Comet Section        




Have the reports of ISON’s death been premature? Is the comet back from the dead? Or do we have a case of a ‘zombie’ or ‘ghost’ comet?

Comet ISON is definitely still there and is rather obvious in SOHO LASCO C3 images. The question is what exactly are we seeing.

The evidence:

- ISON experiences a major outburst of some sort in the day prior to perihelion

- it appears to rapidly fade in the hours before perihelion and its coma became diffuse and elongated

- a diffuse comet with elongated coma and two tails reemerged after perihelion

- the post-perihelion comet appears to be brightening slightly as it moves away from the Sun

Ideas as to what happened (text lifted from my comets-ml post from yesterday):

1 – The nucleus of ISON has disrupted into many smaller pieces. It is this cloud of mini-nuclei that continues to sublimate. Rather than a condensed coma we now have a diffuse extended coma that should continues to spread out and fade as individual mini-nuclei move apart (due to variable solar radiation pressure and velocities from the disruption event) and disrupt further. This is what we’ve seem with other disintegrated comets such as C/1999 S4 (LINEAR) and C/2010 X1 (Elenin). Disruption does not mean the comet disappears instantaneously as the mini-nuclei can last for some days or weeks after disruption. In a way, the comet seems to just fall apart.

2- Much of ISON’s coma consisted on small dust particles that were vaporized by the intense heat of the Sun. We saw something similar with C/2011 W3 (Lovejoy) as pointed out by Joe Marcus, Zdenek Sekanina and Paul Chodas in their papers on the subject. If this is the case with ISON, it should ‘regrow’ a strong coma and tail as it moves away from the Sun.

3 – We are seeing something we’ve never seen before.

While it seems like we may be seeing #3 occurring, I’m wondering if we aren’t seeing a mix of #1 and #2. The coma remains very diffuse with little central condensation.

What this means going forward:

We may very well have a comet to observe in a few nights as ISON moves away from the Sun. Whether it will be a traditional comet or just a diffuse cloud remains to be seen. How bright the comet will be also remains to be seen. All I can say is that the saga of Comet ISON is not over yet.

- Carl Hergenrother (Acting Co-coordinator ALPO Comet Section)

Image of ISON taken with the LASCO C3 instrument on the ESA/NASA SOHO spacecraft on 2013-Nov-29 @ 14:13 UT. Credit: ESA/NASA/SOHO/LASCO.

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