Adam asked:

>For a 40 m^2 aeroshell that's a shockwave temperature of ~ 13,500 K !
>Gary, is that at all feasible for an ablating aeroshell? Galileo's shockwave
>was hotter I seem to recall and we know it made it.

The temperature of interest is the "shocklayer temperature" which is also
the boundary layer edge temperature. For the Galileo Probe, the shocklayer
temperature was about 16,500 deg.K. The Galileo Probe aeroshell was
built by General Electric (GE). As an aside, the General Electric design heritage of the Galileo Probe was based upon the Pioneer Venus Large Probe. The Pioneer Venus Large Probe was based upon the Mk-6 reentry vehicle which carried the W-53 thermonuclear warhead. The Galileo Probe and Pioneer Venus large probe used the same thermal protection material which was carbon phenolic based upon a rayon fiber. The Mk-6 was a carbon phenolic based upon a nylon fiber. All three entry vehicles had a 2:1 base to nose radius ratio (it's almost a hallmark of a GE entry vehicle design). Carbon phenolic is the thermal protection material of choice if the vehicle takes a big heat flux. Carbon phenolic is less desireable if the vehicle takes a big integrated heat flux (heat load) because carbon phenolic is a relatively good heat conductor because of its high density. The Space Shuttle takes a
low heat flux but a high heat load (that's why the reusable silica foam tiles are an option). MER took a low heat flux and a low heat load (which is why we could have used balsa wood). Aerocapture to a gas giant takes a high flux and an extremely high heat load. Designing a thermal protection system for aerocapture to a gas giant is an unsolved problem (we're working on it now). Aerocapture to Triton via Neptune's atmosphere is a problem of extreme interest. It's really hard to do and probably won't happen in our lifetime.

>With that in mind perhaps an NTR powered Jupiter Orbiter/Lander might be
>a better option than JIMO? It could refuel off indigenous ices just like the >"Tsien" had planned in "2010".

Nuclear ion propulsion (JIMO) beats the snot out of NTR. Working technology ion propulsion has a specific impulse of 5,000 seconds. The best that NTR could do is about 1,000 seconds. NASA is working on JIMO right now. There is no current NTR program and none is planned that I know of. NTR becomes interesting only when manned missions to Mars become near term. If the payload is unmanned then nuclear electric is the technology of choice. I'm confident that the United States could build a working nuclear reactor for JIMO (we did it once before with SNAP-10A). However JIMO has formidable political enemies. They don't like the "N" word (nuclear). The enemies have been quiet up until now. The word is that they're waiting in bushes for us to stumble. The political loss of nuclear energy would place a glass ceiling around the Earth. It's very important this does not get screwed up.

Gary