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Ace Hoffman
Trying to avoid the horror of being misunderstood. Is that clear?
Trying to avoid the horror of being misunderstood. Is that clear?

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Regarding Cassini's destruction into Saturn, someone asked me about the plutonium on board. My response is below.
[Hi Ace],

I came across your website looking for information about what can be expected to happen as a result of an RTG being incinerated in re-entry. What would happen to all the PU-238? Incinerated? Vaporised? Distributed around the planet in low quantities? It wasn't clear to me.

Specifically I would like understand what would be the result of the RTG being incinerated in the upper atmosphere of Saturn and, as above, what we would be leaving in its atmosphere?

Thank you for your time

My response:

If Cassini had reentered earth's atmosphere after its Venus flyby (which greatly increased its speed for the long trip to Saturn) NASA estimated that one of the three Radioactive Thermoelectric Generators (RTGs) would have failed completely and incinerated its entire contents in fine particles in the upper atmosphere (particles the perfect size to permanently lodge in human lung tissue). These would take years or even decades to descend to earth (the half-of Pu-238 is about 87.7 years).

As it did so, it would have killed millions of people, but these deaths would have been statistically very hard or even impossible to identify, since they would have been scattered all over the planet. About 55 million people die every year anyway, from all causes (including old age), so an extra few hundred thousand per year is statistically impossible to isolate out. But the generally-accepted theory for radiation harm (known as "Linear, No Threshold" (LNT)) strongly suggests such deaths would occur.

LNT also suggests that a million people are dying right now from a prior "accidental" release of 2.2 pounds of plutonium dioxide. ("Accidental" is in quotes because the system NASA used back then (for a probe known as SNAP-9A) was to intentionally disperse the plutonium at high altitude in the event of an accidental reentry, which occur with surprisingly regular frequency and could have been expected.)

There was about 72.3 pounds of Plutonium Dioxide on board Cassini, and nearly all of that was in the RTGs (about two pounds was in the 127 or so Radioactive Heating Units (RHUs), the rest was in the three RTGs).

The reason one RTG would have incinerated was that the RTGs were located approximately 120 degrees apart from each other, and NASA concluded that that one RTG would get hung up in the debris of the rest of the space probe. The other two, they figured, would break off, decelerate as their fins burned off, and release their smaller internal units (each with two finger-bone sized pellets of Plutonium Dioxide) more or less intact -- unless they fell onto a hard surface, such as concrete, rock, tile roofing, or NASA scientists' heads. The RHUs would also incinerate in the reentry conflagration.

So I assume that essentially the same thing will happen out at Saturn, except that because of that planet's enormous size, Cassini might well be traveling even faster than it was when it did the flyby of earth. Also, Saturn's atmosphere is different so that will have an effect too, one way or another.

Fortunately nobody lives there!

Thanks again, and please feel free to follow-up if you have any additional questions.

Ace Hoffman
Carlsbad, CA

Tomorrow and tomorrow (but faster still),
Creeps in this techno-pace from nanosecond to nanosecond.
To the last phonemes of digitized voice -
And all our backups have slight errors
That will last beyond our time.
Out, out! Brief calculation!
Blogs are but an 8-bit ASCII sequence
A poor representation
Which brightens and displays upon the screen
And then is retrieved no more.
It is a tale
Saved with cut-and-paste,
Signifying a sequence of zeros.

-- (with apologies)

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