Is comet ISON disintegrating – right now?

Discussion if comet C/2012 S1 ISON will survive it’s perihelion started shortly after it's discovery in 2012. It is still well possible that the 5 km wide comet will break apart on November 28, when it almost scratches the solar photosphere. But now, Ignacio Ferrín of the University of Antioquia in Medellín, Colombia, thinks ISON is turning off or breaking up right now, still almost two astronomical units far from the sun. If this holds true, then better say goodbye to the hoped for “comet of the century”. But that might be a little premature.

You can find Ferrín's work in the arXiv and on his website. Ferrín used his concept of Secular Light Curves of comets (SLCs) to determine the brighness history of ISON. Using three independent sets of published brightness measurements for ISON, he found all of them exhibit a Slope Discontinuity Event (SDE) at a distance of 5.1 AU, followed by an u-shaped dip in the lightcurve. After that, the comet failed to increase it’s brighness as it continued to approach the sun. In fact, Ferrín claims the comet is actually loosing brightness since the last few days of september.

SLC of comet ISON. A slope Discontinuity Event appears at -5.1 AU followed by an u-shaped dip and a reduced increase in brightness thereafter. The last three dataponts indicate a sharp drop in brighness at the end of September. Credit: I. Ferrín
Ferrín compared this behaviour with that of other comets and found two that showed similar features: comets C/2002 O4 Hönig and C/1996 Q1 Tabur. Both disintegrated shortly after they experienced similar SDEs. In addition, Ferrín states that ISON has failed to increase it’s activity altough he crossed the frost line and the energy received from the sun has increased significantly.

Ferrín’s bold conclusion: “In the view of the evidence, there is a 100% probability, that comet ISON is turning off or disintegrating.”

Reactions to this claim among experts are mostly negative. They spot serious flaws in Ferrín’s analysis. First, the assertion that ISON is loosing brighness (0.5mag in the last three days of september) strongly contradicts other observations. There are reports the comet actually gained 1.0mag within the last days. On my own images, taken on Sep. 28 and Oct.2, respectively, no apparent decrease in brightness is visible (however, taken with simple amateur equipment and partially under poor observing conditions, you shouldn’t rely too much on these). Comet expert Burkhard Leitner doesn't take no stock in Ferrín's analysis: "Visual observations say: all is well with ISON!" 

Comet ISON on September 28 and October 02, both times imaged with the exact same equipment (8" Newtonian, Canon DSLR, CLS filter, 120s exposures stacked with Fitswork 4). No brightness decrease visible to me. However, observing conditions were much poorer on Sep. 28 (lunar interference and high clouds).
Secondly, both Hönig and Tabur disintegrated much closer to the sun than where ISON is right now. ISON’s SDE occured far outside the frost line, the ones experienced by Hönig and Tabur at around 1.2 AU. This is well within the zone where water sublimates due to the increased solar radiation. It is obvious that one must assume totally different physical processes at work here.

The coming days will show if Ferrín is correct. The amateur astronomer in me sincerely hopes he is not!


  1. Good entry Jan.

    I do not know what is bolder: saying that ISON is presently desintegrating or that "all is well with" the comet. Obviously ISON is behaving in unexpected ways. The most important point that Ferrin's work is evidencing is that comet's expert does not have a proper model to explain the strange behavior of this comet. Neither Ferrin nor e,g. NASA experts seem to completely understand the comet.

    Given such an uncertainty, I actually prefer the "bold" attitude of Ferrin who has the courage to put forward even an unpopular model, the best he can present according to his expertise as an observational astronomer, than the avoiding attitude of other experts that instead of discussing Ferrin's results and demonstrate scientifically he's wrong they just deny that ISON is actually a really strange comet.


    Jorge Zuluaga

  2. David Levy was on Comets-ml Listserv yesterday. He said it was dim but it is brightening. I believe his mag estimate was 11.4 Now who are you going to believe? Someone you don't know or someone you have met with a great reputation?

    1. I would not follow a reputation criterium to evaluate a scientific problem... On the other hand, Ignacio Ferrin (although not American) is well known in the cometary community. So, I believe that he has a good point and even if he was completely strange but data shows us an abnormality we should stick to facts.

      Let's wait for other observations.

      Anyhow, all of us expect that ISON surprises us, either by disintegrating or showing a bright display after water (if any) start at sublimating...

      Kind regards,

      Jorge Zuluaga

  3. ISON is running about 1 magnitude dimmer than original predictions. That is very common behavior for an Oort cloud comet.

  4. I have seen comet C/2012 S1 (ISON) on a few ocassions. I estimated it 13.2 on August 31st, 12.7 on September 8th and 10.9 on October 4th. During the last observation it was much brigther and much better visible than previously.
    As far as I remember, dr Ignacio Ferrin already said that comet C/2012 S1 (ISON) is just disintegrating in February or March 2013. I do not know whethet this comet will disintegrate or not, but for now I could not see any evidence of disintegration.
    When it comes to claims of dr Ignacio Ferrin I would point out, that he is comparing CCD observations (in case of C/2012 S1) mainly in R band with visual observations (C/2002 O4 and C/1996 Q1) which are pretty close to V band. I encourage anyone who does not now the difference between R and V in case of comets to look for magnitude estimates or images of comet 2P/Encke made in those two bands. When it comes to magnitude estimates, the difference is a a few magnitudes. In case of images you may see almost nothing in R (the vicinity of nucleus) against very large, bright coma in V.
    For me, the comet looks pretty healthy and yet, there are no sings or anything "bad" going to the comet.
    Sometimes I think that dr I. Ferrin would like to be the first to claim this comet to disintegrate (the probability of which still seems to be not so small). For now, I do not see any evidence supporting his opinion.

    Piotr Guzik

  5. Thanks for sharing your views!

    As more and more images and visual observations are coming in (see e.g. http://spaceweather.com/gallery/indiv_upload.php?upload_id=87462 and http://groups.yahoo.com/neo/groups/comets-ml/conversations/topics/22028) it seems the comet is doing well - no sign of an "imminent demise"...

  6. The so-called Slope Discontinuity Event (SDE) at a distance of 5.1 AU is largely caused by phase brightening of the coma as it passed through an unusually low phase angle (1.8 deg on 2013 January 12) and is not linked to the activity of the nucleus. It is a common effect re. asteroids but a rarer phenomenon for comets which tend to avoid low phase angle when at opposition (a consequence of the fact that the orbital inclination of comets is on average significantly greater than that of asteroids).