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Karl Battams

Comet ISON Enters the Final Countdown

Posted by Karl Battams

19-11-2013 15:12 CST

Topics: pretty pictures, comets, amateur astrophotos, amateur astronomers

This article was originally posted on the NASA Comet ISON Observing Campaign website and is reposted here with permission.

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, 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.

Spectacular ISON

Waldemar Skorupa

In just four days, comet ISON went from being "pretty" to being "pretty spectacular"!

German amateur astronomer Waldemar Skorupa recorded this image from Kahler Asten, in Germany, on November 16, 2013. While comet ISON is now being reported as a naked eye object in some locations, you should not expect it to look like this in the night sky! Images like the one above are taken with specialized equipment, powerful telescopes and years of experience in processing astronomical images. Most visual (including binocular) observers are reporting comet ISON is mainly a "green fuzzball", with maybe a very faint tail visible.

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 intact, 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. 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.

With only 10 days until perihelion, comet ISON will soon disappear into the Sun's glare. During this time, we will monitor it with an armada of solar satellites including NASA's SDO and STEREO satellites, and the ESA/NASA SOHO satellite. These images will be posted directly online as soon as the data is retrieved from the spacecraft, mostly in real-time, and we will let you know those links nearer the time.

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.  We will guide you through the most perilous part of comet ISON's journey through the solar system. Will it be fried by intense solar radiation? Will it be ripped apart by the Sun's extraordinary gravitational pull? Or will it survive in one piece and become a spectacular night sky object? We have no idea, but we can't wait to find out!

See other posts from November 2013


Or read more blog entries about: pretty pictures, comets, amateur astrophotos, amateur astronomers


Kevin West: 11/20/2013 12:17 CST

Sorry if this has been answered already, but how close to its perihelion will ISON still be visible? Will it be visible right up to that date or sometime before then?

Karl Battams: 11/20/2013 08:46 CST

That's hard to answer as it depends on how bright it gets. With only 8 days to go now, I think we'll lose it very soon. There is a *chance* it will be bright enough for a few hours around perihelion that it would be visible in daytime skies (like C/2007 McNaught, for example). But that requires it to reach ~mag -5 or -6, which could be a stretch for it. That said, it is performing remarkably well right now...

Michael Richmond: 11/20/2013 12:22 CST

You wrote "with a brightness increase of at least an order or two of magnitude," but that's a bit misleading. "An order of magnitude" generally means "a factor of 10". In this case, the brightness of the comet increased by one or two _magnitudes_, which are special astronomical units. An increase of one magnitude means the brightness increases by a factor of about 2.5, while two magnitudes means a factor of 2.5*2.5 = 6.3 or so. Increasing in brightness by one or two magnitudes = factors of several -- yes. Increasing in brightness by two orders of magnitude = factor of 100 -- no.

Karl Battams: 11/20/2013 01:45 CST

Michael: Fair point - a mistaken choice of words on my part. When we comet guys talk among ourselves, we sometimes say "order of magnitude" and we know what we mean, but technically your description is correct. Comet ISON increased in brightness by two or three astronomical magnitudes - so that's maybe 10-15 times brighter. Today it's maybe 40 times brighter than just over a week ago. Thanks for pointing that out though.

Steve Franks: 11/20/2013 10:58 CST

Hmm. An evening comet after perihelion? Seems I saw a chart showing the before and after track putting the comet back on the same side of the sun after a tight turn around at perihelion. Is this possible?

Karl Battams: 11/23/2013 07:46 CST

@Steve After perihelion, ISON is going to be visible in the evening skies -- in the west, specifically, after the Sun sets. (Assuming it survives, of course!)

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