2115: Plutonium

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It's like someone briefly joined the team running the universe, introduced their idea for a cool mechanic, then left, and now everyone is stuck pretending that this wildly unbalanced dynamic makes sense.
Title text: It's like someone briefly joined the team running the universe, introduced their idea for a cool mechanic, then left, and now everyone is stuck pretending that this wildly unbalanced dynamic makes sense.


This comic pokes fun at the properties of plutonium, claiming that it is so unrealistically powerful that it may as well be random science fiction jargon. Indeed, the ability for a metal to radiate energy sounds impossible (this comic leaves out the inherent dangers of highly radioactive material). This is reflected by Megan and Hairy treating Cueball's idea as a joke.

There are devices that need substantial electrical power over long time – on the order of decades – but local sources of energy are insufficient or unavailable, yet constructing a power line or resupplying them with some power source (like fuel, fresh chemical batteries etc.) is either impossible or overly costly. Such devices include maritime beacons and buoys, automatic weather and science stations located in remote areas, and – most importantly – deep space probes and some planetary probes or science packs. Probes sent beyond Jupiter cannot effectively rely on photovoltaic panels for energy, because the great distance to the Sun means that the amount of solar radiation per unit of area is very low, requiring impractically large (and thus heavy) panels to provide enough energy. Carrying a lot of fuel adds mass to the probe, making them more expensive to launch.

Instead, such devices usually use radioisotope thermoelectric generators (RTGs). In an RTG the natural radioactive decay of some unstable isotope (such as plutonium-238 or strontium-90) produces a lot of heat, which is then used to generate energy using thermopiles, which generate electricity directly from temperature differences using the thermoelectric effect. The key element of an RTG, a pellet of radioactive material such as plutonium dioxide, could be facetiously described as a "power orb" – a lump of a substance that gives out heat apparently out of nothing. For example, the Voyager probes used three RTGs, each containing 4.5kg of plutonium-238, each producing at its peak 2400W of heat energy, converted to 160W of electrical energy.

Plutonium-238 must be produced from uranium in a nuclear reactor. Unlike plutonium-239, the alpha radiation emitted by plutonium-238 is relatively harmless, as it is quickly absorbed by surrounding material and turned to heat – but plutonium is still incredibly dangerous if it gets inside a human body unprotected. In pure form it produces a little more than half a watt of heat per gram, which slowly drops as the material decays to lead, emitting a quarter watt per gram after 100 years. Other disadvantages of RTGs include the risk of contamination in the event of a launch failure, and the relatively limited supply of plutonium.

The title text references development of games. A rule or strategy within a game is often called a mechanic, meant as one particular rule (singular) out of the overall set of rules (game mechanics). In this context, the word mechanics is a metaphor referring to the set of rules and interactions that govern the imaginary world of the game. The mechanics of a game define the deterministic or randomized functions of events and/or characters within the game, the outcomes of actions commanded by the players, and so on. This metaphor refers to the mechanics science, and how it describes behavior of physical objects in the real world; However, contrary to real-world mechanics which "just happen" and we only try to describe how things work, in game mechanics every single rule or interaction has to be explicitly defined. The game simulates (to a given extent) an actual world. Game rules do not need to mimic the real world closely and often don't for many reasons; This results in (intended or otherwise) inconsistencies, unexpected behavior or imbalance. Game players complain about “imbalance” when a particular rule, interaction or item present in the game (such as an extremely powerful magical artifact) gives a character exploiting it a great and unjustified advantage. Inconsistencies and possible imbalances can lead to problematic game mechanics being unused or left unresolved, after the creator of those mechanics ceases their participation in the game or game development process.

Things that seem like they shouldn't work but do are the main topic of 2540: TTSLTSWBD.


[Megan, Hairy, Cueball, and Ponytail are talking.]
Megan: How will we keep the spacecraft supplied with heat and electricity?
Cueball: We could use a power orb. They give off thousands of watts 24/7.
Megan: Huh? How do you recharge it?
Cueball: You don't. It's just made of a metal that emits energy.
Megan: OK, come on.
Hairy: Can we please be serious here?
[Caption below the panel:]
For something that's real, plutonium is so unrealistic.


  • Kerbal Space Program, a space simulator game which has been featured in xkcd several times, has its own version of the RTG, running on Blutonium-238. However, due to the lack of a half-life mechanic, the RTG is simply an infinite source power – an actual videogame power orb. Randall has made several references to the game.

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I feel like this is a reference to Kerbal Space Program - the RTG in the game runs on Bluetonium-238, and there is no half-life mechanic, leading to an actual power orb that provides stable power forever. 08:56, 27 February 2019 (UTC)

Even though space is cold, it conducts so poorly that spacecraft would probably have more problems getting rid of heat than keeping heat, considering how isolated they are. Tharkon (talk) 16:43, 22 February 2019 (UTC)

It actually has little to do with conduction; the heat radiates pretty effectively, especially as it gets "hotter" vs the surrounding radiation. 17:35, 22 February 2019 (UTC)
You're right, spacecraft are cooled by radiation. Yet it is far less effective than conductive/convective cooling by blowing the surrounding medium (water, air, whatever) over the hot parts. It's so much easier to cool things down here on Earth! Cooling problems is one of the reasons why nuclear reactors are not very popular in space, they need massive cooling systems.

Reminds me of reddit.com/r/outside Linker (talk) 16:54, 22 February 2019 (UTC)

I put in how the title text makes a probable reference to game development. Netherin5 (talk) 17:41, 22 February 2019 (UTC)

It could, with equal probability, be a reference to parallel universes. There's nothing anywhere that says anything about game development.... 18:29, 22 February 2019 (UTC)
”Cool Mechanic” “Unbalanced” and “Join the team” seemed like hints Netherin5 (talk) 18:33, 22 February 2019 (UTC)
It seemed obvious to me it was a reference to episodic story development, as it looks like that happens with shows and comics all the time. Don't understand how it makes sense for parallel universes (except that this kind of happened with star trek and the introduction of the parallel reality) but recommend updating the article to include all interpretations. 21:20, 22 February 2019 (UTC)
I thought it was more in the line of someone joining a D&D game, doing something crazy with their character then leaving, and the remaining players all have to keep up the story.
(Unsigned: Please sign your posts!)
Definitely refers to gaming, in my opinion: These phrases are used extensively in gaming circles & almost nowhere else.
ProphetZarquon (talk) 23:09, 27 February 2019 (UTC)

This seems to be in the same vein as two other recent comics, Internet Archive and ArkXiv. Perhaps real things that seem unrealistic is a new topic of Randalls? 17:53, 22 February 2019 (UTC)

Nope, it goes back at least to the 331st comic! 21:22, 22 February 2019 (UTC)

Re: "Note that the radioactive material obviously doesn't produce this energy forever, although it can produce it so long the device will break before it gets out of energy." The main problem with Voyager I and Voyager II is not that the devices on board have been broken, exhausted a finite reserve or otherwise failed, but that the power supply can not keep most of the machine powered anymore. In fact, if there was enough electrical power, most of the cameras and other sensors would still work; they might see much that far from the sun and other bodies, but they would work. The plutonium power source undergoes exponential decay, producing proportionally less power each year and each year, the NASA scientists have to decide which devices on the spacecraft need to power-down, never to turn on again, or maybe a device is so important, but needs so much power that they will turn it on for less and less time, sort of like rotating brown-outs. I think the Pioneer probes are in the same boat. Nutster (talk) 04:13, 23 February 2019 (UTC)

Also because of the many restrictions on use of highly radioactive material, plus the basic weight factor, you would only use just the amount of plutonium required to power the craft fully over it's expected lifetime. After that lifetime you no longer have full power due to decay. 05:47, 25 February 2019 (UTC)
Actually, the half-life of the plutonium has a relatively small effect on the loss of power, the degradation of the thermocouples is a more significant cause of the power loss of RTGs on long term spacecraft missions. Plutonium loses about 0.79% of the available energy every year due to decay. TimO (talk) 09:14, 25 February 2019 (UTC)

Could someone perhaps expand on the term "implementing a mechanic"? In New Zealand English at least, a mechanic is a person who works with machines. I thought this was maybe a synonym for "implementing a mechanism", but it seems to have a specific meaning in the team episode writing sense discussed here. 23:41, 24 February 2019 (UTC)

As a non-native speaker, I am familiar with the word "mechanic" on a gameplay level. See https://en.wikipedia.org/wiki/Game_mechanics. I am fairly certain that is what Randal is refering to. 07:34, 25 February 2019 (UTC)
This use of the word "mechanic" is extremely confusing. To most English speakers it means someone who fixes machines, usually cars. Either remove it or explain the unusual jargon meaning in this context
In the US, we also call that a mechanic, or a repairman. But in this context, mechanic is referring to mechanics of a story or game, a synonym to feature. Hope that helps, and sign your comments. Netherin5 (talk) 15:04, 25 February 2019 (UTC)
Tried to explain "implementing the mechanic" (which is actual game developers' jargon) but had to edit out the references to TV shows, comic series and whatever was there before - I have no knowledge how these are created and discussed in their own jargon. The explanation is rather long but the topic is a bit convoluted indeed and hard to explain in plain words (for me at least). Thanks, Netherin5, for trying to fix it up. -- 18:10, 26 February 2019 (UTC)
Mechanic in this case is short for "mechanical function", a method used in games (either physical or video) to allow changes to the game state. For example, a board game may use dice for the purpose of moving a token a number of spaces. That would be referred to as a movement mechanic. In a video game, mechanics might include movement (walking, jumping) or crafting (combining multiple items to produce a new one). -- 16:27, 28 February 2019 (UTC)

The explanation of the title text regarding computer games could be expanded to just games in general. This could also refer to a table top Role Playing Game where the game master who runs the universe through a series of narrative follows the game engine's mechanic. Some guest GM came in and introduced a bunch of stuff but left. The rest of the players continue to play with the current dynamic even if its unbalanced and should not make sense. Long stretch, the character in the comics are playing an role playing game (hence the questions) where the engine requires them to stick to reality. When Cueball suggested the power orb, everyone dismissed it as an unobtanium that Cueball made up and didn't realize that one of such thing exist in real life too.

Don't worry, the game devs nerfed the magic power orb by making it very dangerous to be near. Stickfigurefan (talk) 16:37, 17 August 2023 (UTC)