We hope you're enjoying this roller-coaster of a ride?! When we last spoke we reported on a dramatic increase in dust production and simultaneous drop in molecular emission, potentially indicative of a catastrophic breakup of the comet's nucleus.
That situation may now have changed. (We literally cut-and-pasted that sentence from the last update...)
Now that we have observations of the comet in the NASA STEREO instruments and, more recently, the ESA/NASA SOHO LASCO C3 instrument, CIOC team member Matthew Knight has been able to start recording photometry of the comet. His results seem to imply that the comet may have experienced an outburst during the (approximate) period Nov 21 - 23 with corresponding brightness increase, followed by a leveling off and then dropping back down to "pre-outburst" levels. Since entering the LASCO C3 field of view, comet ISON has increased by at least a factor of four, and indications are it may be closer to a factor of ten. In the most recently available images, the comet appears to be around magnitude +0.5.
It is now the opinion of the CIOC Team that Comet ISON is now behaving like a sungrazing comet. We can not comment on whether the nucleus is in tact or not, but our analyses indicate that its rate of brightening is directly in line with that we have experienced with other sungrazing comets. This has no implication on its chances of survival. We strongly encourage all professional solar observatories who have plans in place for observing the comet, to please do so, and the teams should plan for an object brighter than negative one magnitude (and we are being conservative on this estimate).
Today's tl;dr is somewhat upbeat: we don't know if ISON will survive, and we won't know until it either does it or vaporizes. But the comet is still "alive" and brightening dramatically in accordance with the behavior we expect of sungrazers. Professional observers with solar telescopes should plan for a negative magnitude object, and we urge observation from these facilities.
CRITICALLY IMPORTANT: Comet ISON is extremely close to the Sun and you should NOT attempt to observe it in binoculars or telescope unless you are highly experienced in making these observations. Sunlight through magnifying optics can and WILL permanently damage eyesight.
Light-CurveFrequent 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.