RTGs. This is for two reasons, first was I just did a post all about the Voyager space probes which happened to be powered by RTGs, therefore there is some continuation on a previous topic and continuity is something I like. Another reason is there is also some topical information in that NASA is starting up new fuel production for RTGs with plans that they can be used on future spacecraft.
The fundamentals of RTGs
That said, I have to start at the top, and the first thing to answer of course is… what the hell is an RTG? RTG stands for radioisotope thermoelectric generator (NASA loves using acronyms but in this case it is practically necessary). While that is a daunting name, the device is actually quite simple. Basically an RTG is a cylinder that generates electricity to power the spacecraft by creating heat and making use of what is called the Seebeck effect which is where an array of devices called thermocouples are used to convert a difference in temperatures (heat from the RTG vs. static temperature of space for example) into an electric current. The heat is not transformed into electricity as some mistakenly believe but merely creates the current. The heat still exists and has to be radiated away so the spacecraft doesn’t overheat.
Diagram of a standard RTG, it looks more complicated than it is, but as you’ll see there are reasons for the complexity, safety being one of them.
The heat in an RTG is the key, like many power sources, but what creates the heat that is easy and simple to use in deep space? Radioactive material of course! Various materials undergo a natural process of radioactive decay where an unstable atom will emit particles and thus change. This can be an isotope of an element changing to another isotope but sometimes whole elements can change. As a reminder an isotope is an element with a different number of neutrons, as proton count is what determines what element an atom is. It is possible to make some elements do this exposing them to outside radiation (making them radioactive) but some elements are inherently radioactive and emit radiation spontaneously. They tend to be pretty high on the periodic table. The most famous of these elements is one everyone recognizes but few people really understand, it is called plutonium.
Plutonium was originally a byproduct of uranium fission reactors. I wrote an article about nuclear energy a while back where I explained that in more detail. Plutonium was found to have a lot of other very useful qualities.. some of which were unfortunately very destructive. This has led to plutonium getting a bad rep. Like all atoms though it has its isotopes and the kind of plutonium that is used in nuclear reactors and ultimately nuclear weapons is Pu-239 as I mentioned in the previous nuclear article. However there is another type called Pu-238. As the name suggest it has one less neutron. It is very similar but it has an added benefit of being practically useless for making nuclear weapons (not very useful in standard nuclear reactors either for the same reason). It does have a very noticeable radioactive decay though which make it perfect for RTGs.
This is a pellet of Pu-238 (actually 238PuO2 but those details are unimportant), the radioactive fuel. The red color is due to the heat it is giving off from the decay. It isn’t always glowing red, this image was taken after it was insulated to get this effect. However inside an RTG the pellet may glow like this.
So when Pu-238 emits radiation as it does, this energy loss comes in the form of heat, and voila, we are back to the purpose of the RTG, generating electricity from heat for a spacecraft using literally a block of plutonium and some thermocouples. Very simple, very easy, very useful.
Why RTGs? Why not solar?
This begs the question, why does it matter? Everyone knows solar power creates electricity for a majority of satellites and other spacecraft. A counterpoint would simply be why not? It is good to have options. There are some people though that think RTGs are bad because they use plutonium which is bad because radiation can hurt people. This would be a valid point if there wasn’t rigorous safety precautions used in the handling of plutonium for this purpose. Then there is also the point that RTGs are used on space probes which go out into space and far away from us. Some environmentalist still protest them at that point by saying we are just hurting space or the destination but they aren’t aware that there is enough radiation in space already from natural sources that nothing we could send would do much worse. Basically there is no harm in RTG usage.
RTGs are safe enough for this woman to stand right next to them during inspection. These are the three RTGs that went on Cassini before launch. That probe is flying around Saturn now.
Humoring those ignorant people though, why not solar? It works well enough, doesn’t run out as long we have a sun, which we will for another 5 billion years or so. It is a problem of distance though. Solar works great for spacecraft, but only if near the sun. By near I mean within the orbit of Mars, farther out the sun is dimmer and to generate the same power from the sun you need bigger solar panels. All the spacecraft that use RTGs have gone pretty far out. Galileo went to Jupiter, Cassini to Saturn, Voyager went past both and out into infinity. New Horizons is zooming by Pluto eventually. The Curiosity rover has an RTG on Mars where solar works fine but that was for longevity and because martian dust on the solar panels was a constant problem with the previous two rovers (and in addition to a sand trap killed Spirit).
After some environmentalists protested the use of RTGs on Cassini before it launched, NASA did an analysis of what it would take to power Cassini with solar panels out at Saturn. A graphic, pictured below, was their main excuse for why it wasn’t done. The solar arrays needed to power it properly would have been massive, and therefore hugely expensive. Also it would have decreased the usefulness of the probe. If Voyager used solar panels (still a developing technology at the time it was launched) they’d need to be larger and larger as it traveled out so it could still be used today like it is. So simply put, solar power is not practical out past Mars, and RTGs become the best answer past a fully functioning nuclear reactor which has been done but very rarely due to difficulty (mostly by the Russians).
As can be seen, at Earth or Mars the solar panels are manageable. Farther out though the panels become several times the size of Cassini, itself the size of a bus. No rocket we have could launch that probably.
RTGs have been used terrestrially as well but their space applications are the biggest use. Voyager 1 and 2, the Pioneer spacecraft, Galileo, Cassini, Curiosity, New Horizons, even the Apollo moon landings used them for local power on the moon as well. The RTGs were left on the moon (except for Apollo 13 of course, that RTG is somewhere in an ocean on Earth where the moon lander pieces ended up after re-entry) and if any are still radiating they will succumb to their half-lives eventually and no longer generate power. This means all RTG powered spacecraft have a limited lifespan, but one good enough for most mission lengths. Voyager 1 and 2 are already running on reduced power levels from when they started and are systematically shutting off instruments to conserve power until about 2025 or so when they should eventually die out.
An RTG on the moon from Apollo 14 with an Astronaut’s shadow to the right. This RTG should still be perfectly intact on the moon today.
Somewhat ironically while RTGs can do what solar can’t on many space probes, they end up being even less efficient at generating power than solar panels. Most solar panels depending on the material and several other factors usually have an efficiency of 12 – 30%, the lower end being more likely. This means for all the sunlight hitting the panel only a 10th of it turns into useful power, the rest is mostly waste heat. This problem is one of the biggest roadblocks stopping solar power from being cheap and widespread everywhere and may one day be fixed by future technology. RTGs by comparison have an average efficiency of 3 – 7% due to the already low efficiency of thermocouples despite their usefulness. So a lot of waste heat from that on top of the natural heat from the radioactive decay. Nothing is perfect in this universe, thermodynamics tells us that, but there is always hope that we can do a little better.
Building a better RTG, the SRG
There is a design for a better power source that would improve the efficiency of an RTG to about 23% which is pretty good, and puts it up to the best solar efficiency range. This is called a Sterling Radioisotope Generator, and instead of thermocouples generating electricity from heat, it uses a sterling engine. Sterling engine is a lot like an internal combustion engine in a car, but at its simplest form. A heat source in a medium like a gas or liquid moves a series of pistons to run an electric generator, or what car people will know as the alternator which recharges the car battery when the engine is running. Despite being an engineer I admit though I am not a big car person so if I got that wrong in any way, I apologize to car buffs. So future deep space probes could be powered by small piston engines using RTGs as their heat source, which is actually pretty cool. You can buy or even make simple sterling engines for novelty use by the way if you are interested (not powered by plutonium obviously, can be heated by anything, even a candle).
A diagram of an SRG (actually an ASRG, A for Advanced).
Well I’ve talked about power sources and efficiencies to bore a thousand readers now.. but I hope you did find the topic interesting, I do think it is fun. Now you know more about spacecraft power sources too (that and old Russian lighthouses…), but why did I choose now to talk about them?
The decreasing plutonium supply and NASA’s efforts to make more for the future
The U.S. is running out of plutonium, it has been for decades. This is because all we currently have has been made as a result of nuclear weapons manufacturing, as well as reactors but as my older post explained America cared more about bombs than power. Anyway we stopped creating plutonium for weapons due to various nuclear treaties a while back. So despite how useful it is for RTGs, we don’t have enough of it. Hilariously we have actually been buying plutonium from Russia for use, but that is really expensive like everything else the Russian’s sell us.
This is why I was happy to see this article here: http://www.spaceflightnow.com/news/n1303/20pu238/
That article states how we are now starting to breed a new source of Pu-238, which I remind you is useless for nuclear weapons therefore not a threat to nuclear proliferation. With that we can start stockpiling it for use in future RTGs for missions yet planned, such as the sister rover to Curiosity currently planned for launch in 2020. It is a slow process though as the article describes so we won’t have a stockpile quite yet. With a half-life of 89 or so years it is best to use it as it is made too so you can get the most power out of it when it is fresh.
I think the way America does things when it comes to power production is a little backwards sometimes, but anytime we can end our reliance of a foreign product for energy either at home or in space, the better. Energy independence is important for America if we are going to benefit from space (at least while we aren’t doing much work with other countries past the ISS).
That is all I got for now.. hope you found this all enlightening and not as boring as it may have sounded. Next article will either be about the ISV Venture Star from Avatar or that alien biology stuff I talked about, not sure yet. Stay tuned, and more importantly, stay shiny.
State of Flux 