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Why might Comet 2012 S1 be less bright than hoped?

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#1 Kristen

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Posted 30 September 2012 - 04:18 PM

Sorry, probably a very naive question, but I'm afraid I don't know the answer.

My presumption is that for Comet 2012 S1 (ISON) to have been detected out beyond Jupiter it is relatively reflective, and thus icy / big.

I also presume that, comparatively, it can be estimated if it is bigger / bolder than similar previous encounters (i.e. by how bright it appears at the distance it is at)

If that is the case (which might be a risky assumption!) what sorts of things lead to it being a damp-squib instead of a "once in a civilisation's event" (which would be great to live through, but I realise that Media has to sell copy regardless of the facts ...)

Is there much less spectacle on-approach to the sun inbound than on its outbound journey?

I have read that, as a sun-grazer, it might break up on close encounter with the Sun, but assuming not what makes the difference between Spectacular and Damp squib please?

Many thanks.

#2 Astrodj

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Posted 30 September 2012 - 05:26 PM

I remember the letdown of comet Kohoutek in '73 that was expected to be a "great" comet. The letdown was huge. How a comet will appear visually is just something that remains unpredictable. Especially a new one that hasn't be seen historically.

I hope this one is as good as Hale Bopp; now that was a fine comet!

#3 MikeBOKC

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Posted 30 September 2012 - 05:43 PM

I think one of the biggest unknowable variables determining comet brightness is the amount of ice and other volatiles on board and close to the surface of the comet. If there's a lot then propsects for a good display are high; if not, it's more of a dud. No way to tell, but if this is indeed a first-time passage for the comet thereis hope that it is basically pristine with a lot more stuff to ablate/outgas away, making for a better show.

#4 Rich (RLTYS)

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Posted 01 October 2012 - 05:21 AM

Like I mentioned in another post, we can only hope.

Rich (RLTYS)

#5 Kristen

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Posted 01 October 2012 - 06:33 AM

I think one of the biggest unknowable variables determining comet brightness is the amount of ice and other volatiles on board and close to the surface of the comet. If there's a lot then propsects for a good display are high; if not, it's more of a dud. No way to tell, but if this is indeed a first-time passage for the comet thereis hope that it is basically pristine with a lot more stuff to ablate/outgas away, making for a better show.


I've read that its a coin toss, fair enough!, but the thing I'm struggling with is the thought that if there is a lot of ice at the surface won't that increase the reflectivity - such that its brightness-magnitude at current distance would be a good indicator?

Wouldn't it be darker (too dark to detect at Jupiter distance even?) if it had very little surface ice?

#6 BarrySimon615

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Posted 01 October 2012 - 08:04 AM

To quote Dr. Leonard "Bones" McCoy:

Space is disease and danger wrapped in darkness and silence.

Stuff happens. This comet could be a very fresh loosely packed dirty snowball that will shed a lot of material too soon before it brightens up. Early brightness estimates are just that - estimates. As a "sun grazer" it could graze just a bit too close and if loosely packed....it's gone.

In addition it's brightness curve rises to a sharp peak when it is very near the Sun and either completely obscured by the Sun's light or even if a bit further removed from the Sun lost in morning twilight during November, 2013 or evening twilight in December, 2013. Before it is appreciably affected by morning twilight near 11/21/13 or so, it's brightness estimate put's it at about +2.0 which is nice but not blow away spectacular. After it swings around the Sun and headed back out to the far reaches of the solar system, from our perspective it's movement is mostly in azimuth, not altitude, so it will be low after sunset. On 12/20/13, from southeast Louisiana it will only be 7 degrees up at sunset and will have set well before the end of twilight. This is 3 weeks after it swings around the Sun and it's brightness has dimmed to an estimated +3.5.

(see the charts I created in the thread on this forum under the comet's name)

Barry Simon

#7 Tonk

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Posted 02 October 2012 - 08:19 AM

but the thing I'm struggling with is the thought that if there is a lot of ice at the surface won't that increase the reflectivity - such that its brightness-magnitude at current distance would be a good indicator?


The factor that hasn't been mentioned is fragmentation.

First time visitors from the remote Oort cloud are not always compact and lack cohesive strength. They thus have a tendency to fragment (and fade) once the volatiles start to melt/sublime when the comet approachs the sun - usually ramping up internal stresses between the orbits of Jupiter and Mars. This is one major mechanism whereby a comet fails to perform

The most recently documented case was Comet Elenin which completely fragmented.

Comets with a orbital element "e" with a value very close to 1 (parabolic) are thus treated with extreme caution when it comes to predicting light curves and other behaviours.

Comet ISON has e very very close to 1 - so thats the caution

The converse is with periodic comets which are "tested" and prior apparitions give a good idea how the next one might behave. However even these confound as in the relatively recent break up of 73P

By the way Kahoutec did not dispoint me - it was my first observed comet and it was magnificent with a decent tail readily observable both by eye and binnoculars. Over hypeping did not detract from the actual performance

#8 Astrodj

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Posted 02 October 2012 - 10:14 PM

By the way Kahoutec did not dispoint me - it was my first observed comet and it was magnificent with a decent tail readily observable both by eye and binnoculars. Over hypeping did not detract from the actual performance


It was also my first observed comet, and brother, I wasn't let down either. I was ecstatic. The "letdown" I refer to was perceived by the public at large who were media-hyped to a comet of the century level, blazing across half the sky and visible in daytime,which it wasn't.



Here is a great article from Nasa history archives detailing how it all went down.

#9 Tonk

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Posted 03 October 2012 - 09:02 AM

The latest measurements of ISON (reported to todays Yahoo comets-ml group) are indicating that the initial predictions are a little conservative! It appears to be unusually dust active already. However the caution re the risk of fragmentation still holds and probably won't go away at any point for this apparition. If it holds together until it at least passes the orbit of Mars in its post perihelion phase then we should be in for a visual treat!

#10 Tonk

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Posted 03 October 2012 - 09:16 AM

Here is a great article from Nasa history archives detailing how it all went down.


That was a good read - thanks. Also it seems the hyping thing was USA centric. The first I read about the comet was in the UK Sunday Times newspaper in the first week of 1974! I simply went out the next clear evening to our local park and observed the comet - rather short notice for me. Certainly not 6 months of escalating hype.

#11 Dave M

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Posted 03 October 2012 - 10:22 AM

The latest measurements of ISON (reported to todays Yahoo comets-ml group) are indicating that the initial predictions are a little conservative! It appears to be unusually dust active already. However the caution re the risk of fragmentation still holds and probably won't go away at any point for this apparition. If it holds together until it at least passes the orbit of Mars in its post perihelion phase then we should be in for a visual treat!


Sounds like good news to me :D

#12 operascope

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Posted 23 November 2012 - 04:18 PM

“Comets are like cats: they have tails, and they do precisely what they want.” -David Levy

Lance

#13 Achernar

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Posted 23 November 2012 - 06:37 PM

It is not visible because it's big with a bright surface, it's visible because there are a lot of very volatile chemicals that are vigorously outgassing to space to form the coma. That is also releasing a lot of dust, which is further enhancing it's visibility. Comet nuclei are jet black, and that is why we couldn't even see them beyond Jupiter until super sensitive CCD cameras and big telescopes were teamed up to look for them. It is comparatively untouched as oppossed to a periodic comet and it could very well be a very large comet too like Hale Bopp, which will give it a LOT of ice and gasses to generate it's coma and tail.

It could of course fall apart and fade away, many sungrazers perish even before they reach perihelion. Or it could get all of it's ices and gases baked out of the outer layers, leaving the inner core untouched but also leaving it a faint comet.

Even if it never gets very bright, an eighth or ninth magnitude comet is worth looking at with a telescope, and a fourth or third magnitude comet will be spectacular through larger binoculars and small telescopes. It will not be getting nearly as close to us during the inbound leg, the best opportunity to see it will be after perihelion which will place it high in the evening sky from the northern hemisphere. So even if it does not match McNaught, it will be in a good position to see for weeks much like Garrad, Hyakutake and Hale Bopp were. The time to start watching for it in earnest is when it comes out from behind the Sun and hurtles past us back into deep space. There is a chance we will see an outburst where the comet suddenly flares up dramatically as Comet Holmes and more recently Hergenrother did. Anything is possible with a comet, since they are after all like my cats, they have tails and they do whatever they want. Sungrazers often are very bright with long dramatic tails, especially when they are whipping around the Sun at 500,000 mph or more. I would expect at some point to see the dust and ion tails at large angles to one another, so you will definitely want to have a camera handy to record this one if it is as bright as Hale Bopp. That one was easy to see from my city. Let's hope this will be THE great comet of the early 21rst century, one we can tell our grandchildren about when Halley's comet is heading in for it's next appearance in 2061.

Taras

#14 Jure Atanackov

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Posted 08 December 2012 - 09:17 AM

Comet Kohoutek and some other comets that disappointed (remember C/2002 T7 (LINEAR)?) are dynamically new comets. This means they are on their first journey towards the Sun from the Oort clouds. Such comets are very rich in highly volatile substances that begin sublimating at large heliocentric distances, inducing rapid brightening of the comet. Comets that have already made a close pass of the Sun have lost much of these substances and the main driving forces of brightening are water and CO2. These two substances begin sublimating more readily at smaller heliocentric distances. Assuming a dynamically new comet is actually an 'old' comet and that water and CO2 are causing the brightening, high brightness at large heliocentric distances means that the comet is unusually large. Such a comet would indeed become very bright closer to the Sun. But dynamically new comets usually experience a reduction in brightening rates at <2 a.u. from the Sun, where water and CO2 sublimation takes over as the main brightening mechanism of the comet. This is exactly what happened to comet Kohoutek in 1973.

By the way, it actually reached magntiude -1 to -3 and there was a daytime sighting in Japan!

There is reason to believe comet ISON is not dynamically new, as sungrazing orbits are usually very evolved orbits and it is quite unlikely that a comet would make such a close approach of the Sun on its first inbound trip. Under this assumption, comet ISON is doing very, very well at the moment. Plugging the CCD photometry that is available into the standard photometric formula, it appear this comet's absolute magnitude is between +5 and +7. It will be interesting to see when exactly it comes into visual range. It is not unusual for CCD magnitudes to be 1 to 2 (sometimes more) fainter than visual. Should this be the case, then we'd be dealing with the intrinsically brightest sungrazer since the Great september comet of 1882. Hmmm...

Clear skies!
Jure

#15 BrooksObs

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Posted 08 December 2012 - 12:13 PM

The question(s) surrounding Comet ISON's potential future development number more than a few, just as Jure has pointed out in the preceding post. However, I will try to add here some of the situations that will likewise govern its future evolution.

As Jure has pointed out, the current orbital eccentricty of Comet ISON is very close to e = 1.0, implying that it is a dynamically "new" comet coming sunward from the Oort cloud for the very first time. He similarly and correctly indicated that dynamically "new" comets very often fail to live up to their initial brightness predictions.

The case with Comet ISON, however, is not necessarily so straight forward concerning its exact orbital eccentricity. At present the elements suggest the orbit is ever so slightly hyperbolic. However, the trajectory of sunskirting comets (the rarest of types that there are) are almost rectilinear at large heliocentric distances making precise determination of the parameter "e" rather more difficult to determine than usual. An ellipse of extreme eccentricity would likely fit the positional observations just as well.

Then, too, the fact that the orbit of Comet ISON and that of the Great Comet of 1680 (whose e = 0.99999) have many peculiar similarities raises additional questions. Although the directions of aphelion for the two comets diverge significantly, the perihelion points of their orbits almost exactly coincide! This to me raises the distinct possibility that the two objects might once have been a single body perhaps one or two revolutions ago. Any current differences in their orbital parameters can be accounted for by the direction and velocity of a break-up near the time of perihelion passage. The very fact that their perihelia are almost identical, while sunskirting comets are so exceedingly rare, indicates to me that the alignment is much more than simple chance. Now the Great Comet of 1680 put on an extraordinary show and if Comet ISON is indeed a large fragment from it, this bodes very well for it. And likewise, if ISON is a piece of the 1680 comet, then it is not dynamically "new" and it becomes a case of what you see is what you get with regard to its current intrinsic brightness.

I do, however, wish to point out that the speculation regarding ISON's great brightness near the time of its perihelion passage has likely been decidedly exaggerated. At the same time, it is likely that most observers have little or no appreciation of how the brightness of comets with extremely small perihelia actually behave as they race past the Sun.

Following up on that thought, I would hasten to point out that even were Comet ISON to attain a peak magnitude of -6 to -8 that this level of brightness would be exceeding brief and the comet would be significantly fainter just HOURS before and after the event. Likewise, when at its greatest brightness the comet will be within a degree or so of the Sun's limb!

BrooksObs






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