The extremely cool NASA JPL video Triumph at Saturn (Part I) is really worth a watch and/or listen.

At about 17:40 it discusses Cassini's RTG and at 18:36 there is a shot of four RTGs in a row being checked for radiation levels.

RTGs have been in short supply, and because they start degrading as soon as they are assembled (or as soon as the radioisotope has been refined) they're not something one might just have a stock of laying around, or so it seems to me.

Question: Why are there four RTGs in a row sitting in this room? What are they waiting for? Were they built together and stored for separate launches?

screenshot from the extremely cool NASA JPL video "Triumph at Saturn (Part I)"

cued at 17:40

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    $\begingroup$ I like the monoliths behind the RTGs too. Cue "Also sprach Zarathustra" $\endgroup$ Oct 25, 2021 at 22:04
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    $\begingroup$ @OrganicMarble I saw the movie when it first came out in theaters (which somewhat dates me) and when I had learned to read better I read the book. I discovered math not in school, but in the dimensions of the monolith: the squares of the first three prime numbers. They pulled it off in Seattle back in 2001 (also here) and again more recently. These are better looking, but a bit on the thin side for Clarke's tastes. $\endgroup$
    – uhoh
    Oct 27, 2021 at 0:29
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    $\begingroup$ Yeah, I bugged my parents to take me to see it when it first came out. My dad slept blissfully through most of it. The soundtrack is the first vinyl LP I bought too. i.imgur.com/3syd5pL.png $\endgroup$ Oct 27, 2021 at 1:17
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    $\begingroup$ @JamesErvin a radiation safety-trained person knows what to expect, and (as shown) carries instrumentation to check radiation levels, as well as a personal dosimeter. 238Pu is primarily an alpha particle emitter and those have a very short range (microns). In that sense it's like the 241Am alpha sources in smoke detectors, but a lot more powerful. There is a small neutron background, so you would not want to stand that close for a long time. My guess is that this is a unoccupied storage area to minimize large doses of thermal neutrons, and folks only visit for short periods of time. $\endgroup$
    – uhoh
    Oct 28, 2021 at 6:46
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    $\begingroup$ @uhoh it was a joke that I wanted to make but didn't have quite the right wording, regardless, I'm going to take a look into what you sent me. It looks very interesting! $\endgroup$ Nov 2, 2021 at 23:36

1 Answer 1


There were seven General Purpose Heat Source - Radioisotope Thermal Generators (GPHS-RTGs) assembled (there were parts for an eighth).

The seven would eventually fly in space on four different spacecraft.

Perhaps we are looking at the 3 Cassini flight units and the New Horizons one.

The flight units used by mission, with power levels at launch, were:

  • Galileo: Flight Units 1 (289 We) and 4 (288 We)

  • Ulysses: Flight Unit 3 (289 We)

  • Cassini: Flight Units 2 (296 We), 6 (294 We) and 7 (298 We)

  • New Horizons: Flight Unit 8 (245.7 We at bus instead of connector pins)

The listed power levels are electrical power levels in watts (We),


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    $\begingroup$ Wow, why did Cassini need three times the power of some of those other probes? $\endgroup$
    – Nick S
    Oct 26, 2021 at 17:47
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    $\begingroup$ @NickS that would be another question ... feel free to ask it :) $\endgroup$ Oct 26, 2021 at 18:46
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    $\begingroup$ What happened to Flight Unit 5? $\endgroup$
    – nick012000
    Oct 27, 2021 at 3:26
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    $\begingroup$ @nick012000 From the first link: "[...] the Cassini and Ulysses backup RTG (Flight Unit 5, never flown) [...]", "[...] Flight Unit 5 was completely fueled and ready to be exchanged for a faulty unit, but was never needed." $\endgroup$
    – Will Chen
    Oct 27, 2021 at 21:20

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