Archive for November, 2013

Remnants of Comet ISON fading fast

Saturday, November 30th, 2013

As I surmised yesterday, it appears that the nucleus of Comet ISON was mostly destroyed during its close approach to the sun, and what remains is fading fast, now estimated to be magnitude 5 in brightness:

According to Karl Battams’ recent blog post,

“…during its passage through the Sun’s million-degree corona, its dusty/gassy coma got very much burned away, though clearly some fine dust survived (which is the fine cloudy stuff you see being pushed away from the Sun).”

(His post also has a couple of very cool animations from the STEREO spacecraft, so I encourage you to take a look.)

Why does the comet appear the way it does now, after perihelion? Here’s a semi-technical explanation…what I’ve surmised based upon my knowledge from atmospheric science.

Basically, the material being flung out sideways from the comet’s normal orbital path is very fine, whereas the particles in the “head” are larger. Although the reasons for this are not discussed in other blogs posts about the comet, I suspect it is the same reason why tiny particles of rock can float in the air as aerosols, whereas large particles would fall to the Earth. Or, why fine sediment particles are suspended in a river, while large particles fall to the bottom.

Gravitational forces act on the mass of an object, whereas atmospheric or liquid viscous pressure forces act on the physical size (cross-sectional area) of the object. Because mass increases as the 3rd power of the particle radius, and physical size (or cross-sectional area) increases as only the 2nd power, the larger a particle is, the greater the gravitational effects are relative to pressure forces.

In the case of the comet, the pressure forces are from radiation pressure and the solar wind. As the nucleus of Comet ISON got pulverized during perihelion, the tiniest particles were more affected by the radiation pressure and solar wind than by gravity, and they got “blown away” from the normal orbital path. As I mentioned in my last post, they appear to be “flung outward”, away from the normal gravitationally-dominated orbital path of the comet nucleus.

If you watch the above video, you will see this fine material being blown in the solar wind, away from the nucleus. Presumably, what is left in the nucleus are somewhat larger particles whose path is still dominated by gravity, but there is so little left that there is not much there to reflect sunlight that we can see.

I suspect that in the coming days only the better telescopes will be able to see what is left of the comet. Astrophotographers like me will see very little if anything at all.

A Comparison Between Comets ISON and Lovejoy (2011)

Friday, November 29th, 2013

The experts are still trying to figure out just how much of Comet ISON has survived during Comet ISON’s flyby of the sun yesterday.

I’m not a comet expert, but I’m going to venture a guess based upon the following comparison to another comet (Lovejoy) that also surprised astronomers when it survived perihelion in December, 2011.

Here’s a time lapse video of imagery from the SOHO LASCO C3 instrument when Lovejoy flew around the sun on 16 December 2011. Note three things:

(1) the horizontal flaring (sensor overdriving), which indicates a very bright object, both before and after perihelion;
(2) how compact the comet nucleus remains after perihelion,
(3) how the comet gradually grows a new tail pointing away from the sun after perihelion.

Now let’s look at Comet ISON. Compared to Comet Lovejoy, note:

(1) there is not as much flaring before perihelion, and NO flaring (yet) after perihelion (ISON is not as bright as Lovejoy),
(2) the nucleus is much more diffuse (larger, but dimmer) after perihelion,
(3) there appears to be comet material “flung outward” after perihelion, even before a new tail grows.

Now for some wild speculation by a rank amateur. I think the nucleus mostly broke up during perihelion, and what we now see is diffuse material that will rapidly dim in brightness over the coming days and weeks. I hope I’m wrong, of course…I would love to do more time lapse video of a brilliant pre-dawn comet. But at this point, I’m not hopeful.

Comet ISON, RIP…or Zombie ISON?

Thursday, November 28th, 2013

Is Comet ISON just a a residual pile of cosmic dust? Or did it survive?

Shortly after passing perihelion, it looked like little was left. This SOHO LASCO C2 (narrow field) time lapse video shows what was believed to be the dusty remnants of ISON, without a bright nucleus, emerging from its close encounter with the sun:

But now, a few hours later, whatever is left is gradually getting brighter in the LASCO C3 (widefield) imager. From what I’ve read, the experts are totally confused (click image for the latest):

UPDATE: At 8:46 p.m. CST, Karl Battams Tweeted that he and his associates now believe ISON has survived with some portion of its nucleus intact.

Comet ISON Hours Away from Perihelion

Thursday, November 28th, 2013

Here’s the latest SOHO spacecraft time lapse video of ISON approaching the sun, with perihelion expected today at 1:44 pm EST:

…and a more recent SOHO image, from 10:37 a.m. EST:

The following is from Karl Battam’s blog this morning, where he discusses the tremendous variations in brightness ISON has been undergoing..the rapid dimming in just the last few hours, and what it might (or might not) mean:

Last night I was optimistic that comet ISON would continue its dramatic brightening trend, and soar into the negative magnitudes. This morning it is indeed with a heavy heart that I show you the image opposite, in which we clearly see that ISON has faded rather dramatically in the past few hours. It is still likely around -1 magnitude, but this number is falling fast.

The question on everyone’s lips is “will it survive perihelion?”, and now I’m reluctantly thinking it seems very unlikely to survive at this point. I do think it will reach perihelion, and reach the NASA SDO field of view, but based on what I see it doing right now, I will be very surprised to see something of any consequence come out the other side.

BUT… at every single opportunity it can find, comet ISON has done completely the opposite of what we expect, and it certainly wouldn’t be out of character for this dynamic object to again do something remarkable. Read more.

Blow-by-blow tweets by Karl are posted at @SungrazerComets.

Here’s the NASA Solar Dynamics Observatory near-real time coverage SDO website.

Here’s the Solar and Heliospheric Observatory latest imagery from the SOHO website.

And here’s the Comet ISON News Twitter blog I also follow to get the updates from others who are watching all of the various ISON news and data outlets.

Comet ISON, if it survives perihelion intact, should become visible again the the Northern Hemisphere pre-dawn sky, near the eastern horizon, around December 3. How visible it will be is, at this point, unknown.

Here’s a photo I took the morning of Nov. 20 (I used a Canon 6D, Canon 200mm f/2.8 (at f/5.6), ISO 1600, stack of 70 15-sec exposures for a total of 17.5 min exposure time):

The Magical Mystery Climate Index: Luis Salas nails it.

Monday, November 25th, 2013

In my post from earlier today, I showed the following mystery climate index plot with the challenge to readers to figure out what modes of variability it contained:

Several commenters were actually very close in their explanation…but Luis Salas gave the actual equation to explain the above plot (and it looks like an “honorable mention” for CatJ). It’s the sum of 3 terms: a linear trend, an annual cycle, and a 6.5 year cycle:

Why did I do this? As a couple of people already guessed, it was mostly to show how a linear trend superimposed upon a cycle can yield periods of rapid change, followed by no change, then rapid change once again. In other words, a linear trend combined with a sinusoidal cycle can lead to plateaus.

Is that what is going on in today’s globally averaged temperature? A warming trend with a natural cycle producing our current warming plateau? I don’t know…but I don’t think we can rule it out. If that is what’s happening, then when warming returns it should be about as strong as before. But….

…but a couple people also alluded to another possibility: that what I have shown as a linear trend is (in nature today) just part of lower frequency oscillation…say the ~60 year cycle in ENSO strength, related to the Pacific Decadal Oscillation (PDO). In that case, it would be possible for there to be a long period of downward trend in temperatures in our future.

I’m really not advocating what the forcing mechanisms are: solar, internal variability, etc. I’m just trying to get people to think in terms of these superimposed signals. (Which, of course, are just mathematical simplifications of what could be the net effect of very complex physical processes).

Only time will tell which is closer to the truth, or whether the real situation might be even more complicated that the possibilities listed above…which would not surprise me at all.

The Magical Mystery Climate Index – What does it show?

Monday, November 25th, 2013

I’ve been having discussions with a physicist-friend about how to analyze time series data: what kinds of smoothing or filtering should be used, etc. The blogosphere is filled with discussions of various climate datasets and what people think they “see” in them.

Time series analysis is nothing new, and has a rich history. But it is easy to be fooled by data. So, as a learning exercise, I would like readers to examine the following 20-year plot of monthly data…I’ll call it the Magical Mystery Climate Index. I would like you to tell me what you see.

For those so inclined to do some data analysis, here are the data in an Excel spreadsheet: Magical-mystery-climate-index

I suppose what I am asking is this: What modes of variability do you see in the data? I happen to know the answer, because I’m the one who defined those modes of variability. I just want to see what other people come up with. I’ll post the real answer when I stop getting new ideas from readers.

A White Thanksgiving for New York City?

Friday, November 22nd, 2013

[UPDATED with 12Z Nov. 22 model plots, now putting the storm just offshore.]

I’ve been watching the setup for what could turn into a white Thanksgiving for much of the Northeast..and maybe a travel nightmare for the day before Thanksgiving.

From what I can tell, the last white Thanksgiving in NYC was about a quarter century ago (in 1989), and before that was a half-century prior to ’89. Maybe my friend Joe Bastardi will correct me.

Predicted cold air outbreaks for the Northeast and mid-Atlantic in the coming days, combined with a low pressure system moving across the northern Gulf of Mexico, are consistently combining in the GFS model to produce an early-season nor’easter, with significant snow from the mid-Atlantic up through New England.

From last night’s this-morning’s GFS run, here’s the sea level pressure and 12-hr precip plots for Thanksgiving eve and morning:

And here are the corresponding 850 mb forecast plots, which also show the cold air mass associated with the system:

Now, for nor’easters to form and impact the East Coast, timing of the cold air arrival versus the low pressure approach from the southwest is everything.

If cold air arrives a little early, the system remains off the East Coast, with only windy and colder weather for the East. If the cold air is late, the storm moves inland with rain for the East, and snow for the Ohio Valley and eastern Great Lakes. Given this is all a week away, things could change significantly by then.

But those planning on travel to the East Coast for Thanksgiving should keep an eye on this situation in the coming days.

Oh, and if NYC does get hit with significant snow for Thanksgiving….let me be the first to blame global warming for it. ;-)

Spica & Comet ISON Rising, Nov. 20, 2013

Wednesday, November 20th, 2013

Time lapse video of Spica and Comet ISON rising, Nov. 20, 2013, over northeast Alabama. It was very windy, so there is a little camera shake as it zooms in from full frame to 130% crop. Composed of 326 frames from a Canon 6D, Canon 200mm f/2.8 lens (@ f/5.6), ISO 1600, 30 sec exposures. Tracking with AstroTrac. Note the movement of the comet past the neighboring stars late in the video.

Comet ISON & Moonlit Clouds, I & II (time lapse)

Monday, November 18th, 2013

I took these this morning (Nov. 18, 2013), again near New Market, Alabama. I was hoping the clouds would clear in time to reveal the comet, which is just what happened. The first video is from ~300 20-sec exposures, the 2nd video used ~150 5-sec exposures (shot just after the first). I used a Canon 6D with 85mm f/1.2 lens and an AstroTrac for tracking of the stars. All of the lighting of the landscape was provided by the moon and the very sensitive camera and lens…by eye, it was much darker.

Comet ISON Rising, 14 Nov. 2013

Thursday, November 14th, 2013

UPDATE (11/16/2013): I’ve replaced the video with one that pans and zooms…much better. (The video will keep looping after it loads):

This is my latest (and best) attempt at a time lapse video of Comet ISON, rising over a heavily forested area of northeast Alabama.

The video starts without tracking, so the stars are streaked. Then I turned on star tracking when the comet was approximately centered in the frame, and the frame of reference “launches” to keep up with the comet.

I was finally able to see the comet for the first time in my Canon 10×30 binoculars….I could barely make out the faint tail. But you need dark skies and know just where to look. To find it, I’m now using the iPhone app “Star Walk“, which is absolutely amazing.

This time I used my new Canon 85mm f/1.2 lens, stopped down to f/2.5, 30 sec exposures. This very fast lens is awesome…it seems to show more detail than my 200mm lens extended to 400mm with a 2x extender. The video is cropped to 120% of full pixel resolution, so the effective magnification is about 6x or 7x.

There is an interesting satellite which passes to the left of the comet, from top to bottom. It crosses the sky much slower than most satellites, suggesting a very high orbital altitude. Based upon the direction and the angular speed (the 30 sec satellite streaks are same length as the star streaks at the beginning of the video), it appears to be geostationary (wow! Imaging a geostationary satellite with a 85 mm lens!).

PLUS…if you look closely, in one frame a meteor streaks by the geostationary satellite!

UPDATE: It appears the geostationary satellite passing by ISON was either Intelsat 905 or 907, which passed by (as seen from from my location) close to 4:30 a.m. These satellites are near the Equator off the coast of Africa, around 25W longitude, at an altitude of ~22,000 miles.

UPDATE #2: The bright satellite that whizzed by ISON just before the geostationary satellite appears to be an Atlas 5 Centaur booster rocket from a March 10, 2007 DoD NEXTSat launch.