Gary wrote: "However the Galileo Probe was able to enter Jupiter's atmosphere at such high speed because that atmosphere was mostly hydrogen which has enormous heat capacity. Had the probe entered the Earth's (or Titan's) mostly nitrogen atmosphere at that speed then it would have vaporized."

Gary,

Could you explain a bit more about the physics of how a hydrogen-rich atmosphere allows more heat loading? I can see a couple of ways that it *might* work, but I'd appreciate advice about how it *does* work.

For example, hydrogen is very low molecular weight and can be "knocked aside" easily, so would carry energy away (like a human jumping 5 metres into water, rather than lead shot)

Alternatively, the answer may be in the radiative properties of hydrogen - does it radiate the energy away from the heat shield, or a at least is it transparent and permit the heat shield to radiate, whereas other gasses may be opaque?

I tried, but I don't know enough. can you add some more detail about your fascinating area of expertise?