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Explain xkcd: It's 'cause you're dumb.
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Plutonium
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.

Explanation

Ambox notice.png This explanation may be incomplete or incorrect: Created by a Power Orb. Please mention here why this explanation isn't complete. Do NOT delete this tag too soon.

This comic pokes fun at the properties of plutonium, claiming that it is so unrealistically powerful that it may as well be random sci-fi jargon. Indeed, the ability for a metal to radiate free energy sounds impossible (this comic is likely a simplification). This is reflected by Megan and Hairy treating Cueball's idea as a practical joke.

There are devices that need substantial electrical power over long time – in 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 large 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 also adds mass to the probe.

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 thermocouples, which generate electricity directly from heat by employing 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.

The title text references development of team-written episodic stories, such as video games, television shows, or comic series, and how after joining a team and implementing a mechanic, a writer can leave, and give others working on it little or no knowledge of how to handle a specific plot element or design.

Transcript

[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.


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