Hi Greg,

The probe was crashed into Jupiter because its fuel was running out - in this case the depletion of electricity due to the plutoniom-238 decaying. So most of that 48 pounds of plutonium was on its way to being more and more inert.

Even were some of the PU238 to reach the inner atmosphere before melting it would not create a nuclear explosion. These folks are confusing PU238 with PU239. There is a BIG difference. Uranium-238 captures a neutron and become U-239. It decays and becomes Neptunium-239 in about 23 minutes and which further decays to become Plutonium-239 after 2 and 1/3 days. Plutonium-239 is fissionable.

But PU-238 decays via alpha radiation to become U-234. So many of the atoms in those pellets heading into Jupiter are U-234.

I could go into a long explanation of nuclear principles but I'll keep it simple - when the target nucleus of an atom has an even mass number and atomic number - it is not fissionable by slow neutrons. When the mass number is odd and the atomic number is odd - it is still not fissionable by slow neutrons. Only when the mass number and atomic number are even and odd (or vice-versa odd and even) is the nucleus fissionable.

You wanted a formula and I'll give you a simple one: Z2/A

Z is the atomic number and A is the mass of the nucleus. When Z2/A is less than 35 the critical energy for fission is so large that only super high energy neutrons could cause a fission reaction.

PU-239, U-238, U-235, U-233 and Thorium-232 are fissionable and have Critical Energy numbers at around 7.5 (Mev) and below.

PU-238 and its decayed state U-234 are not fissionable. PU-238, 94 and 238 (even, even) & U-234 92 and 234 (even,even)

That is why PU-238 was chosen to power spacecraft because it is less fissionable material and subsequently much safer. It also keeps spacecraft from being targets for folks seeking relatively easy access fissionable material for terrorist weapons.

Hope that helps clear it up for you.

It is far more likely that the recent intense solar activity interfaced with the Jovian atmosphere to cause the black spot.

SDAI-Tech1