Comet ISON's Current Status

Latest Update: November 20th, 2013

The latest light-curve for Comet ISON (compiled November 19, 2013 by Matthew Knight).
The flood of new images and surprising changes in appearance in Comet ISON have slowed since yesterday’s update. This isn’t because everyone has suddenly stopped caring about ISON, but because it is getting harder and harder to image it.

As of this writing ISON is only about 25 degrees from the Sun, meaning it can only be observed in twilight. Even worse, there is a nearly full moon up at the same time, making it very difficult to get the kind of detailed images like we saw over the weekend. This is not to say that there aren’t nice images coming in, as you can see here, just fewer than we were getting last week. (Note that that photo was taken by ISON’s co-discoverer Vitali Nevski!)

While the Earth-based images are slowing down, space-based imaging is just getting going and the next week should yield a trove of fascinating and unique results. As mentioned late last week, MESSENGER is scheduled to observe Comet ISON right now from its orbit around Mercury. Due to constraints on getting data back to Earth, it may be a few days before we see the results, but last week’s first images (when ISON was much farther away from MESSENGER and the Sun) are encouraging.

ISON should also make its long awaited appearance in STEREO-A’s HI1 camera tomorrow, although it will be a few days before the images get transmitted back to Earth. Next week things really get interesting when ISON passes through numerous other cameras - eight in total - on STEREO-A, STEREO-B, SOHO, and SDO beginning on November 26 and going through until November 30th.

If that seems like a lot of spacecraft imaging to try to keep track of, we have good news! We will be blogging right here at the CIOC website as often as we can and, since two of our CIOC members (Karl Battams and Matthew Knight) regularly use these data in their own research, we will have plenty to say about it.

We will leave you with one final tidbit for the day. For those of you who remember C/2011 W3 Lovejoy’s beautiful sungrazing passage in December 2011, Lovejoy had an estimated total magnitude of about mag 10 when it was eight days before perihelion, and went on to reach approximately mag -3 at peak. ISON is now eight days from perihelion and is estimated to be around magnitude 4.5. I’m not going to speculate on what that means for its peak brightness, but note that ISON is about 100 times brighter than Lovejoy was at about the same distance!

To sum up with today’s tl;dr, it has been a relatively slow day for new reports on ISON due to more challenging observational circumstances, but expect new data from space-based observatories in the next few days.


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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