Hanging On By Its Fingernails...

Once again comet ISON raises our hopes and then dashes then... perhaps for the final time. It has clearly started to fade dramatically, and this does not bode well for survival. [Image credit: ESA/NASA, annotations by Karl Battams]
Last night I was optimistic that comet ISON would continue its dramatic brightening trend, and soar into the negative magnitudes. This morning it is indeed with a heavy heart that I show you the image opposite, in which we clearly see that ISON has faded rather dramatically in the past few hours. It is still likely around -1 magnitude, but this number is falling fast.

The question on everyone's lips is "will it survive perihelion?", and now I'm reluctantly thinking it seems very unlikely to survive at this point. I do think it will reach perihelion, and reach the NASA SDO field of view, but based on what I see it doing right now, I will be very surprised to see something of any consequence come out the other side.

BUT... at every single opportunity it can find, comet ISON has done completely the opposite of what we expect, and it certainly wouldn't be out of character for this dynamic object to again do something remarkable.

So, assuming ISON is indeed on its way out, what might have happened here? Well there's a clue in something we said yesterday on our Current Status page when we said that "Comet ISON is now behaving like a sungrazing comet". By that statement, we meant that it was no longer following the typical pattern of brightening that an more typical comet would follow, and instead was brightening at a dramatic rate as we see with sungrazing comets such as the Kreutz sungrazing comets that we see.

The ESA/NASA SOHO satellite discovers a new sungrazing Kreutz comet about every three days on average, but they are typically very small objects only tens of meters in diameter, that brighten rapidly in the C3 field of view and the quickly begin to vaporize and fade out a few hours before perihelion.

Wait... read that last sentence again: "brighten rapidly in the C3 field of view and the quickly begin to vaporize and fade out a few hours before perihelion". Does that sound familiar? It should, because that's exactly what comet ISON appears to be doing! We have been assuming - quite validly - that because ISON is (was?) a large sungrazing comet, it would behave more like Comet Lovejoy in 2011, and continue to brighten up all the way until it reaches perihelion.

But what if ISON's nucleus is no longer the same size that Lovejoy's nucleus was at this point? We know that a few months ago, ISON was at least two or three times larger than Lovejoy's ~500m diameter. But a lot has happened to ISON since then, including some enormous outbursts. It is certainly conceivable that ISON has simply lost too much mass on its way in, and has a relatively small nucleus, leading it to behave like a small sungrazer, rather than a large sungrazer.

It's useful to understand what happens to small sungrazers and why we tend to see them begin to fade out at around 11-14 solar radii from the Sun (~8-10 million kilometers, 4-6 million miles). Recall that as comets approach the Sun, their ices begin to vaporize ("sublimate"), releasing dust and gas into the comet's fuzzy coma. But when we get really close to the Sun, the surface temperature on the comet starts to get very hot, and not only do the ices sublimate, but the rocky material begins to boil off too! We think it's this process that really kills of the small sungrazers, and it is certainly well within the bounds of reasonability that ISON simply can't withstand the complete vaporization of its surface.

Is there any hope?

YES! Don't give up now! There is only a few hours until perihelion, and anything could happen. ISON may have been outbursting in the LASCO field of view, and a rocky nucleus could still exist at the center of all that. If there is still something solid there, and it reaches perihelion, then even if it falls apart at that point, we will still see something emerge from the solar atmosphere. And even if the comet does get completely destroyed in the next few hours, I hope that we will get a spectacular show in the SDO images. I also want to note that we are analyzing the SOHO data very carefully and we do see some evidence that maybe ISON's gas production is completely finished but dust production is continuing at a steady rate. Indeed Matthew just said to me, "I have never seen light-curve behavior like this! It's doing completely different things in the different color filters!

He has looked at well over 1,000 sungrazing comets, so when he says a comet is weird... it's weird! So the bottom line is that we have contradictory evidence and we're still trying to interpret it. There is hope for comet ISON!

Regardless of what unfolds today, please remember this: we're watching a truly unique astronomical event for which we have no similar occurrence on record, and we're getting to watch it unfold live on the internet. That's worth restating: this is one of the more extraordinary astronomical events to happen in modern history, and we get to sit in our comfy chairs and watch a giant ball of 4.5 billion year old ice hurtle through the Sun's million-degree outer atmosphere at 0.1% of the speed of light, 93-million miles away from us. Regardless of sizzle, fizzle, or a victorious reemergence, comet ISON's perihelion is a truly spectacular event!

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.