In our November 23, the overall message was that not much had changed, and the comet appeared to be keeping it all together as it approached the Sun.
That situation may now have changed.
We are seeing reports online that molecular emission from the comet has fallen dramatically, meanwhile dust production seems to be enormous. What this could indicate is that the nucleus has completely disrupted, releasing an enormous volume of dust while significantly reducing emission rates. Fragmentation or disruption of the nucleus has always been the highest risk factor for this comet so if this has indeed happened then while unfortunate, it would not be a surprise.
However, these reports are new, and while they are undoubtedly valid, we do still need to keep observing the comet to be sure what it happening. Remember: Comet ISON is a dynamically new sungrazing comet, fresh in from the Oort Cloud, and the last time we saw an object like this was never! Furthermore, a sungrazing comet just three days from perihelion has never been studied in this kind of detail - we're breaking new ground here! When we factor in your standard "comets are unpredictable" disclaimer, what we have is a huge recipe for the unknown.
Accordingly, we urge observing teams to factor in the changing nature of comet ISON, and attempt to obtain all data they can and share those with us so that we can keep the community informed. We appreciate that observing ISON now is extremely challenging as it is only three days from perihelion. Comet ISON is now very much in the realm of the solar observing fleet, and indeed the NASA STEREO-A spacecraft is returning some amazing views of both comets ISON and Encke.
To keep up to date on the latest reports from professional observers, we recommend you regularly check our Observation Logs page, which you can find under the "Observing" tab, above. Also, we have once again updated our light-curve with new observations.
Today's tl;dr is perhaps a little worrying. Reports of dramatic changes in comet ISON's dust and molecular emission rates may possibly be an indication that the nucleus has disrupted. We may not know for sure until ISON reaches (and hopefully) passes perihelion, though complete disruption - if that is indeed the case - would lead us to think that survival may be unlikely. In three days' time, we'll know...
Frequent visitors to this page will notice that the above lightcurve plot has been overhauled. It is now a bit busier, but is also conveying more information and is hopefully easier to see exactly what's going on right now. In a nutshell, here is what is new.
First, we are now plotting two sources of data. The first, shown as red circles, are the magnitude measurements being reported by the Minor Planet Center. As we've discussed before, these measurements vary widely from observer to observer based on each person's technique and instrumentation. Most are only measuring a relatively small region near the comet, yielding a fainter brightness than if they measured the whole coma.
The second, plotted as blue triangles, are from the International Comet Quarterly (thanks to Alexandre Amorim for pointing this out to us). These data had not previously been plotted here, but are quite useful. The ICQ data are "total magnitude" estimates that try to encompass all of the light from the comet. Many of these estimates are made by very experienced observers using binoculars or even (soon) their naked eye who compare the comet to nearby stars of known brightness. It takes a lot of work to be good at this technique, but as you can see from the relatively small scatter in the blue points, yields quite reliable estimates.
You will notice that the ICQ data fall much closer to the trend line we've been plotting all along, but back in the first half of 2013, the red and blue points roughly overlap. This is because back then Comet ISON had a small angular size, so in effect all brightness measurements were "total" measurements. Now that ISON is much closer to the Sun, its coma is much larger in an absolute sense. Furthermore, it is also closer to the Earth, so its apparent size is also larger. The result is that what worked very well early on (measurements using a small aperture on a CCD) has begun to diverge systematically from the total brightness. So now the blue points do a better job answering what everyone cares about: "How bright is Comet ISON?"
The other big change we've made is to create a smaller, zoomed in plot that only shows the most recent data. This gives a much better idea of how closely the current estimates are following the trend line. The immediate takeaway is that the huge brightness increase over the last few days isn't that surprising, and we can probably hope to see similar brightness gains on a daily basis over the next two weeks (assuming ISON hasn't broken up)!
It will, of course, become harder for people to estimate the brightness as ISON gets closer to the Sun because the sky will get brighter when ISON is observed, and will therefore be harder to detect the full coma or to compare it to nearby stars. At some point we will likely stopping getting new ground-based brightness estimates and will have to start relying on observations from the SOHO and STEREO spacecraft. But rest assured, we will plot data here as frequently as we can, so please keep checking back ( via isoncampaign.org ).