Responding to Adam's comments:

>[Main reason why I asked was to confirm that very fact. Tom Brosz linked to some idiotic
>ramblings about Galileo's RTGs causing Jupiter to detonate and I wanted to challenge the
>doom-sayer with some real facts. Like Pu238 not being fissile, hence not a fission-trigger for a
>fusion blast. But chiefly I compared the vapourisation of Galileo to the vapourisation of
>Shoemaker-Levy, and made the point that the Pu238 would be spread very far and wide.]

One should not waste too much time worrying about flatulence from idiots. Jupiter is really, really big. Mankind's entire nuclear arsenal could be detonated at once in Jupiter's atmosphere and the explosion would probably be undetectable from Earth. The radiation of the Pu-238 in Galileo's RTGs is insignificant compared to the trapped radiation surrounding Jupiter at the orbital altitude of Io. Anyone worrying about the RTG's effect on Jupiter is obviously clueless beyond hope and should be ignored.

>The peak deceleration for the Galileo Probe was 230 G. Had the Galileo orbiter not been
>vaporized, it would have been crushed.
>
>[Forgot about that... what would the ambient pressure have been? The dynamic pressure
>would've been huge???]

The stagnation pressure peaked at about 9 atm. while the dynamic pressure peaked at about 5 atm. Most of the decelleration took place in the upper atmosphere where the gas density (hydrogen-helium) was very low, i.e. dynamic pressure = 1/2 * density * velocity * velocity. The dynamic pressure was a worry in terms of thermal protection design because it could cause chunks of material to spall off. However carbon phenolic is pretty tough stuff (think kitchen counter top material).

>[Wow! That's an amazing heat reduction. Makes me wonder if a dynamic carbon heat-shield
>wouldn't be possible for a photon drive [U238 plasma radiator temp ~ 25,000 - 50,000 K]]

Carbon phenolic is normally used in the throats of solid rocket and reaction control rocket motors, e.g. the SRBs on the Space Shuttle. This is not widely known but for quite a while the carbon phenolic used in the Shuttle's SRBs was improperly made. They missed losing a Shuttle by a gnat's eyelash due to nozzle throat spallation. Fortunately the problem was discovered and fixed before anyone got killed. Photon drives are typically based upon antimatter-matter reactions. That sort of process would have to be contained within a magnetic bottle. A big issue would be the annihilation radiation (ridiculously high energy gamma rays). Shielding against that would be very difficult.

>[Jupiter radiates heat too and has lots of cloud layers. I doubt its atmosphere is purely
>isentropic. If JIMO ever makes it off CAD screens we might get some real details on Jupiter's
>atmosphere...]

Our current President seems committed to JIMO. Obviously there's a hidden agenda but I'm not certain what it is. I'm hoping we can sneak an atmospheric probe or two onto JIMO (there are payload mass issues with JIMO because the propulsion system mass takes up most of the launch mass budget).

>[Don't know. Doesn't the hydrogen liquefy before it gets that hot? Average density is pretty high
>down there too. But no doubt Galileo liquefied too...]

Boiling is determined by temperature AND pressure. For example you can keep water liquid at 700 deg.C if it is under very high pressure. Ultra high temperature liquid water is one of the main mechanisms for minerialization on the Earth (that's how gold gets concentrated within quartz rock). Jupiter almost certainly has a metallic hydrogen core that has a temperature of thousands of degrees. Material phase change is a fascinating branch of physics.

Gary