Comet ISON Enters the Final Countdown


This image, taken by Juanjo González Díaz on November 14, 2013, shows clearly some of the beautiful structures that have suddenly appeared in comet ISON's tail. Is this the beginning of a spectacular show, or the beginning of the end for comet ISON?
We’re now less than two weeks away from comet C/2012 S1 (ISON) reaching perihelion and, if we’re honest, we are still none the wiser as to how the situation might play out!

Last week, the CIOC’s Matthew Knight wrote an excellent blog post detailing the three possible outcomes for ISON in the coming days: imminent disintegration, disintegration at perihelion, or survival. Since he wrote that, and indeed in just the past 24-hours or so, comet ISON has undergone a dramatic shift in nature, with a brightness increase of at least an order or two of magnitude, and soaring rates of dust and gas being released from its nucleus. In light of this new development, we can now say with absolute certainty that comet ISON will definitely do one of those things that Matthew said it might do…

OK, that was vague. But the truth is, although this new development is tremendously exciting, it still doesn’t help us answer either of the questions that everyone wants to know: namely, how bright will comet ISON be, and will it survive perihelion?

The problem is that we don’t yet know why ISON has so suddenly and dramatically changed in brightness and production rates.

It could simply be that ISON has realized it’s a near-Sun comet (soon to be a sungrazer) and is beginning to “turn on” accordingly. After all, if it is to reach the lofty brightness goal of magnitude -3 to -5 that we long predicted (OK, guessed) that it might, then it has a lot of work to do in the next two weeks! This is actually a very likely scenario, and if true means that the first of Matthew’s scenarios – imminent disintegration – will not be the case.

But this requires the comet’s nucleus to remain in tact, and this brings us nicely to the second possibility: comet ISON’s nucleus has fragmented. We always said this could happen, and it perhaps has. If so, it will still be several days before we know for sure. When comet nuclei fall apart, it’s not like a shrapnel-laden explosion; the chunks simply drift apart from one-another at slightly different speeds. Given that ISON’s nucleus is shrouded in such a tremendous volume of light-scattering dust and gas right now, it will be almost impossible to determine this for at least a few days and perhaps not until the comet reaches the field of view of the NASA STEREO HI-1A instrument on November 21, 2013. We will have to wait for the chunks to drift apart a sufficient distance, assuming they don't crumble first.

If ISON’s nucleus has fragmented, the chances of any substantial chunk of nucleus surviving the extreme close brush with the Sun on November 28 are really quite small – but still not impossible. And even if ISON’s nucleus does fall apart completely, Matthew painted us a promising picture in his second scenario with a comet that still graces our December night skies with an extensive and beautiful tail.

So even in the face of adversity and the unknown, there is still hope that we will see a vivid naked-eye comet in the night skies in Northern Hemisphere. And even if not, we have already obtained unprecedented scientific knowledge of this comet with data from ten different spacecraft to date, and a vast wealth of ground-based observations from professional and amateur astronomers. In this sense, cometary science has already won. It just remains to be seen how big that win will be.

Follow us on the CIOC website through perihelion week as Matthew and I will be feverishly working, analyzing, blogging and tweeting as much as possible from atop Kitt Peak in Arizona.

Keep up-to-date on the latest ISON and sungrazing comet news via my @SungrazerComets Twitter feed. All opinions stated on there, and in my blog posts, are my own.