Comet ISON's Current Status

Latest Update: November 30th, 2013



The latest light-curve for Comet ISON (compiled November 24, 2013 by Matthew Knight).
Despite the promise that our last report held, comet ISON began to fade significantly in the hours immediately prior to perihelion. Before passing by the occulting disk in the center of the LASCO C2 field of view, there was no longer an obvious discernible nucleus, but the tail remained quite broad and "dense" compared to other sungrazers we've witnessed in this field of view.

During perihelion, no positive detections of comet ISON were made by the ESA Proba-2 SWAP instrument, or the NASA SDO satellite. More complex image processing methods may yield a signature but you should refer to the relevant instrument teams for that information and those details.

Following perihelion, a dusty trail appeared from the LASCO C2 occulter and, for the following approximately 24-hours, and very diffuse and dusty fan-shaped tail was visible with a diffuse central condensation that may or may not have contained a small remnant nucleus, or "rubble pile". A great many questions remain to be answered.

At time of writing, the remnant of comet ISON is a faint, diffuse, cloud with no obvious central condensation. An approximate visual magnitude estimate is around +7.5, but this is value obtained "by eye" as no central condensation exists to enable aperture photometry. It is currently leaving the LASCO C3 field of view and entering the Heliospheric Imager 1 (HI-1) instrument on the STEREO-A satellite. While it may appear brighter in this image, observers need to keep in mind the increased sensitivity of HI-1 versus LASCO C3. The latter has a limiting magnitude of around +8.5 while the former has approximately +13.5. There also exist small but non-negligible bandwidth differences.

The tl;dr is quite simple: comet ISON likely catastrophically fragmented at or near perihelion.

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-Curve

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.

Now we're around perihelion time for ISON, we will update this "Current Status" page more frequently -- in some cases daily. Check back regularly for updates, and follow the CIOC's Karl Battams '@SungrazerComets' Twitter feed.

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