The 2012 "SWAN-grazer": Comet Lovejoy version 2.0?
This page will be a somewhat live-blog of the unfolding events of Comet SWAN -- a newly discovered Kreutz Sungrazer that I suspect will turn out to be very impressive. This page is in reverse chronological order, with the most recent entries at the top of the page, and will be updated periodically as new data and information becomes available. Scroll to the bottom to read from the start.
March 16th, 2012
1500UT: Sadly, this is another obituary for another bright comet. I didn't expect this one to survive, and indeed it didn't, but it didn't disappoint either. But this comet didn't turn out nearly as spectacular as I expected.
I got very excited last week when it was first reported that a Kreutz-group comet had been spotted in the SOHO/SWAN instrument. We have seen 1,900-or-so Kreutz group comets with the SOHO/LASCO camera, and none of them -- including Comet Lovejoy -- have ever been visible in the ultraviolet images that the SWAN camera takes. To me, this hinted an object of significance, and my comet scientist colleagues were all equally worked up about it. But Comet SWAN (now officially known as C/2012 E2 (SWAN)) surprised us by being... unsurprising! We have no idea why it flared up in the SWAN images, but most likely it underwent a flare up (outburst) of some kind, and then settled itself down for the remainder of its journey into the Sun. I made sure we took lots of filtered (i.e. different wavelength) images of it as it travelled through the LASCO C3 and C2 cameras, so hopefully we will learn something when we analyze those.
But regardless of all this, Comet SWAN was still a very bright, very beautiful comet, particularly in the LASCO C2 images in the hours preceeding its toasty demise. Peak magnitude was probably around mag 1, which was much lower than I thought it would be, and it could be that it exhausted a lot of volatiles in that theorized outburst that alerted us to its existence. That's something we probably won't ever know.
I'll wrap up this mini-blog with a movie (in 1080p goodness!) of the final few hours of Comet SWAN's existence. There are a couple of things I want to point out here, to address questions I frequently get about this stuff. First, the comet does not ever hit the Sun. Perihelion distance (closest point to the Sun) for this object was about 350,000km above the solar surface (~0.0069 solar radii), though in reality I don't think this comet even existed in any appreciable form by the time it got to that point. Second, this comet did not cause the eruption you see in the upper-right of the movie a few hours after it disappears. I have covered this before, as have others such as the Bad Astronomer, and there's just no link that we know, no evidence of a link, and no physics we know of to support a link. We also have a simple statistical answer that is far, far more reasonable than speculative physics. And as if we needed it, the final nail in the coffin is that Comet Lovejoy, which was far bigger than this comet, and got much closer to the solar surface, caused the Sun absolutely no concern whatsoever and certainly didn't set off a CME.
A final comment: I still stand by my prediction that we will see another big Kreutz-comet object within the next few years. This one didn't turn out to be it, but we will certainly remain vigilant and you will definitely hear all about it here when if happens!
March 14th, 2012
1900UT: I'm being delinquent here. Lots of technical problems today. "Poor workman" and all that... Anyway, before we get any more updated images of Comet SWAN (very soon, as I type this) I wanted to make a comment on the "survivability prospects" of the comet. We saw with Lovejoy that it survived reasonably well, although there is a strong argument to be made that the nucleus of that comet actually disintegrated at or near perihelion, because ground observers were completely unable to find any central condensation in the center of the coma. But regardless, something survived and so now we have to ask: "Will Comet SWAN survive?". I touched on this in an earlier entry but despite lessons learned from Lovejoy, I don't think this comet will make it. We have seen SOHO-discovered Kreutz comet almost as big as this one that have certainly not made it, and this one is much closer in size to those objects than it is to Lovejoy. So the odds are stacked against it. Also, if it did have an outburst in the SWAN imager, then you could infer it's a pretty volatile thing -- more so than your typical Kreutz -- and maybe it will vaporize or fragment catastrophically at perihelion.
That said, we do not yet know the orbit of this thing. I have actually submitted some measurements now that should allow for a reasonably good orbit calculation, but it might be a while before they are published. The critical parameter will be perihelion distance -- the measure of how close it will get to the Sun. If this value is on the extreme upper edge of the typical Kreutz-comet orbit, then it could be as "far" as two solar radii (which equates to one solar radii from the Sun's surface, since perihelion distance is measured from the Sun center). In this case, it does have a better shot at making it. On the other hand, it could be extreme in the alternate direction and have perihelion distance a little less than one solar radius... Yes, that would mean it would literally hit the Sun!
Most likely, it will be the typical 1.2 - 1.4 solar radii that most of these comets have. And most likely it will not survive. New data should start to flow very soon, and I think that we're probably only a few hours (maybe 6?) from perihelion, so there's not too long to wait to find out!
1230UT: I'll kick off with a couple of movies today.
On the left are highly-processed images of Comet SWAN in the STEREO/SECCHI HI-1B images on March 11 and 12 (the dataset was incomplete at the time I made this movie). There are some very nice tail dynamics going on with the comet, but otherwise it appears to just be a very typical, though extremely bright, Kreutz-group comet. The movie on the right is a SOHO/LASCO C3 movie in which you can see Comet SWAN enter the images in the lower left, brightening rapidly as it heads towards the Sun. At around 1800UT in this movie, there's a powerful coronal mass ejection (CME) resulting from an m7.9 flare that occured. This event temporarily floods the image with high-energy particles (protons) that were accelerated away from the Sun by the powerful eruption. The CME was not directed towards the comet, so et won't affect it at all.
Still I'm scratching my head. Just 48hrs before those HI-1 images were taken, this comet was shining brightly in the UV images taken by the SOHO/SWAN instrument. We still do not know why this comet was visible in the SOHO/SWAN images when none of the other ~1,900 such objects we have seen, including Comet Lovejoy, were visible in that camera. The leading theory right know is that the comet had some kind of outburst that exposed fresh materials that the SWAN instrument is particularly sensitive to. That idea does make some sense, but I still don't understand why it looks no different to any other Kreutz-group comet we've seen other than being much brighter than usual.
I'll continue to monitor the comet, and will post updated images/movies when we get a new set of them. I might also try and upload a slightly higher-quality version of the above LASCO movie, too.
March 13th, 2012
1730UT: The theory was great, but it looks like this SWAN flies alone. I only see one comet, albeit a very nice one! And I thought it was rather fun to throw out a theory and test it "live". It is possible that there was an outburst or a fragmentation event of some kind several days ago, but any traces of evidence that may exist are not resolvable in the LASCO C3 images (or the STEREO HI-1B full-res images, which I just now got my hands on).
So what does that leave us with? Well, we still have one of the brightest Kreutz-group comets observed by SOHO in 17yrs, and we have a remaining mystery of just why this comet was clearly visible in images that require an extremely big, and very bright object. We can now sit back and watch Comet SWAN plunge (I refuse to say "SWAN dive"...) into the solar atmosphere where I am sure that it will evaporate completely!
I will wait now until we have a more significant data set, and I will process a movie and get that online. There is obviously something different about this comet -- we just don't know what it is (yet). So keep a close eye on it over the next couple of days as it races towards the Sun.
1530UT: So what does "SL9" mean? It means Comet Shoemaker-Levy, that spectacularly crashed into Jupiter back in 1994. Not only was it awesome to watch that happen, but even more amazing was that prior to reaching Jupiter, the comet fragmented into many pieces. My theory (shared with a couple of other scientists) is that Comet SWAN was until very recently a much larger comet that catastrophically fragmented several days ago. If true, such a fragmentation event would release a huge amount of cometary material, much like the Comet Holmes outburst I mentioned yesterday. It would also mean that instead of just one large comet giving off dust and ice, there could be a large cloud or swarm of much smaller comets, with their combined dust/ice output enough to show a signature in the SWAN images. For example, if we take one large comet and break it in half, for example, that gives us much larger surface area of comet exposed to the solar wind. Do that a few times and you have lots of fresh comet surface exposed.
Now, this is just a theory, but it would be a good explanation of what we are observing. And it's an easy one to test, because all we need to do is wait! In about half an hour from now, we should start to get near realtime LASCO C3 data. If this theory is correct, it should be quite apparent to us in the new images.
Continue to hang around to experience the ups and downs of real-time astronomical discovery!
Comet SWAN just peeking into LASCO C3.
1500UT: Well the good news is that someone owes me a beer. (See previous entry.) Comet SWAN has arrived and it is bright, but the bad news is that it's no rival for Comet Lovejoy.
Honestly, Lovejoy has kind of spoiled things for us. We had gotten so used to seeing tiny sungrazers, that if one like Comet SWAN were to turn up, we would be all kinds of excited because -- don't get me wrong here -- this comet really is a bright object. It entered the field of view at ~0800UT with a brightness of probably mag 6.5 (I'd guessed 5.5...), so not quite as bright as I expected but if we forget for a moment that Lovejoy existed, then this is a very bright comet. It will probably peak around mag -1, making this one of the brightest Kreutz-group comets ever observed by SOHO. But it's no Comet Lovejoy.
Here's what's interesting to me about this comet, and we can't lose sight of this fact: until now, we had never seen a Kreutz-group comet in SOHO/SWAN data. That included Comet Lovejoy. Something fundamentally different happened with this comet that we have not seen in ~1,900 similar objects over the past 17-years. SWAN has a huge field of view and produces tiny images from that, so for something to show up in that data it needs to be both big and bright, and have a particularly strong presence of hydrogen (or Lyman-alpha, to be exact). This comet does not appear to be huge, is not extraordinarily bright, and doesn't appear to have an extended gassy/dust coma or any other feature that would lead to such a strong signal in the SOHO/SWAN data.
This is truly puzzling! I said yesterday that having something dispel all of your scientific preconceptions is an amazing feeling for a scientist. Well this is a very similar thing. I don't think any of us were expecting the comet to be relatively small like this, with some scientist I spoke to speculating on a possible 1km+ nucleus (absolutely not the case here!), or even that Lovejoy was just the "small precursor" to this one (again, clearly not the case). We are going to have to think long and hard about this.
I do have a theory though (it was independently suggested by a fellow comet researcher too, so I can't claim all the credit for the idea)... It will be a fun exercise to test this out in real-time, so let's do that. This is real-time science: Observe ->> Postulate ->> Test! We get more data in about an hour from now, so that gives me just enough time to write a short post about this theory and then we can wait and see if it's correct. If you want to play along, I'll give you hint that you can research now: SL9. Check back soon...
Comet SWAN in the very low resolution STEREO HI-1B images (click for a larger version).
1200UT: What the...?? Where's the comet?!
Valid questions, and I have answers! You'll notice that there's a significant lack of 'pretty pictures' of this comet thus far. All we have is that one animation from SWAN that I posted yesterday. This is not only a problem for trying to present the comet to you all, it's a problem for doing even a basic scientific analysis to allow us to get an orbit for the comet. All we have right now are a handful of those SWAN images, and some ultra low-resolution "space weather beacon" images from the STEREO-B satellite. I've put an example of one of these images opposite. The comet is there... but it's hard to see. The beacon images are a highly compressed data product that get sent down from STEREO in near real-time. They're no good for comet hunting or star watching but they are good enough to show up large coronal mass ejections (CMEs), and that's what STEREO was designed to study. It will be another day before we get high-resolution versions of those images because it takes a long time to pull all the data down from STEREO (it's a long way from Earth).
But that's getting away from the question at hand which is "why isn't the comet in LASCO yet?". Well, given that we only have a few poor quality observations of the comet, it is really hard to get any useful kind of orbit for the comet, and hence the timing of it is tricky. Yesterday, I saw a bright blob at the very edge of the
12:18UT LASCO C3 image, right by the occulter pylon (the arm that holds the solid occulting disk in the center of the telescope) at the lower-left corner of the data. I stuck my neck out and said that I thought that blob might be the head of the comet. For neither the first or last time, I was wrong, and it was just a cosmic ray (energetic particle) hit on the detector. We've had a
lot of those recently so in hindsight it was dumb of me to jump to conclusions.
If I had seen the STEREO HI-1B images (e.g. the one opposite) sooner then I would not have jumped to that conclusion. Given where the comet is in those images I would say it was still 16-20hrs from reaching the LASCO C3 data. So by my estimation, we should see it in LASCO C3 by about 1100UT today. We do not have a spacecraft contact today until 15:45UT, which should give us processed data by ~16:15UT. I will be very surprised if the comet is not in C3 by then, and I'll be scratching my head if it's not.
There is one caveat that I want to warn about though. The current orbital track of the Kreutz-group comets in LASCO C3 brings them into the field of view in the lower-left corner, very close to the occulter arm in LASCO C3, which they will then more or less follow all the way into the Sun. Depending on the exact orbit of this comet, which we don't know, it is possible that at least some of its passage through the images will be completely blocked by the arm. If we're particularly unlucky, it will actually enter the image behind that arm, and won't pop out from behind it until late in its passage. There's nothing we can do about that unfortunately; SOHO has antenna issues that mean we can't safely roll the spacecraft a bit to move the arm.
So hang in there. A bright comet is coming and I'll share it as soon as I see it! I will then be able to give a better idea of survival chances, peak brightness, and size comparison relative to Comet Lovejoy (though I did already bet a beer with a well-known comet scientist that this will not be as big as Lovejoy...).
March 12th, 2012
1700UT: I'll start with timing. When I said that we only had a couple of days, I meant until perihelion (closest point to the Sun), not until appearance. I knew this morning that I had maybe a few hours before it arrived in the data. We do not currently have spacecraft contact with SOHO -- that will pick up again at 21:40UT today. By the time we get our first
LASCO images back this evening, Comet SWAN should be bright and obvious, though it might be somewhat masked behind the occulter arm pylon (the radial thing in the lower-left corner). It will be very late tomorrow (March 13th), or more likely the 14th that Lovejoy will reach LASCO C2 and then the extreme ultraviolet (EUV) imagers of STEREO and SDO.
Now I've got time for a quick Q&A to respond in advance to the typical questions I will get about this comet:
How bright will Comet SWAN get?
I'll be hugely speculative here and say that it won't quite get as bright as Lovejoy did, which was about magnitude -4, but I would not be surprised to see it get to at least magnitude -2. I think it will be the second brightest Kreutz-group comet seen since the 1970s.
How close will it get to the Sun?
We don't have an orbit for the comet yet because we have had very few useful observations. However, we do know that it's a Kreutz-group comet, and so its perihelion distance (closest point to the Sun) will be between one and two solar radii. In other words, it could literally skim the Sun's surface, or be close to that.
Is the comet related to Comet Lovejoy?
Yes, it's a member of the Kreutz group of Sungrazing comets. We have seen around 1,900 of them now, and they all originate back to a parent object that would have broken apart close to the Sun at least a couple of thousand years ago.
Isn't it rather a coincidence that we get two big sungrazers in 3-months?
Very much so, and this ties in to the previous answer somewhat. Large Kreutz comets tend to be "clumpy", and by that I mean when we get one really big one, there tends to be another turn up within a few months or years of it. We saw this in the late 1800, in the 1960's/1970's, and now. I would say that prior to this revolution around the Sun (on a ~900yr orbit), Comet Lovejoy and Comet SWAN were part of the same chunk. ("Comet SWANjoy"??)
Will Comet SWAN survive passing by the Sun?
Ugh. I hate this question. I rather famously commented that I did not think Comet Lovejoy could survive perihelion, and clearly it did. I was happy/delighted to admit I was wrong about that, but it does now make me very hesitant to comment on the fate of this comet, particularly as we still do not know how close to the Sun it will get. For now I'm going to tentatively say "probably not", but I'll be willing to make a slightly less tentative response tomorrow morning.
Will we be able to see Comet SWAN from the ground?
That depends on the previous question. If it does survive, for a few days at least it will likely be quite impressive, as Lovejoy was. But there are a lot of "if's" and "maybe's" here. Comet Lovejoy really threw a lot of my preconceptions out of the window (which as a scientist, I can tell you is a wonderful feeling!), so now I'm far less sure than I would have been before.
I'll shoot for another update later on today when we see it in LASCO C3.
Comet SWAN in the SOHO SWAN images (click for an animated version). I have annotated the image over on the right-hand side to point out the comet. It's not much to look at but is a "first", nonetheless.
1500UT: I'll start at the beginning, which was all the way back on March 8, 2012... Ukrainian Amateur astronomer, citizen scientist, and SOHO Comet Hunter Vladimir Bezugly
reported a bright blob on images taken by the SWAN instrument on SOHO. He noted that he couldn't correlate it to any known bright comet in that part of the sky, and that it appeared to be on the track you would expect to find a Kreutz-group comet. He only had two images to go on, though, so his claims were tentative. However, as the SWAN images flew in at a rate of one-per-day, it became apparent that the comet was real, and was probably a Kreutz-group object with a solar rendezvous within the next few days. Word was sent out to the comet community, and observatories were encouraged to try and locate the comet. Sadly there were no reports of success, and that left us head-scratching a little. Most likely, with such a short lead-time on this, the comet is already far too close to the Sun to be viewable from Earth.
"So why are we getting excited here?", you might think. "SOHO has discovered a Kreutz-group comet. Big deal. Add it to the list of 2200+ discoveries it has made." That's all very true, but this one really is different. This one was seen and discovered in the SOHO SWAN instrument. In 17yrs of operation, after ~1900 Kreutz-group discoveries, this is the first Kreutz-group comet to ever be seen in SOHO/SWAN... including Comet Lovejoy! Yes, that's right, this comet has done something that Lovejoy did not -- it has shone brightly enough for the SWAN instrument to see it, and that makes me sit up and think twice about it.
We have two possibilities here: either this comet is bigger and brighter than Lovejoy, and every other Kreutz-comet we have ever seen, or for some reason it has a very strong emission of a type that SWAN is sensitive to. Regardless of which it is, this comet is different, and that makes it very, very interesting to us.
So I should say a little something about the SWAN instrument and its images. First, SWAN is an "all-sky" imager, which means it has a 360-degree view around it. There are only two areas it can't look at -- the Sun and the Earth -- and so those are the blacked out parts of the image/animation opposite. SWAN does not take images like an ordinary camera either. The
full explanation gets a little technical, but basically SWAN is looking at the imprint of the solar wind (the Sun's constant outward flow) upon hydrogen in our solar system, and it sees what are called solar "Lyman-alpha" photons, which is basically light that is reflected at a certain wavelenth in the ultraviolet part of the spectrum. What this means for us and comets is that a comet needs to have a pretty hefty amount of hydrogen in order for us to see it (as we see the solar wind "reacting" to this hydrogen). Bigger comets tend to have enough of a gassy coma that we can see them in SWAN images, but Kreutz-comets are usually far too small for that. So why are we seeing this one? Remember, not even Comet Lovejoy showed up in SWAN data. Right now all we can do is speculate, but fortunately that's a particular strength of mine so here goes!
Speculative theory #1: this comet is bigger and gassier than Lovejoy. Well if that's true then we are in for a crazy show, and a comet that would almost certainly be visible in the midday sky within the next couple of days. That seems a tad farfetched to me though, so I'd rather go with...
Speculative theory #2: an outburst. Comets do this occasionally -- a chunk of particularly feisty volatiles will get exposed to the solar wind and it'll blow up in a big icy cloud around the comet. A recent famous example was Comet Holmes back in 2007, which erupted into a truly enormous cloud. So if Comet SWAN has undergone a similar (though likely less dramatic) outburst, it could have formed a hydrogen-rich coma around itself.
Those are my two leading theories right now. By the end of today we should have a much better idea, as this comet's appearance in the SOHO/LASCO C3 images is imminent. At that point I will be better positioned to comment on this comet relative to Comet Lovejoy. My inkling (more speculation...) is that it is not going to be quite as bright as Lovejoy, but will be a very, very bright object.
Coming up.... STEREO observations, timing, and SDO potential...
1300UT: I promised myself I wouldn't do this again, and I didn't think I would need to any time soon. Yet once more, here I am, live-blogging a Sungrazing comet. So, without further ado, I'll take a deep breath, straighten my tie (OK, my sweater...) and make my grand opening statement: Welcome to the beginning of the end for Comet SWAN!
I said a very similar thing not too long ago, and we all know how that turned out. I'm referring of course to the spectacular Comet Lovejoy that thrilled and delighted us last December, and only recently faded away to the point that it's no longerbarely visible from Earth. For two weeks I chronicled the Comet Lovejoy saga, providing latest information, updates, movies, images, fact and opinion. (You can read that story here [part 1] and here [part 2].) At the very end of the ordeal I made a prediction, based very carefully on scientifically precise and accurate quantities of speculation and guesswork, that "we will see another large Kreutz Sungrazing object, much like Comet Lovejoy, within the next five years". I was intentionally vague in that statement, and gave myself a 5-yr window in which I'd either be correct and smug, or be wrong and just not say anything. What I did not anticipate is that within just three-months, my smugness potential would be so high! Flippancy aside, I'll now give a brief summary of the situation we're looking at, but with some words of caution. After that I'll get into more detail in a later post.
As I type this, a very bright Kreutz-group "Sungrazing" comet is heading towards the Sun. How bright, we are not sure; how big, we are not sure. I do not think that this is another Comet Lovejoy, but I do think that it will be a very impressive object, and quite possibly one of the brightest Sungrazing comets that we have seen over the past couple of decades. Its informal name is Comet SWAN, named after the Solar Wind ANisotropies instrument on the ESA/NASA SOHO satellite in which the comet was first detected. It does not have a formal name yet because we haven't been able to get a good enough view of it in order to calculate an orbit. All we know is that it is a Kreutz-group comet, it is a very bright one, and it will reach the Sun's outer atmosphere within the next couple of days.
I will start compiling more information and images now, and will make a more detailed post a little later on. We only have a couple of days advance notice for this comet, so my blogging won't have the same intensity and sense of anticipation as the last one, but it will hopefully be fun nonetheless. For intermediate updates you can follow the @SungrazerComets Twitter feed.
Credits: All data presented/offered here is free for public use, so you can take it and use it. We ask that for STEREO/SECCHI images you credit "STEREO/SECCHI image courtesy NASA/NRL", and for SOHO/LASCO images you credit "SOHO/LASCO image courtesy NASA/ESA/NRL", or something along those lines. Email firstname.lastname@example.org if you're not sure.
Karl Battams, NRL (March 2012)
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