Comet ISON's Current Status

Latest Update: November 18th, 2013

The latest light-curve for Comet ISON (compiled November 15, 2013 by Matthew Knight).
We've never made any secret of the unpredictability of comets, or of comet ISON, and the dramatic events of last week simply served to underline that. Here's a quick recap for those that are late to the party...

Just over a week ago, around November 10th or 11th, comet C/2012 S1 (ISON) was simply a reasonably bright magnitude +8 object visible to observers with small telescopes or perhaps binoculars in dark sky regions. All this changed, however, around November 13th when ISON very abruptly and dramatically begin to flare up in brightness. Within four days it suddenly became a spectacular object, naked eye visible to observers with very dark skies and easily visible to urban observers with binoculars or telescopes.

The reason for this flare-up remains unknown. It could simply be the result of increased activity as it enters the inner solar system and has to deal with increasing amounts of solar radiation. It could also have fragmented. All of these are within the realms of the possible outcomes that we have talked about extensively on this site.

For now, all we can do is wait and observe, though time is running out fast for the latter. Comet ISON now shines as a 4th magnitude object but ground observations are limited to an increasingly brief window just before dawn. The moon is also interfering with observations, making it harder to get reliable photometric data, and it's not going to get any easier at this point.

At time of writing, Comet ISON is at 0.505AU (47,000,000mi, or 75,500,000km) and racing towards the Sun at 59.3km/s (132,650mph, or 213,480kph). With only ten days until its extremely close brush with the Sun, there is very little ground-based observing time left and for most people, it may already be over... for now!

We still have no idea whether ISON will survive past the Sun in one piece, in many pieces, or not at all. We will be monitoring it in near-realtime with the LASCO instrument on the ESA/NASA SOHO satellite, and this should help tell us how the comet is doing. We plan to be blogging and tweeting throughout this time, so don't stray far from the CIOC website. If ISON does survive, then visual observers should begin to pick it up towards the end of the first week of December, and it will gradually climb higher into the dark night skies as we move further into December.

But that's all in the future, and a lot of water has to pass under the bridge (or through the corona?) before then. The tl;dr for today is that ISON still appears to still be in one piece, is a 4th magnitude object, but is now increasingly hard to see in the brightening dawn skies.


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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