Only a dim "ghost of ISON" survived the comet's November 28th passage around the Sun. The comet's head dwindled away as it raced through the Sun's greatest heat, but a headless streak emerged into spacecraft view out from the other side of the encounter. It's traveling along the comet's originally prescribed track but fading steadily, with no sign of cometary activity. Nothing will be visible by eye from Earth.
December 1: Goodbye from SOHO, and a CBET summary. The dimming wisp of ISON's dust departed from SOHO's LASCO C3 field of view early today; it's at the 1 o'clock edge of this image.
If it appears a little brighter in images from the STEREO-A spacecraft, shown here, that's only because the camera is more sensitive.
Dan Green of the International Astronomical Union's Central Bureau for Astronomical Telegrams published a long summary (Electronic Telegram No. 3731) early today. Excerpts:
The comet's nucleus apparently disrupted near perihelion, with the comet's head fading from perhaps a peak brightness of visual mag –2 some hours before perihelion to well below mag +1 before perihelion. M. Knight, Lowell Observatory... [adds] that the brightest feature in the coma faded steadily after perihelion from about mag 3.1 in a 95"-radius aperture when the comet first appeared from behind the SOHO coronagraph occulting disk on Nov. 28.92 to about mag 6.5 on Nov. 29.98. . . .
K. Battams, Naval Research Laboratory, writes that, based on the most recent LASCO C3 images (Nov. 30.912 UT) . . . what remains is very diffuse, largely transparent to background stars, and fading; it appears that basically a cloud of dust remains. . . .
Z. Sekanina, Jet Propulsion Laboratory . . . finds that the comet's production of dust terminated about 3 hours before perihelion . . .
The strong forward-scattering effect (phase angles near 120-130 deg) has tempered the rate of post-perihelion fading of the comet, but the merciless inverse-square power law of increasing heliocentric distance is necessarily the dominant factor in the comet's forthcoming gradual disappearance . . .
Hubble will have a look for the remnant once it moves far enough from the Sun for Hubble to safely point at it, in mid-December.
November 30, 6 p.m. EST: More fading, and Bortle revises his forecast. John Bortle withdraws his relatively optimistic forecast posted yesterday. He writes,
The comet's appearance only suggests a progressive decline now.
While ISON's photometric behavior up to and around perihelion did seem to mimic that of 1962's impressive Comet Seki-Lines, the failure of ISON's current cloud-like coma to exhibit any area of condensation as the cloud thins, and its fading and growing diffuseness, do not correspond to Seki-Lines' post-perihelion regeneration. . . .
To be similar in behavior with C/Seki-Lines, ISON would be developing an increasingly small, dense, very bright condensation within the coma. Likewise, the intensity and length of the tail(s) would be rapidly growing, just as we saw in the case of 2011 W3 Lovejoy following its perihelion. . . .
Following a 2- to 4-day blind period after the comet leaves spacecraft view, Bortle expects only a "large, low-surface-brightness, diffuse cloud showing just a trace of tail" — in other words a challenge object for skilled astrophotographers and image processers, not visual observers — if anything at all.
November 30, 9 a.m. EST: Fading continues; no activity. Filip Fratev posts again (at 8:15 UT this morning), "just measured another drop of 0.5 magnitude for the last 11 hours (20:30 – 7:18 UT). Thus it is in the range of magnitude 3.1–3.6, probably more close to 3.5." The later LASCO C3 image from 12:54 UT shows the fading very obviously.
He posts again at 15:29 UT; "ISON's brightness has dropped rapidly. I estimated the comet as between about 4.5-5.0m. Magnitude decrease was almost 0.1m per hour."
Jacob Czerny notes that the comet remnant is fading at the rate expected of a simple, inactive debris cloud moving farther from the Sun's illumination. In addition it is expanding, which means its surface brightness is dimming even faster than its total brightness.
As for what looks like a new comet tail? That turns out to be well modeled by the trajectories of particles that ceased to be emitted at perihelion two days ago. Explains Hermann Böhnhardt (Max Planck Institute for Solar System Research), "If we assume in our calculations that the comet has emitted dust at perihelion, we can reproduce the current images quite well."
Here's the latest blog article from the indefatigable cometary scientist Karl Battams: In ISON's Wake, a Trail of Questions. With great new movies from the STEREO A and B coronagraphs. Key takeaway:
It does seem unlikely that there will be much to see in the night sky. I suspect that some of the outstanding astrophotographers around the world will be able to get something, but I doubt it will be as spectacular as before perihelion. I hope I'm wrong though.
November 29, 10:30 p.m. EST: A fading ghost. At 19:54 UT Filip Fratev (Bulgarian Acadamy of Sciences) wrote, "ISON [has] started to fade.... [In] the last four hours it faded by more than 2 magnitudes and obviously is less bright... I estimated the comet to be between 2.6 and 3.1 magnitude now."
Four hours later Karl Battams of the Comet ISON Observing Campaign tweeted, "We can't tell if #ISON is in one piece or many. It's about mag 5 now and fading."
November 29, 3:30 p.m. EST: A bold forecast. Longtime Sky & Telescope comet writer John Bortle, who's been covering these objects since the 1960s, now writes us:
"Here are some things I garner from the 12:30 UT image taken today by the SOHO LASCO C3 camera.
"1. Since yesterday's 'resurrection,' the comet's overall dust output has clearly increased dramatically as compared with its pre-perihelion situation.
"2. Likewise, early yesterday the dust production appeared to consist almost exclusively of 'very heavy' dust particles. My impression this morning from the latest SOHO and STEREO images is that the comet is now experiencing a huge and ongoing dust release in a broad range particle sizes. If this continues, it is a very hopeful sign for visual observers in days to come.
"3. I've mentioned several times that 1962's Comet Seki-Lines is the one other modern, dynamically 'new,' truly sunskirting comet whose photometric behavior is known in some detail. It likewise experienced a strange 'disappearance' right about the time of an anticipated brightness surge at perihelion, which had been expected to bring it to naked-eye daylight visibility at around magnitude –7.
"4. A quick glance at ISON's appearance in the 12:30 UT SOHO image jogged my memory that it looks very much like another comet photo I saw long ago . . . of none other than Comet Seki-Lines! One of the earliest post-perihelion photos of Seki-Lines was one taken by, I believe, the great comet photographer of the 1960s and 70s, Alan McClure. It showed a strangely wedge-shaped, large, brilliant coma forming the beginnings of a huge, broad, tail created by an obviously wide range in dust-particle sizes. Sound familiar?
"5. Although ISON's overall magnitude and degree of coma condensation appear reduced from last week, it still looks like a rather bright object that could well put a nice, if likely rather truncated, display between December 1st and 15th for visual and imaging folks alike. If (still a big if this early) ISON continues to exhibit intense dust production and retains sufficient ices to drive activity for a short time (perhaps less than a week could suffice), a broad tail of considerable size may quickly develop.
"As with 2011-12's sungrazing Comet Lovejoy, it would be impressive mainly from dark-sky observing sites due to rapidly diminishing surface brightness as the object withdraws from the Sun. The comet's 'head' will probably be tiny in comparison with the tail and might even be difficult to define with the unaided eye."
November 29, 11 a.m. EST: "Reports of my death are exaggerated." After the comet seemed to vanish at perihelion yesterday, no one expected to be looking at a picture today like the one above!
It sure looks like a comet from here. The big question: is that bright thing just a cloud of dust and very fine rubble, soon to disperse? Or will it hold together, traveling along along the comet's orbit, long enough to be seen when it gets far enough from the Sun to be viewed from Earth? And do chunks remain within it that can keep shedding stuff to make a coma and tail in proper comet fashion?
Many around the globe have been trying to detect this "ghost of ISON" in the bright sky near the Sun. We've heard no positive report so far. The comet is moving north of the Sun. Put the Sun safely behind the edge of a building or something; don't endanger your vision, especially not with binoculars or a telescope. As a rough brightness comparison, the bright star at lower left in this image is 1st-magnitude Antares.
Want to watch where a surviving comet would appear in the dawn sky in coming days? See our charts.
What a puzzle for comet scientists! ISON has defied predictions every step of the way.
Watch this space for developments.
November 28, 10:00 p.m. EST: Comet ISON emerged from today's close brush with the Sun as a headless ghost, as seen in the animation at right. Notice how the brightest strand, somewhat back from the leading point, is shedding material radially away from the Sun, as if vast numbers of debris chunks in the cloud are all behaving as tiny comet nuclei. The cloud is expected to disperse.
Yet the cloud looks more heartening in these 30 most recent images from SOHO's C3 coronagraph, as it moves farther from the Sun.
Clearly astronomers didn't quite know what to think of the reappearance. "Our excitement built as we saw a feature emerging from behind the occulting disk at the expected location and time . . . and grow brighter into a fan shaped feature," says Padma Yanamandra-Fisher (Caltech). "The question is: did the comet fragment into multiple pieces, or is it dust and cometary debris? Right now we saw the only one feature and so cannot answer the question."
Bruce Betts of the Planetary Society tweeted, "It is now clear that Comet ISON either survived or did not survive, or … maybe both. Hope that clarifies things."
Carey Lisse, a comet specialist at JHU's Applied Physics Laboratory, comments, "It is very possible you are seeing ISON's trail shedding mass and emitting small particles. Another, less likely possibility, is that the comet fragmented into a number of bigger pieces, like Comet Shoemaker-Levy 9 [which struck Jupiter in 1994], and they are emitting dust. The post-perihelion dust fan we are seeing now is
what you would expect from dust experiencing solar radiation pressure and gravity forces while making a U-turn around the Sun at perihelion."
Matthew Knight (Lowell Observatory) has a slightly more optimistic outlook. "My best guess is that there are a few pieces of rather large debris remaining, but that they continue to shed dust. So far there hasn't appeared to be a strong central condensation like I would expect if there was a substantial nucleus. By eye, it does seem to be looking more like a 'comet' in the C3 images, which suggests that there is something left."
Read NRL observer Karl Battam's take on what he's calling "Schrödinger's Comet".
November 28 update #2: ISON is dying before our eyes! As of 2 p.m. EST November 28th, just after perihelion, ISON's head — which swelled to brilliance just hours ago — seems in the latest spacecraft images to have faded right down to nothing at all.
What's left is a long, thick streamer of a dust tail. The tail is destined to be flung widely across the sky as it comes out the other side of the Sun — unless it vaporizes away first, which is possible and even likely.
The Solar Dynamics Observatory (SDO) spacecraft should have picked up the comet's head now by now, in its narrower, zoomed-in view compared to SOHO's. So far it is seeing nothing at all.
3:30 p.m. EST: SDO's scientists were able to extract nothing of the comet as it should have crossed SDO's field of view at perihelion.
4:10 p.m. EST: The LASCO C2 coronagraph saw only a sparse, headless debris trail come out from behind its Sun-occulting disk after perihelion. The "Ghost of ISON," it's being called.
November 28 update #1: Perihelion Day Begins. For most of yesterday Comet ISON (short for International Scientific Optical Network) brightened dramatically. In images from the SOHO spacecraft's LASCO C3 coronagraph, its overexposed head grew ever-wider horizontal lines of pixel blooming. From these, members of NASA's Comet ISON Observing Campaign estimated its brightness at magnitude –2.
That was then, this is now. A new image from LASCO C3 shows a big drop in brightness between 9:06 and 12:54 UT (4:06 and 7:54 a.m. EST) November 28th, as shown at right. For comparison, the bright star at the 7 o'clock position is 1st-magnitude Antares.
Perihelion is around 1:40 p.m. EST today. I'll be watching NASA's Google+ Hangout: Comet ISON Live, which will run for 2½ hours right around perihelion, from 1:00 to 3:30 p.m. EST (18:00 to 20:30 UT).
Read CIOC blogger Karl Battams' update from 9:06 a.m. EST, Hanging On by its Fingernails, with a better comparison of the two images.
Later images up to 15:37 UT show the head fading way down as it reaches the camera's occulting disk around the Sun. (Note, the public SOHO site is getting slammed.)
Meanwhile the comet has entered the narrower, higher-resolution field of the LASCO C2 coronagraph. Latest C2 image. Note how insignificant — okay, non-existent — the head appears now with respect to the tail! This is looking now like a big version of the common little SOHO sungrazer comets that often vanish near the Sun.
November 27: One day to go. Last night Comet ISON entered the view of LASCO C3, a wide-field coronagraph on the Solar and Heliospheric Observatory (SOHO) spacecraft. This instrument takes a new image about every 20 minutes. The comet should enter the narrower, higher-quality field of SOHO's LASCO C2 camera around 9 a.m. EST on the 28th (14h UT) and exit around 6 p.m. (23h UT).
The good news, according to Lowell Observatory astronomer Matthew Knight, is that Comet ISON increased in brightness at least fourfold in a matter of hours since entering the C3 coronagraph's field of view.
Update 4:30 p.m. EST: "ISON is definitely into negative magnitudes as seen in LASCO C3 in the 27 November 20:42 image [3:24 p.m. EST]," posts Robert D. Matson, a longtime LASCO user. "I estimate somewhere between magnitude –1 and –1.5."
Update 6 p.m. EST: Writes Karl Battams, "I'm going to go ahead and say it's about magnitude –2 now. This makes ISON a very bright comet, and one that we are truly very lucky to be witnessing. We have less than a day until ISON's close brush with the Sun, and even in that short window we could conceivably add another three magnitudes to its current brightness. But — on the flip-side — ISON could still start to fade away at any moment. We still don't know if it has an intact nucleus, but we do tend to think that there must still be something coherent at the center of its dusty coma."
It's conceivable that the comet will become bright enough to glimpse in broad daylight in the hours immediately before or after perihelion. To attempt to see it, you'll need to be exceedingly careful and have a clear daytime sky free of haze. Tips and a position chart for attempting this challenging observation.
It's important to remember that even if the nucleus has completely broken up, the rubble swarm will take time to expand enough to affect how the (overexposed) comet head appears in these low-resolution images. The comet's brightest part is not really half as wide as the Sun!
In other news, there's talk that the lack of a visible gas tail (which would point radially away from the Sun) may mean the nucleus has disintegrated or otherwise gone dead.
- Info from Phil Plait on NASA's Thanksgiving Day Google Hangout
- Latest SOHO Images
- Latest STEREO-B coronagraph images
- Latest STEREO-A HI STARS Images (left column)
- NASA's Comet ISON Observing Campaign (CIOC)
- Yahoo Comets Mailing List
- Karl Battams Twitter Feed
November 26: More uncertainty! Mixed news today. On the one hand, longtime comet expert Jakub Czerny posted:
No good news from [the] STEREO A [spacecraft imaging the comet; see Nov. 25 below]. Matthew Knight has sent me his photometry of spacecraft images to 15h UT Nov. 25. After a strong fade, the comet has started gaining brightness again, unfortunately with a speed far below (n = 0.66) the brightening for a simple reflecting body (n = 2).
In other words, the comet's head was physically dwindling when it should have been growing like crazy. Czerny notes that this is consistent with no nucleus — no dirty iceball — remaining in the comet's head, just a slowly dissipating swarm of inactive rubble.
But now, more recent images from STEREO A show ISON perking up and looking more back on track. For instance this pic taken a few hours ago, at 12:49 UT on November 26th. ISON is the obvious comet; Encke is lower left of it. More pix. Here's a movie of them:
Some comets dwindle and burst often. "This is a dynamic situation," Karl Battams (Naval Research Laboratory) said during a NASA press conference. "We don't have any past experience to predict what's going to happen to this object."
November 25: Grim news in millimeter waves? Michal Drahus posts this morning:
Comet ISON has been closely monitored at the IRAM millimeter telescope in Spain by Israel Hermelo (IRAM Granada) and myself (Caltech/NRAO) for the last 6 days. We observe consistent, rapid fading of the molecular emission lines between Nov. 21 and Nov. 25 by at least a factor of 20 (likely more). This may indicate that the nucleus is now at best marginally active or that . . . it no longer exists.
. . . What we're observing are the HCN molecules emitted within about 1 hour of the moment of observation. In other words, our data show what's going on with the comet [nucleus] essentially "now" and not hours or days ago [as is the case with visible light].
Parent molecules have very short lifetime, so they are a good indicator of the real-time activity level of a cometary nucleus. Even if there is no nucleus remaining, we would continue to observe the comet on STEREO images for a day, two, or even a bit longer.
Too soon to panic, but there are other ominous signs. ISON failed to brighten much as it was disappearing into the dawn. There was a big dust release around the 19th, perhaps hinting of disintegration, followed by less dust production despite the increasing heat. And by the 22nd there were reports that the nucleus's center of brightness was lagging behind its predicted position by about 5 arcseconds, as if most of the light were now coming from trailing rubble.
November 25: Gone from Earth view; spacecraft take over. The comet has moved too close to the Sun for practically any visible-light observations. But now it's entering the fields of view of solar observatories in space, which are watching the Sun from various directions far from our home planet.
Several days ago ISON entered the very wide view of NASA's STEREO A, one of two twin craft nearly on the far side of the Sun from us. Here's a spectacular movie, four days long (November 19–23), of Comets Encke and ISON making their way with tails flapping in gusts and waves of the solar wind. A labeled frame is above. The images are processed to display differences from each frame to the next. More about the movie and comet tails in the wind, from Karl Battams of NASA's Comet ISON Observing Campaign (CIOC).
Here's a less-processed closeup movie.
NASA plans many studies of ISON around perihelion date. Here's a schedule of when solar-pointing spacecraft will be watching, and the instruments to be used (note: these observations might take hours or days to be streamed to Earth):
- Nov. 21–28:
STEREO A, Heliospheric Imager.
- Nov. 26–29:
STEREO B, coronagraphs. Stereo B will be the only craft to watch ISON transit the face of the Sun.
- Nov. 27–30:
Solar and Heliospheric Observatory (SOHO), coronagraphs.
- Nov. 28–29:
STEREO A, coronagraphs.
- Nov. 28:
Solar Dynamics Observatory (SDO); for about 3 hours around perihelion.
- Nov. 28: Hinode, Japan's X-ray space telescope; for about 55 minutes around perihelion.
Lastly, on November 28th — when relatives are gathered for Thanksgiving Day — set the living-room computer to display NASA's Google+ Hangout: Comet ISON Live. The hangout will be on for 2½ hours right around perihelion, from 1:00 to 3:30 p.m. EST (18:00–20:30 UT). The organizers are billing it as "Fire vs. ISON: Watch the Epic Battle Live." Beats football, say I.
November 23: Visually gone; photographic splendor. I can't detect anything of ISON any more with 10×50 binoculars. It's too low in the bright dawn, at least seen from here at sea level at 42° north latitude (farther south would be better).
So imagine my amazement at the image below, taken by Michael Jäger in the Austria Alps on what must have been the very clear morning of Friday the 22nd. I saw nothing just a few hours later. This is a testament to the power of careful electronic imaging and processing to pull things out that the eye can't see.
November 22: Still 4th magnitude, according to an estimate this morning by imagers Jean-François Soulier and Alain Maury. Here's their collection of images, the most recent on top. (The date of each is in the bottom data lines below the black-and-whites.)
The comet's diminishing altitude in the brightening dawn is shutting out more observers every day. But images taken by Sun-observing craft in space should start coming in soon. Read what's expected in Karl Battams' November 20 update at NASA's Comet ISON Observing Campaign site.
November 21: Tail disconnection. Astro-imagers around the world continue their work even as the comet moves lower in the moonlit early dawn. This morning several recorded a tail disconnection event — a blob of the ion (gas) tail being pinched off by a gust of the magnetic-field-bearing solar wind, and blowing downstream.
Denis Buczynski of the British Astronomical Association put together these photo links in time order, showing the feature moving down the tail. (The last one shows the feature best). All times are on November 21st:
- 2:49 UT: Feature not seen. Gerald Rehmann image.
- 6:06 UT: Feature seen Juanjo image.
- 6:28 UT: Feature seen. David Storey image.
- 12:23 UT: Feature seen. Joseph Brimacombe image.
November 20: Brightening fast. Even as ISON drops lower every morning at the beginning of dawn, it's now brightening fast. This morning it was immediately obvious in my 10×50 binoculars through the moonlight and suburban light pollution: a fuzzy "star" with a hint of tail, bright greenish white and showing a tiny, starlike pseudonucleus. The comet head was much brighter than 5.5-magnitude 86 Virginis 3° above it.
Vitali Nevski, ISON's co-discoverer, estimated it at magnitude 3.7 this morning in Russia. John Bortle in New York State calls it 4.5 but was nevertheless struck by its new brilliance. He writes:
I was amazed at its intensity as seen with my 15x70s. This morning C/ISON reminded me very much of the way Ikeya-Seki looked during one of my last pre-perihelion sightings in early October of '65.
Once clear of the trees, with twilight already having begun, it was surprisingly bright with an intensely condensed coma roughly 2.5′ in diameter. Total magnitude after accounting for atmospheric extinction was 4.8 [later revised to 4.5], although in my opinion the available comparison stars were less than first class, most for lack of choice being of spectral class K. The comet could be easily followed in binoculars long into twilight... a good sign for things to come! No tail clearly evident against the twilight sky.
Don't wait. ISON is rapidly moving lower every day.
Meanwhile, Comet Lovejoy 2013 R1, is very high before dawn behind the hind legs of Ursa Major. It was also instantly obvious to me through the moonlight in the 10×50s. It looked very different: much bigger; dimmer and more diffuse overall; with definite signs of a big, dim tail trailing a degree or so upward from the big head.
November 19: Another outburst returns ISON to 5th magnitude. After fading a bit from its outburst on November 13–14, Comet ISON was in the midst of another flareup of gas and dust production early this morning (November 19.4 UT), reports Emmanuel Jehin at the European Southern Observatory's TRAPPIST (TRAnsiting Planets and PlanetesImals Small Telescope) project. This would be ISON's third such event, counting one on November 1st.
Various other observers are reporting that the comet's head is about magnitude 5.0. It's moving lower at the beginning of dawn each day, and waning moonlight will continue to light the pre-dawn sky.
But don't be daunted by moonlight or even big-city light pollution! S&T's Tony Flanders spotted both ISON and Lovejoy this morning from a city park in Cambridge, Mass., looking through the bright skyglow over neighboring Boston with 10×30 binoculars and a 70-mm refractor. He writes,
This is the first time I’ve definitively spotted ISON in binoculars: a very faint star barely visible in preposterously bright skyglow, slightly fuzzy if you look carefully, and perhaps just a hint of a tail.
Through the 70-mm refractor at 60×: Pretty nice, with a well-defined 2′ or 3′ coma with a brighter pseudonucleus and a faint, medium-narrow tail about 20′ long.
ISON was right next to the 5.5-magnitude star 86 Virginis and indisputably brighter.
Comet Lovejoy was vastly easier to see despite probably being fainter [others are calling Lovejoy magnitude 6.0], because it’s now very high in the sky. It was instantly obvious in binoculars. It had a much bigger coma with a broad, diffuse, 20′ tail.
Meanwhile, astronomers in Germany report on their analysis of the U-shaped hood that ISON was displaying three days ago, as noted in the November 16th entry below. They say the wide "wings" forming the U might indicate that one or more large fragments broke off the comet's nucleus a few days ago. Such wings often result from the interaction of gas blowing from two separate chunks. Such an event could account for the comet's brightness surge around November 13. See press release for details and image.
November 18: An evening-sky possibility? When Comet ISON (or its remains) emerges from the glare of the Sun after its November 28th solar flyby, the best time to look for it will be at the beginning of dawn, low in the east-southeast, as we've been emphasizing.
But if the comet exceeds expectations, you might also be able to get a poorer glimpse of it low in the west at dusk in December. The farther north you live (Canada, northern Europe) the better your prospects for this. But the comet will be lower at dusk than at dawn in December, and its tail will be lying nearly flat along the west-northwest horizon rather than pointing up high.
This applies to observers at north temperate latitudes. In the tropics and Southern Hemisphere, the comet after perihelion will be visible poorly or not at all.
November 17: Long tail. Some observers are reporting that ISON's head was a little dimmer this morning following the last few days of outburst (maybe just due to moonlight returning?), but what a tail! Imagers who were able to extract the tail from the moonlit sky found a thin blue gas ribbon extending 7° or longer, as in the wide-field view above by Michael Jäger. Click the image for full-size view.
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