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There's this interesting story: Did NASA Accidentally “Nuke” Jupiter?

Long story short:

On September 21, 2003 Galileo is deorbited in order to prevent eventuality of contamination of Europa. It descends into Jupiter atmosphere.

October 19, 2003 an unexplained "blotch" appears on surface of Jupiter.

The speculation is that Galileo gradually sunk into thicker layers of Jupiter body, and eventually reached level where pressure squeezed its RTG battery so much the plutonium went supercritical. The plasma bubble takes some time to expand and reach the surface, producing the spot.

How likely is this scenario? Is this just a hoax, a baseless speculation, or is this a fact? Can someone confirm or debunk this story?

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    $\begingroup$ Quite fascinating! A bit sad though that it ends with conspiracy-thinking: Is it possible that the professional “insiders,” those at the major observatories (which get most of their funding from NASA anway) -- including NASA’s own Space Telescope Institute -- know perfectly well how this atmospheric spot arrived on Jupiter … and have been quietly told not to “immortalize” another monumental miscalculation by NASA … particularly, at this politically sensitive time?. That part I certainly don't believe. Probably the rest is not true either, but still an entertaining read. $\endgroup$
    – gerrit
    Jul 31, 2013 at 12:26
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    $\begingroup$ Also, see some other work by the same author: His writings claim that advanced civilizations exist or once existed on the Moon, Mars and on some of the moons of Jupiter and Saturn, and that NASA and the United States government have conspired to keep these facts secret. $\endgroup$
    – gerrit
    Jul 31, 2013 at 12:27
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    $\begingroup$ If you want professional questions and answers on space exploration on this site, please take care to avoid nutjobs and conspiracy theorists. This is a question possibly fit for Skeptics.SE. $\endgroup$
    – Deer Hunter
    Jul 31, 2013 at 12:44
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    $\begingroup$ Note that the RTGs of the Galileo probe carried Plutonium-238. That stuff is different from Plutonium-239, the isotope used for nuclear bombs. $\endgroup$
    – Philipp
    Jul 31, 2013 at 13:06
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    $\begingroup$ @gerrit To be fair, that's an ad hominem argument which doesn't actually address or debunk the issue at hand (not that I believe the claim, of course). $\endgroup$
    – asteri
    Jul 31, 2013 at 13:30

3 Answers 3

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The speculation is that Galileo gradually sunk into thicker layers of Jupiter body, and eventually reached level where pressure squeezed its RTG battery so much the plutonium went supercritical.

Galileo was powered by RTGs. That is a heat source powered by the decay of Plutonium-238.

Nuclear decay rate does not change. It isn't affected by pressure, density, or literally any chemical process, because the decay is a nuclear process. Since the decay is constant, the heat production rate is constant. That shouldn't cause anything like a bubble detectable from space, although admittedly, it could cause bubbling.

Nuclear criticality is a process of a fission chain reaction. So the question becomes:

  1. Does Pu-238 have a fission cross section sufficient to go critical?
  2. Are there any other radioisotopes that could go critical? (other Pu isotopes probably)

The answer to both of these is pretty handily "no". The only fissile Pu isotopes are Pu-239 and Pu-241. While I don't doubt that someone might be able to detect some Pu-239 atoms in an RTG, its fission cross section would have to dominate over the other cross sections (neutron dead-ends) within the material. Looking up the data, the neutron absorption cross section of Pu-238 looks sufficiently large to shut down any reaction. This mechanism would preclude criticality regardless of size.

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    $\begingroup$ Ah, I see. By "mechanism" you meant the neutron absorption. (Now clear in hindsight.) Well, at least I still "earned" 2 points of reputation! :-) (I had thought of leaving a comment since it was such a minor edit and I was not entirely clear on what was intended. 20/20 hindsight indicates I should have followed my first inclination.) Thank you for quickly correcting my edit. $\endgroup$
    – user56
    Aug 1, 2013 at 0:09
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The Galileo probe carried two radionucleid batteries, each one carrying 7.8kg of Plutonium-238. But Plutonium 238, while radioactive, is not the kind of Plutonium used for nuclear bombs. That's another isotope: Plutonium-239.

That's a big difference, because only the kinds of Plutonium with odd isotope numbers are fissile. Fissile means, that they are capable of sustaining a nuclear chain-reaction. Plutonium 238 is not fissile. That means that it is impossible for Plutonium-238 to undergo nuclear explosion.

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As both @AlanSE and @Philipp said, Pu-238 won't go boom no matter what you do to it. However, lets pretend it was Pu-239 and see what happens.

First, the battery puts out only about 1/2% of the power of a Pu-238 battery, Galileo doesn't work at all.

Second, while it is just barely possible to make a mass supercritical with pressure, you would have to start with a mass that's just a hair below critical and then subject it to some really extreme pressure. While we think of metal as incompressible, it will squash a bit under planetary core type pressures.

Finally, if you manage to do this, you end up with a criticality accident, not a mushroom cloud. The one I recall reading about knocked a nearby worker off a catwalk – he lived long enough to run out of the building. If you want a boom, you have to convert the subcritical mass to a very supercritical mass very quickly – which is why all the complex mess with explosives and very precise detonators.

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  • $\begingroup$ Are you sure here? This is Jupiter core we're talking about here, I don't know about pressures of conventional charge in a nuclear bomb, but we're talking about 3500GPa of Jupiter core here, and the plutonium would be protected in a very strong case (you don't want the plutonium scattered all around the launch area in case of launch failure!) which might snap rapidly under extreme pressure. $\endgroup$
    – SF.
    Jul 31, 2013 at 23:28
  • $\begingroup$ @SF. But it descends smoothly(-ish) to that pressure. Good point about the case snapping, but I imgaine that starting with relatively slow cracks and degradation is more likely - and that even if you got a 'sudden snap', it would be before the pressure was strong enough (let's say halfway down is where the case fails - still not enough to act as an implosion trigger). $\endgroup$
    – hunter2
    Aug 1, 2013 at 4:30
  • $\begingroup$ @SF Nuclear bombs use multiple sub-critical masses, and slam them together using explosives to make a single well above critical mass. $\endgroup$
    – aramis
    Aug 1, 2013 at 5:13
  • $\begingroup$ @aramis You are describing a gun-type bomb. You can't make a Pu-239 bomb that way, the assembly isn't fast enough. Pu-239 requires an implosion. $\endgroup$
    – Loren Pechtel
    Aug 1, 2013 at 18:06
  • $\begingroup$ @SF NASA isn't going to lift a case that can take anything like planetary core pressure. Besides, the pressure is just going to crush the case along with the plutonium. $\endgroup$
    – Loren Pechtel
    Aug 1, 2013 at 18:07

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