By its past evolution I’m actually convinced that what we are now observing in case of comet ISON is its “swan song” last performance at end of its long cometary life. I would like to introduce my opinion about what is happening now with it.
Originally when it was discovered, this comet seems to be active enough near level of many other dynamically new comets and it was hard to imagine that this comet may not survive its perihelion passage. Few months later its activity become to leveling off and in spring of this year, photometric parameters of this comet pointed it near its surviving limit, defined by John Bortle. Based on his study ~70% of comet under this line end their life near perihelion passage. His study was based on 84 comets where 16 totally disintegrated. Reliability of his conclusion could be tested in some past comets, and out of 4 comets bellow this line, all totally disintegrated too. When comet ISON passed 2 AU distance from Sun its activity pointed it still deeper and deeper under this level.
First I tried to make a quantitative study of this comet compared to comets that demise and others which survived their perihelion passage. Comet ISON seems to not fit well both groups, in part of its orbit its activity pointing to larger nucleus then disintegrating comets usually have, but in other hand ISON is going much closer to Sun then any of compared comets and many of these which survived would probably not make the approach as ISON going to do.
I have found that ISON was with its photometric behavior and absolute activity very close to few of them. First one was comet C/2000 WM1 (LINEAR) which survived, but undergone a large outburst with 3 mag amplitude shortly after it passed its perihelion. I suspect that comet ISON may also show such behaviour, because both comets seems to be very comparable bodies. And this thing exactly happens in similar distance from Sun (ISON ~0.65 AU, LINEAR ~0.55 AU), but while comet LINEAR was already moving away from Sun its nucleus was no more stressed by increasing heat flow from Sun. Maybe it this comet was continue move to very proximity of Sun its entire nucleus will erode and disintegrate.
Also very similar object was another comet LINEAR – C/1999 S4, one of best described case of comet which entirely disintegrated. When it reached its perihelion (0.77 AU from Sun), the glue of nucleus – frozen water and volatiles, was exhausted and whole nucleus turned to dust cloud and several larger fragments with diameter from 20 to 50 meters, which could be something as original “cometesimals” – basic building blocks of cometary nuclei. They were found by HST 12 days after terminal break-up of nucleus, and some of them may remain intact and fly away from Sun. It was only case when they were found, but simply that can be just because this was only one demised comet which were examined by large telescopes. No other disintegrated comets was researched by instruments that may allows to reveal such small objects.
Nucleus size itself is crucial for surviving prospect of comet, fast rotational speed, local structural instabilities and irregular shape may cause disruption of larger nucleus, but simply if nucleus is very small it will fast erode and most of it mass will sublimate, when it gets too small, heat will reach very deep inside and gas pressure may finish their fragmentation. In other hand, larger nuclei will have its interior intact and any destructive forces will affect only their surface.
Now what happened to comet ISON and what we may expect? It seems to be truth that nucleus of ISON is larger then nuclei of past disintegrated comets and simply it ISON would move in their orbits, it will loose large portion of its mass, but definitively it will survive its passage around Sun relative intact. But because it is approaching much closer then any of them, it would need even larger nucleus for us to be sure it will survive intact.
The main difference between small (sub-kilometer) and large (kilometer sized) nuclei is in stability of their photometric behavior. While brightening of larger comets is very stable in long-term, the smaller nuclei showing large fluctuations in their brightness.
Reason is obvious, see and examples:
2 km diameter nucleus have 12.6 square km surface, if a 50 meter fragment will split from it, total surface will increase by 0.01 square km. We will not even notice such increase which is smaller then 0.1%.
200 meters diameter nucleus have 0.13 square km surface, it it split to near 200 (lets simplify it) and 50 meters fragments, their total surface will grow by almost 8% (and so its activity), such change is already noticeable as small outburst.
The increase in brightness is of course higher, because there is released dust and fresh surface rich in volatiles and water. While for larger comet the change will be smaller then 0.1 mag to total magnitude, same change for smaller nucleus can cause a magnitude outburst.
Beautiful example was two comets originated from one spitted comets. Large fragment C/1988 A1 (Liller) showing very stable and calm brightening, while smaller fragment C/1996 Q1 (Tabur) show very disturbed light curve with at least 3 outburst followed by 3 declines and after last one it totally disintegrated.
Another very nice example is fragmented comet 73P/Schwassmann-Wachmann. Main fragment C of original comet show very calm and stable light curve, while smaller fragment B during its 2006 return exhibited at least 4 peaks followed by fast fading. Smaller fragments also tends to continue the disintegration process very fast.
Now back to ISON, what is going on with this comet? As its said before, this comet seems to be definitively larger, then usual disintegrating comets, but maybe to small to be a surviving sungrazer. Mass of its nucleus is somewhere near limit to survive one perihelion passage, most probable smaller.
N. Biver estimated that until first outburst, comet could release mass similar to sphere with 200m diameter. Assuming density 0.5 g/cm3 that means 5*10^5 tons mass.
13. November outburst lasted 2.5 days and had amplitude almost 3 mag. It could be caused by splitting of small nucleus to two or more fragments but generally leads only to increase out-gassing and natural growth of dust production. Unfortunately there was no or poor evidence of possible fragments, maybe because poor resolution. Non-gravitational forces reached extreme high level, large reaction force of jets was pushing nucleus on its orbit against gravity of Sun.
19. November another very slow outburst occurred with amplitude 1 mag. 6 days later gas emissions of comet dropped by factor 20. Part of nucleus or maybe fragment/s completely disintegrate and later, during 26. Nov a resulting synchronic feature appeared. By out-gassing and dust production comet may lost totally another 5*10^5 tons of material. The mass of dust in synchronic feature is unknown for me. But if its same as occurred in case of comet Lovejoy it can be 2*10^6 tons (assuming density of dust 2g/cm3).
22. November, dramatic decrease of comet activity occurred due effects named before.
24.-26. November activity of comet has stabilized. If we assume that this is normal non-outburst activity level of this comet and we will compare it with stable activity of another two comets in similar distance from Sun – C/2006 P1 (McNaught) and C/2011 L4 (Panstarrs), and also assume that all comets have spherical nucleus and similar activity per square meter. Another assumption (I know it seems to be too much, but this are comets) that both of that comets had nucleus in diameter 1 to 4 km giving us result, that ISONs activity if there is one intact nucleus corresponds to diameter between 100 and 630 meters. That corresponds to total mass between 1.5*10^5 to 3.7*10^7 so the comet may already lost from 95% to 8% of its original mass.
27. November another outburst happened with amplitude at least 4 mag so far. Comet will definitively loose another large partition of its mass, lets assume 5*10^5 tons again. In case of minimal mass this means final disintegration, but apparently remaining mass is at least slightly larger then 1.5*10^5 tons.
28. November ISON is passing its perihelion. In work of Z. Sekanina about comet Lovejoy, he used for calculations this evolution of its diameter caused by erosion – 400 meter 0.6 days before perihelion, 280 meters in diameter at perihelion, and 150 meters in diameter 1.6 days after perihelion. This means 9*10^6 tons of mass lost. In case of maximal mass for ISON estimated before deducted by last outburst and erosion during approach to Sun, the comet would totally loose at least 25% of its entire original mass (therefore small chance for surviving, if it wont fragmenting). To survive and disintegrate as comet Lovejoy it should by now have at least similar diameter (400 meters).
Even comet ISON seems to be larger then regular disintegrating comets, it is still very small comet. The last outburst may be also the last one for this comet, if not, simply erosion of nucleus may consume all of its mass within hours during close approach. It seems that there exist small chance for remaining nucleus disintegrating after perihelion, but chance to survive entire revolution is extremely small. However its “swan song” can be still very impressive to see.
And at the end, here is nice example of comet, which actually fragmented, but its nucleus was large enough to remain intact, fragmentation caused permanent increase of comet C/2001 A2 (LINEAR) activity, and not only transient as we can see for ISON.