The predictions of Walt Brown

Coragyps - Yes I think you are right that the vacuum in space is somewhere like the best that we can achieve on the ground. That means we have to assume around 1E-10 mBars or better. It isn't easy to find data on systems that low.Your bell jar is probably powered by a single stage rotary vane pump. Right?I use two twin stage rotary vane pumps as the backing pressure (about 2E-2 mBar) for a bank of 3 turbo molecular pumps in order to reach these very low pressures.PY

And it was probably a pre-World War II model at that.

On the question why haven't comets sublimed totally?One issue I wonder about is the amount of energy striking the comet as it approaches the sun. Since they are very small in relation to a planet, just how much radiant energy will they get even relatively close to the sun? Is it simply a case that only the leading side gets exposed to radiation while the rest remains at very low temperatures?IIRC, the side of Mercury away from the sun is at very near absolute zero.The only means I can see for heat transfer would be a slight amount that would be spread by the material driven off and a small amount transfered directly through the body.

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Aslan is not a Tame Lion

Comets should get plenty of radiation when close to the Sun - say, Mars distance or so. What must happen is that any surface volatiles - water, carbon dioxide, etc. - get cooked off leaving a non-volatile crust which then at least partially shields the ice below from more radiation. Comets invariably rotate, so they would get cooked, sooner or later, from all sides.

The stuff that sublimes off is what makes the tail that always faces away from the sun. (following the solor wind)[speculation] I guess comets must continuously get smaller with each circuit. The only thing saving them is that they spend a vast majority of their lives out where the sun's radiation is too small to give the ice that little bit of energy that it needs in order to change phase. [/speculation]FYI I really know bugger all about comets.PY