[X] My Assistant

[Explorer1]

Might be good to put this on for closest approach coverage tomorrow morning

http://www.youtube.com/watch?v=NQXVzg2PiZw

[Mr Valiant]

Can we possibly imagine what you would be experiencing, what your eyes would see, body feel, maybe even ears hear, if we where riding in the saddle of ISON.
The ring side seat is unbelievable in its self.
Go ISON

[Explorer1]

Yep; and we probably won't know for a long time, if ever (Solar Probe Plus notwithstanding). Even the size of the solar disc from that distance, previewing Earth's fate in a few billion.

Maybe a probe to Icarus or 2000 BD19 could give us an idea of the experience, but I'll bet rendezvousing with one of them would make MESSENGER's trajectory look like a cakewalk.
6 million miles and counting, new update on Emily's blog: http://www.planetary.org/blogs/emily-lakda...-live-blog.html

[Ant103]

A little work with the last pictures from C2 and C3 coronographs onboard SOHO. Merged the two pics, harmonized, remove some noise and artifacts, colorized.



I think to make other versions when the comet will be a little closer to the Sun

[machi]

Nice!

[Mongo]

Oh-Oh

Sungrazer Comets @SungrazerComets 2m

Oh, this is not such good news: the very latest images show that #ISON is considerably fainter...

[craigmcg]

http://sohowww.nascom.nasa.gov/data/realtime-images.html seems to be down and http://lasco-www.nrl.navy.mil/ has the message below.

Data Outage Announcement

The realtime LASCO images (JavaScript movies, javagif directories, etc, are currently offline while we restore a server. We apologize for the inconvenience and direct you to the SOHO Website for realtime images and data.

[Mongo]

It seems that one theory of cometary behavior can explain ISON's dimming without assuming that it has disrupted:

posted by Jakub Cerny at 8:44 this morning at comets-ml:

Re: ISON brightening in SOHO C3 - small chance for daylight observat

Hi Filip,

seems that at least someone was expected it:

CBET 3723: 20131127 : COMET C/2012 S1 (ISON)

" J. N. Marcus, St. Louis, MO, U.S.A., writes: "Considerable sublimation of submicron-size silicate dust grains can be expected to occur in the coma and tail of comet C/2012 S1 near perihelion, if its behavior is similar to that of sungrazing comet C/2011 W3 (Lovejoy), whose nucleus survived for a few days beyond perihelion (see Sekanina and Chodas 2012, Ap.J. 757, 127). If C/2012 S1 can survive until its perihelion passage, then its brightness, based upon my preliminary photometry of C/2011 W3 in the SOHO LASCO C3 and C2 coronagraph fields, should start to fall below any power-law formula at about r = 0.1 AU (Nov. 27.8 UT), peak at magnitude -1 to -3 at r = 0.03-0.04 AU (Nov. 28.5-28.6), and then decline by 2 or more magnitudes by the time of perihelion passage on Nov. 28.8 (details of my study of photometric behavior of near-sun comets will be submitted to the ICQ soon). Thus, a strong falloff in brightness at these times should not necessarily be construed as disintegration of the nucleus. After perihelion, the comet's brightness would temporarily increase as submicron grains re-populate the coma and dust tail."

Best regards,
Jakub

So a sungrazing comet with similar properties to comet Lovejoy produces a lot of submicron-sized dust (ISON has certainly released a lot of dust, as we know). When it gets too close to the sun, the dust sublimates away and the comet's brightness falls. When it recedes from the Sun beyond the dust sublimation line, it re-brightens as newly released dust starts to contribute to its brightness. But it could be producing dust at its established rate the entire time.

Also, keep in mind that none of the pre-perihelion tail was ever expected to survive perihelion, it was always going to need to grow a new tail. What will be important for its visibility in December will be its gas and dust production rates after perihelion.

[scalbers]

I see a new LASCO C3 image from 1430UTC. Looks nice with two tails, however just minuscule blooming. Back into positive magnitudes?

http://sohowww.nascom.nasa.gov/data/realtime/c3/

A question though. If the solar corona looks equally bright (while at least fainter stars disappear) as one approaches the inner radius of the image, is some type of filtering being applied? Could this make the interpretation of things different?

[fredk]

That's right, the corona is vignetted inside of 10 Rsun - see description here. You can also tell because there are much fewer stars visible close to the sun.

But the question of course is how much is ISON darkened by the vignetting. I have no idea, but hopefully those familiar with the telescope will chime in and give us some sense of the expected darkening. Of course it will be Rsun-dependent, ie more darkening the closer ISON gets to the sun.

[scalbers]

Interesting about the vignetting. When the 1430UTC image was taken the comet was 1.6 degrees from the sun, or about 6.5 times the radius. Thus the actual magnitude is somewhat brighter than would be implied by the simple bleed line comparison. How much? Difficult to say unless we can get some comparisons with planets/stars at close in distances, or someone can tell us what the vignetting function is.

More is at this web site.

http://lasco-www.nrl.navy.mil/index.php?p=.../lascocal_index

Some plots are here:

http://lasco-www.nrl.navy.mil/index.php?p=...vig/c3vig001025

We can probably count the pixels offset from the sun to get a result on this plot:

http://lasco-www.nrl.navy.mil/content/leve...igpylonprof.gif

Maybe a factor of 2 or 3 (1 magnitude)? The plot assumes we are offsetting perpendicular to the pylon holding the occulter.

On the twitter feed, they mention seeing ISON in the McMath solar telescope on Kitt Peak.

[MahFL]

The tail looks magnificent on the SOHO image.

[fredk]

For an example of what the vignetting might do to a comet, check out the C3 videos of Kudo-Fujikawa here.

Still, it's not clear whether other factors (exposure, processing, real dimming of comet) contributed to the dimming of Kudo-Fujikawa.

[scalbers]

Interesting discussions at Sky and Telescope and Twitter, and comets-ml mailing list.

First is a claim of a ground based image yesterday from Slovakia. Hard to say one way or the other. A bit more plausible than the one from the other day, though if it was around 0.5 magnitude it would need some very clear air (as they may have had) to spot.

Second is latest C2 image has it "cigar shaped" indicating the head is fading.

Third is that phase angle effects would nominally place peak brightness after perihelion, and brightness would be suppressed before perihelion due to phase angles around 45 degrees. I'm unsure what the phase functions actually look like for comets, and whether they are monotonic with respect to phase angle. Generally though high phase angles enhance scattering.

[Mongo]

Third is that phase angle effects would nominally place peak brightness after perihelion, and brightness would be suppressed before perihelion due to phase angles around 45 degrees. I'm unsure what the phase functions actually look like for comets, and whether they are monotonic with respect to phase angle. Generally though high phase angles enhance scattering.

I would think that there is no one phase function for comets. Even for a specific comet, it would depend on its dust production rate and dust size distribution (both probably varying with time).

ISON does seem to have an "all or nothing" nucleus. It's either pumping out gas and dust at a furious rate, or it's almost dormant, with nothing in between. Its behavior so far has been unusual to say the least, but this is the first time we've seen a large first-time sungrazer, so we don't know what to expect. Plus, as David Lavy said, "Comets are like cats: they have tails, and they do precisely what they want."