Starship Asterisk*

Comet ISON Before and After

Explanation: Sungrazing Comet ISON reached perihelion, its closest approach to the Sun, yesterday, November 28, at 18:45 UT. The comet passed just over 1 million kilometers above the solar surface, a distance less than the diameter of the Sun. These two panels follow ISON before (right) and after its close approach, imaged by the LASCO instrument onboard the Sun staring SOHO spacecraft. Overwhelming sunlight is blocked by LASCO's central occulting disk with a white circle indicating the Sun's positon and scale. The bright comet is seen along its path at the bottom of the before panel, but something much fainter exits near the top of the after panel, potentially a dust tail reforming from the debris left from ISON's perihelion passage.

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Based on the current C3 photo at 00:18UT on the 29th, maybe we are seeing shades of Comet Shoemaker–Levy 9 in Comet ISON?

Woo-hoo!!!

The continued visibility of what remains of Comet ISON is currently yet another great unknown for this enigmatic comet. Remember, though, when interpreting new ISON images, that comet tails usually point AWAY from the Sun.

- RJN

RJN wrote:The continued visibility of what remains of Comet ISON is currently yet another great unknown for this enigmatic comet. Remember, though, when interpreting new ISON images, that comet tails usually point AWAY from the Sun.

- RJN

I have to assume what we are seeing is a debris trail. There is also faint evidence of ISON's tail/path before perihelion. ISON appeared to shed some extra material on November 24(EDIT:Maybe it was Nov. 23 I saw that image) after encountering an X-Flare. The debris are less blown away by the solar wind. Is the newer image less focused than the one on the right? There is more sharpness in the grain and pixel grid of the image recorded 2013 11/28 - 17:00. Hope we see better images of the nucleus area soon.C_S

RJN wrote:The continued visibility of what remains of Comet ISON is currently yet another great unknown for this enigmatic comet. Remember, though, when interpreting new ISON images, that comet tails usually point AWAY from the Sun.

Ion tails point away from the Sun. Dust tails lie substantially in the orbit of the nucleus, but pushed somewhat outwards by radiation pressure. Because of the high speed of ISON near the Sun, I'd expect the dust tail to be quite close to the parent orbit (both ejection velocities and perturbation velocities are small in comparison with the orbital velocity).

It remains to be seen whether the material in the post-perihelion image is simply rubble or if it's actually something still outgassing from a surviving piece of the nucleus. All of the outer volatiles could have been vaporized, leaving a temporarily inactive body. So many interesting questions. Even if this isn't a "comet of the century" in the public eye, it's going to be very important to the space dust community for refining models of cometary bodies. I expect to see quite a few papers over the next year analyzing this event.

It appears from more recent images that quite a lot of the comet survived:

http://www.nasa.gov/content/goddard/com ... piyx3d0nAk

I hope that it will still be visible in our sky!

Fausto

Possible explanation of what happened…


A – Comet nucleus outgassing increases markedly as it approaches the sun.

B – Comet nucleus progressively emits more and more gas and dust.

C – Maximum output of gas and dust on final perihelion approach as nucleus is exposed to maximum solar gain and maximum gravitational stresses.

D – Comet nucleus heats to melting point but does not explosively disintegrate as shielded by its own gaseous coma, and too loosely bound for expanding gases to form pressurised pockets. Almost no gaseous emissions from this point as most solid parts of nucleus liquefy, rather than sublimating or vaporising. As ice is likely to be lighter than liquid form, the centre might melt first thus reducing gaseous output at this point. The action of melting might actually absorb heat from the local surroundings of the nucleus and inner coma, reducing gaseous and dust emissions drastically in the process.

E – Comet nucleus is completely liquefied, its outer surface is starting to boil away as the comet passes its closest approach and heads away from the sun.

F – As the comet moves away from the sun a liquefied nucleus continues to boil away and a much reduced cemetery tail is produced.

What happens next will likely depend on how much the comet’s coma shields the liquefied nucleus from the sun’s radiation and how large the remaining body is and hence how quickly it might start to cool.

If very small the remaining liquid ball might evaporate completely very rapidly, and the comet’s tail will fade in the next few hours / days.

If medium sized it might cool rapidly forming a solid crust that may rapidly prevent any further major out gassing, or may produced a reduced sublimating tail.

If large enough, the liquefied nucleus may retain enough heat to continue to boil away, producing a visible tail or tails, lasting days or weeks, before succumbing to the cold of space and forming a new solid crust of ice.

I'll try to post an image that goes with the text later today.

Regards

©CastleGarden

CastleGarden wrote:Possible explanation of what happened…

D – Comet nucleus heats to melting point but does not explosively disintegrate as shielded by its own gaseous coma, and too loosely bound for expanding gases to form pressurised pockets...

I don't think there was anywhere enough energy to melt the nucleus in the short time it was very close to the Sun, unless that nucleus was already substantially disrupted.

I didn't understand the tail's direction, either. It's not a trick of perspective?

geckzilla wrote:I didn't understand the tail's direction, either. It's not a trick of perspective?

What is it you don't understand? We don't see an ion tail in the images near the Sun. The dust tail is pretty much doing what we'd expect- substantially following the same orbit as the nucleus, but pushed slightly outwards into a slower orbit. Of course, it's hard from a single perspective to see exactly how the tail is directed, but I don't see anything unusual.

The tail is in the orbit, sure, but it looks like it's pointing the wrong way. This video recently submitted in the comets gallery compares Lovejoy to ISON:

Video сomet ISON slowed to 2 times compared with the comet Lovejoy: ISON = 12 fps and Lovejoy = 25 fps.

I didn’t understand the lack of an outward pointing tail either until reading this statement from Chris:

Chris Peterson wrote:

RJN wrote:The continued visibility of what remains of Comet ISON is currently yet another great unknown for this enigmatic comet. Remember, though, when interpreting new ISON images, that comet tails usually point AWAY from the Sun.

Ion tails point away from the Sun. Dust tails lie substantially in the orbit of the nucleus, but pushed somewhat outwards by radiation pressure. Because of the high speed of ISON near the Sun, I'd expect the dust tail to be quite close to the parent orbit (both ejection velocities and perturbation velocities are small in comparison with the orbital velocity).

Remembering that at perihelion a comet is moving at its fastest, this object is outrunning its tail But we can see a tail beginning to swing out to the left as the comet slows and the debris starts catching up.

Bruce

BDanielMayfield wrote:Remembering that at perihelion a comet is moving at its fastest, this object is outrunning its tail! But we can see a tail beginning to swing out to the left as the comet slows and the debris starts catching up.

In the Stereo-Behind imagery you can definitely see the tail swinging outwards as the comet slows down.

If ISON truly survived and turns out to be naked eye visible, maybe we should rename it Comet Timex - it takes a licking and keeps on ticking!

Geckzilla's videos shows, to me at least, that it is still viable as a comet...it is still very bright on the other side of the swing...hope we will be able to see it...

:---[===] *

Here are two of the latest videos downloaded a few minutes ago from the SOHO site:
http://sohowww.nascom.nasa.gov/hotshots/index.html/
I will add some comments in a bit. - RJN

&& &&

Crossing fingers and toes hoping for a good show....

There we go, the tail is indeed swinging around.

Here are some speculative comments I have on Comet ISON's encounter with the Sun, given the latest SOHO images. Note that although I have published on comets once before, I have never published on sungrazing comets and don't really know much about how comet tails interact with the solar wind, particularly when near the Sun. Anyway, here goes:

Something has Survived

The SOHO images posted previously indeed do indicate that some part(s) of ISON survived.

ISON's Ion Tail

Why the ion tail will point somewhat away from the Sun. By eye, it seems to me, that ISON might well be outrunning the solar wind when within a solar diameter or two of the Sun's surface (a possibility mentioned above). But ISON's speed is almost completely horizontal and not out from the Sun at that time, therefore the ion tail which responds to the vertical wind should still point outward.

Since ISON is moving so fast, however, the ions expelled will move away from the place they were expelled, which will trail the comet's coma. The glowing ion trail should still appear to connect to the comet so long as it is expelling ions -- but now move outward from where ISON used to be -- but move outward from the Sun faster that the coma is moving. Therefore this ion tail will appear to "swing around" the coma as ISON recedes from the Sun. I think the newly developing ion tail being, for a while, nearly perpendicular to ISON's motion out from the Sun shows this effect in the SOHO C3 video. As ISON's motion shifts to more outward than sideways, the ion tail will keep on "swinging" until it (nearly) appears to point out from the coma in the direction away from the Sun.

ISON's Dust Tail

Why some of the dust tail will still point somewhat toward the Sun. Dust already trailing the nucleus(es) will continue to trail the nucleus(es) even after it rounded the Sun. This dust will not magically jump over the comet nucleus(es) after perihelion. That is also possibly why the orbit of the comet faintly persists (particularly in the SOHO C2 image) even hours after the comet has passed -- trailing dust is still trailing.

One caveat is that dust chunks small enough to evaporate and exposed to direct sunlight and solar wind may turn into plasma and no longer orbit as dust but as plasma.

After perihelion, however, newly emitted light dust will be pushed out from the coma by light pressure and so will not trail the coma. This dust will roughly follow ISON's orbit but be slightly further out from the Sun and move slightly faster.

Yes I am Making This Up

Again, almost all of this is speculation. I include it here partly to inform those interested and partly to invite discussion that may help to better illuminate what is actually going on with ISON and even possibly lead to timely and telling observations of this unique and brief opportunity in the life of this unusual comet. (e.g. There is no time for peer review in this case.)

- RJN