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Plutonium powered RTGs are encased to survive re-entry. According to the Wikipedia article on Apollo 13

RTGs were used to power … the scientific experiments left on the Moon by the crews of Apollo … Because the Apollo 13 Moon landing was aborted, its RTG rests in the South Pacific Ocean[6]

The RTG was carried on the LM, which separated from the CM more than an hour before splash down and re-entered on its own about 500 miles uprange from Apollo 13.

Apollo 13's final midcourse correction had addressed the concerns of the Atomic Energy Commission, which wanted the cask containing the plutonium oxide intended for the SNAP-27 RTG to land in a safe place. The impact point was over the Tonga Trench in the Pacific, one of its deepest points, and the cask sank 10 kilometers (6 mi) to the bottom.

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Google Earth

enter image description here https://honeysucklecreek.net/msfn_missions/Apollo_13_mission/a13_re-entry-groundtrack.html dashed red line is Apollo 13 ground track to splashdown

The splashdown of the CM was very accurate: the “miss distance” was 1.85 miles https://en.wikipedia.org/wiki/Splashdown The RTG landed 500 miles away, over the deepest spot I could find in the Tonga Trench. Good shooting, or good luck? The CM can be steered during re-entry, but re-entry and break-up of the LM should be less predictable. How much control did NASA have over the splashdown site of the RTG?

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    $\begingroup$ Cue spy-thriller plot where the entire reason for the Apollo 13 malfunctions was sabotage intended to facilitate somebody getting their hands on the fuel without anyone the wiser... $\endgroup$
    – Perkins
    Commented Nov 10, 2023 at 19:04
  • $\begingroup$ @Perkins ... directed by James Cameron? $\endgroup$
    – Woody
    Commented Nov 10, 2023 at 19:47
  • $\begingroup$ @Perkins Except what would they do with it? RTGs use Pu-238, all the isotopes that would work in a bomb are odd. $\endgroup$ Commented Nov 12, 2023 at 5:56
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    $\begingroup$ @Woody When the multiplication factor hits 1 you have a chain reaction--useful for power. You have a little wiggle room because not all neutrons are emitted immediately. Once you can sustain a reaction on prompt neutrons you have a Chernobyl. For a bomb you must very, very quickly convert a subcritical mass into one with a multiplication factor of 2 or thereabouts--the reaction has to be fast enough that the critical mass doesn't go flying apart before it's done. $\endgroup$ Commented Nov 14, 2023 at 19:40
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    $\begingroup$ @Woody. Yup--and note that those critical mass numbers are to get a multiplication factor of 1. The fancy engineering of a plutonium bomb is to convert it from a mass with a multiplication factor less than 1 into one something like 2 very quickly before it vaporizes itself. Just pouring too much plutonium into a container produces a bang that nearby witnesses have walked away from (but died later.) $\endgroup$ Commented Nov 15, 2023 at 23:49

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Good shooting.

Chuck Deiterich, lead retrofire officer, was responsible for the impact point.

From Henry S. F. Cooper's book Moonwreck aka Thirteen: The Flight That Failed:

Deiterich, however, insisted that it was important, because the LM carried a metal cask full of radioactive fuel, which had to be aimed so that it landed in deep water. The radioactive fuel had been intended for powering scientific instruments left on the moon, and its cask was the only part of the LM that was strong enough to remain intact during the heat of reentry; it had been designed this way because during a previous mission such a cask, aboard an unmanned satellite, had scattered its radioactive contents through the upper atmosphere, and four years later these could still be detected around the world. Earlier in the Apollo 13 flight, when it had seemed possible that the spacecraft would come down in the Indian Ocean rather than in the Pacific, the Atomic Energy Commission had sent anxious word that the fuel cask would land uncomfortably close to a populated area of Madagascar. Now Deiterich had assured a representative of the A.E.C. who was present that the controllers would see to it that the cask landed in deep water a couple of hundred miles off the coast of New Zealand

(emphasis mine)

later after some unexpected maneuvering

Deiterich was still worrying about the fuel cask, possibly because he felt the gaze of the A.E.C. man on the back of his neck. He rapidly figured out where the cask would come down under the new circumstances, and he was able to assure the A.E.C. representative that it would still come down in deep water.

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    $\begingroup$ How much control did NASA have over the impact location? My understanding is that separation occurred after the final course correction, after which the LM did not perform any burns. Was there any control other than the separation timing? Was NASA just satisfied that the debris field was somewhere in pelagic waters? Was it just coincidence it splashed as it crossed the trench? $\endgroup$
    – Woody
    Commented Nov 8, 2023 at 21:47
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    $\begingroup$ Separation timing is all you need, physics takes care of the rest. $\endgroup$
    – GdD
    Commented Nov 9, 2023 at 13:33
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    $\begingroup$ @Woody: The Apollo 13 crew set the switches so NASA could do a final burn by ground control. Not clear if they used it or not. (I have the book Apollo 13.) $\endgroup$
    – Joshua
    Commented Nov 9, 2023 at 17:56
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    $\begingroup$ @Woody: In theory NASA could send a ground command to use the RCS thrusters to make a burn. (The ascent engine was already inoperable due to a blown disc.) Also, the way they separated gave the LM a nudge backwards. $\endgroup$
    – Joshua
    Commented Nov 9, 2023 at 21:01
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    $\begingroup$ @Ludo a US Transit navsat in 1964. en.m.wikipedia.org/wiki/Radioisotope_thermoelectric_generator $\endgroup$ Commented Nov 10, 2023 at 13:12

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