Comets can spend billions of years out in the Oort Cloud, and then a few brief moments of terror orbiting the Sun. These are the sun grazers. Some survive their journey, and flare up to become the brightest comets in history. Others won’t survive their first, and only encounter with the Sun.
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Dr. Pamela Gay: This episode of Astronomy Cast is brought to you by Swinburne Astronomy Online, the world’s longest running online astronomy degree program. Visit astronomy.swin.edu.au for more information.
Fraser Cain: Astronomy Cast episode 324: Sun Grazers. Welcome to Astronomy Cast, our weekly facts-based journey through the cosmos where we help you understand not only what we know, but how we know what we know. My name is Fraser Cain. I’m the publisher of Universe today. And with me is Dr. Pamela Gay, a professor at Southern Illinois University, Edwardsville, and the director of Cosmo Quest. Hey Pamela, how are you doing?
Dr. Pamela Gay: I’m doing well, how are you doing, Fraser?
Fraser Cain: Doing really well. And, thanks to everybody who went onto iTunes last week and put nice reviews for us. Really appreciate it. That’s a way that new listeners can find out more information about what we do and get us incidents of podcasts worth subscribing to and listening to and getting involved in all of the other activities that we’re doing. So, we really appreciate it. So if you haven’t already, go to iTunes and write us a review.
Dr. Pamela Gay: And don’t forget, the other way that people find us is through social media. So, don’t be afraid to share out an episode on Facebook, Twitter, LinkedIn. Heck, you could probably even find a way to Pinterest us with screen capture, although that might be a bit strange. But, we’re just here trying to get science out to as many people as possible, and we love your help.
Fraser Cain: Pinterest is actually great because I put all these astro photos on there. I really like it.
Dr. Pamela Gay: I like it, it’s just sharing Astronomy Cast on it because we’re an audio show is a bit strange. But, go for it. We’d love to see how you do it creatively.
Fraser Cain: Yeah, no, it works just great. No, totally do it. Share it. Share the video. It works great. Okay, great, so let’s get moving then.
Dr. Pamela Gay: This episode of Astronomy Cast is brought to you by 8th Light Inc. 8th Light is an agile software development company. They craft beautiful applications that are durable and reliable. 8th Light provides disciplined software leadership on demand, and shares its expertise to make your project better. For more information, visit them online at www.8thlight.com. Just remember, that’s www.8thlight.com . Drop them a note. 8th Light; software is their craft.
Fraser Cain: So, comets can spend billions of years out in the Oort cloud, and then a few brief moments of terror orbiting the sun. These are the sun grazers. Some survive their journey and flare up to become the brightest comets in history. Others won’t survive their first and only encounter with the sun.
And, we are recording this episode two days after comet Ison went kablooey, and I guess that’s why you picked the topic today.
Dr. Pamela Gay: It’s true. I think it’s actually been four days. It went through on Thursday, and so here we are on Monday and it was only today that it was finally pronounced dead on perihelion. It took a while to sort out what happened. And, what’s kind of amazing is how our ability to, well, follow the progress of these sun grazers as they orbit has changed over the history of knowing that they’re out there.
Comet Ison is perhaps now one of the modern era’s most famous sun grazers, but it’s certainly not the most famous over history. What’s kind of awesome is there’s this long history of amazingly bright sun grazer after amazingly bright sun grazer popping up through history, and many of them have been bright enough to see during daylight, where you can actually see, essentially, a tail emanating away from the sun across the daylight sky.
Fraser Cain: Yeah. And I think, well with Ison, it was really surprising across the whole process, from when it was discovered, to it didn’t brighten up, and then it flared up, and then when it finally did make its sort of turn around the sun, it disappeared, but then there was this puff ball that came out the other side.
So, it was funny how even NASA were going back and forth like, “It’s dead. No, it’s alive. No, it’s dead. Okay, it’s dead.” So, you can see that a lot of things about the behavior of these sun grazer comets that was quite surprising even to the people who know all about these things, making the predictions and even what happened afterwards.
Dr. Pamela Gay: And, part of this was because the majority of the sun grazers that we see all belong to one family, the Kreutz sungrazers. And, these are all comets that descended from one comet that probably met a very ill-fate several orbits ago.
So, at some point in the past, likely hundreds and hundreds of years ago, there was a great comet that, as it went around the sun, fragmented into many pieces, and periodically those pieces – it was probably two giant pieces originally. Those pieces have continued to fragment and continued to fragment, and have led to the great comet of 1843, the eclipse comet of 1882, the great comet of 1882, and then they started cropping back up in ’45 through the mid ‘60s. And now, they’re cropping up again just in time for SOHO to start watching them, well into the mid-‘90s forward.
So, we’re used to seeing sun grazers that all belong to one family of objects. Comet Ison was different. It was originally thought it might have been related to the great comet of 1680, but the more we looked at it, the more we realized, no, this is actually a first timer. This is a raw piece of ice that is on its first journey past the sun, and we’re just not used to seeing that. And, clearly we were not able to predict how it would behave.
Fraser Cain: Right. So, I think that’s sort of part of the surprise here, is that so many of these sun grazers source back to one object. That, can you just imagine what that must have looked like after it went past the sun?
Dr. Pamela Gay: Well, yeah. It’s hard to imagine, specifically when we start looking at what its fragments were able to do. The great comet of 1843, its tail was two degrees wide. Its length was 45 degrees across the sky. So when you looked at it – so, the moon is half a degree. So, its tail was four moons wide on the sky.
Fraser Cain: And, it was like halfway from the horizon up to the very top of the sky.
Dr. Pamela Gay: Right. And, it was visible during the day, which meant that you could actually see it for a while as this tail growing away from the sun. So, that’s really kind of an amazing thing to think about, and this was one of the fragments of the original comet.
So, yeah. Whatever it was initially must have been something absolutely amazing. It’s thought that maybe it was the great comet of 1106, but it’s just not quite clear. But, the fact that the great comet of 1106 occurred in 1106 and there’s records of it – because, there’s not a lot of astronomical records from that era. But, on February 2nd, all across the world, people began to notice that particular comet. And, well, hopefully we’re starting to understand how that one fragmented, the orbits decayed, the fragments spread out, leading to more and more of these sun grazers that we’re now seeing today.
Fraser Cain: So, what do scientists define as a sun grazer?
Dr. Pamela Gay: It’s simply a comet that gets well within the Mercury sun distance, and often it gets closer to the sun than the moon is to the Earth. And this isn’t to the center of the sun. That would be inside the sun. But, the distance between the surface of the sun and the surface of the comet is often less than the Earth moon distance.
Fraser Cain: Right, and that’s like 350,000 kilometers, and the sun is 1.4 million kilometers across. So, it’s close.
Dr. Pamela Gay: Yes.
Fraser Cain: And so, you can kind of – and so what happens to these poor comets? What are the forces, the stresses, what’s going on to them?
Dr. Pamela Gay: Well, so it starts experiencing three different factors. On one hand, you do have the tidal forces. This is the difference in the gravitational pull on the near side and the far side of the comet that is working to pull it apart, just shred it. These comets are held together with both gravitational force and chemical properties, and so they’re trying – the tidal forces are working to overcome both of those forces. The tidal forces don’t have very long because these comets are often traveling as fast as 500 kilometers per second. But, that can be a long enough time to break a comet up that way.
The other thing that it’s dealing with is the sheer heat. The radiation of the sun is transferring energy, which is raising the temperature at the surface of the comet, and, well, comets are made primarily of frozen stuff. Frozen oxygen, frozen water, frozen carbon dioxide, frozen nitrogen. All these different frozen things. Ammonia, that’s where the nitrogen is hiding.
All these different frozen things, when they heat up, they make the transition from solid to gas. And, so this is where you can see jets sticking out the sides of comets as they fly. Those jets are actually really icy particles, are sublimating away, and any dust that was trapped in the ice is then acting to reflect the sunlight back at the observer.
And then the third thing these things are dealing with is they’re getting beat with the solar winds. And, there’s actually this constant flux of particles coming off the sun, and that’s also beating. Although, that’s by far probably the least effect that they’re dealing with. All of those things are also beating on the surface of the comet.
So, this causes the volatiles to react, it causes organics to form on the surface, usually at greater distances, and depending on the structure of the comet, if it’s a lumpy comet that’s only kind of held together with gravity and actually basically a rubble pile, just the jets from the volatile sublimating can blow the thing apart. If it’s a more solid, iceberg like comet, it may have a better chance of surviving, of glazing over with organics and protecting itself a little bit better.
But, it’s certainly a harsh ride, no matter how the comet is formed.
Fraser Cain: Yeah, I mean, when we think about comets versus asteroids, we think of asteroids as balls of metal or balls of rock. They would handle that kind of a pass, depending on if they – they might get torn apart depending on their piles of rubble or whatever. But, they might just reform again. They’ll come back together.
But in this case, you’ve got this terrible tidal force is tearing apart, and then this enormous stream of material that’s trying to radiate it away. And, so they just can’t pull themselves back together. They can’t handle it.
Dr. Pamela Gay: Well, and you have the gas expanding. It’s – imagine the comet is filled with a bunch of exploding balloons, and that’s pretty much what’s happening.
Fraser Cain: Again, just imagine what it’d be like just to be standing on the surface of the comet and seeing this stuff happening, all of these vents opening up and streams coming out, and things bubbling and boiling away. Amazing.
Dr. Pamela Gay: This is actually something that you can simulate. And, if you go over to Cosmo Quest, we actually have the recipe. The stuff that comets are made of are all things that we can find here on Earth. And, in fact, most of us can find most of the ingredients just by going down to the local grocery store, if your grocery store happens to have dry ice. Or, most towns have an ice store that has dry ice. And, you can mix together corn syrup, ammonia, water, dry ice, and pack it together, add in some dirt from the yard, and set it in front of a light and a fan and watch what happens and essentially watch the evolution of a comet sitting on your desktop.
Fraser Cain: Yeah, our good friend Nicole Galucci did this experiment, made a comet, during our Ison special, and it was great. It just worked out perfectly. She brought all these ingredients together, and packed up the big snowball. I wouldn’t want to get hit by it. But yeah, it’s really cool. So, if you can dig that up, look at our Ison special that we did a couple weeks back in.
Dr. Pamela Gay: The links are on cosmoquest.org/blog. Just check out or blog and find all the stuff that we’ve been doing so that you can recreate your destroyed comet.
Fraser Cain: Now, one of the things that I think people are quite surprised at is that the SOHO mission is actually the most prolific comet discoverer out there. And, it’s fining comets moments before they’re destroyed, which I think is kind of hilarious.
Dr. Pamela Gay: So, the problem that we normally deal with is the sun is kind of a bright thing, and these comets are often tiny, meters to dozens of meters across, in some cases. And, as they get close to the sun, they grow a tail, they become highly reflective, and that tail is spread out over a great deal of space, and that’s why we’re able to see them.
But, all of this is happening within a few degrees of the sun on the sky, and we’re not gonna see them with our eye. And, one of the crazy things is there’s actually – one of the 1882 comets was only discovered because it was seen during a solar eclipse in Egypt.
Well, we don’t have to wait for solar eclipses to see the region around the sun when we use SOHO. It has a corona graft. And that plate that’s blocking the sun’s light allows us to see fainter objects that are down near its surface. So, we’re able to catch all of these icy fragments as they make their suicide journey around the sun.
So, for the first time, we’re actually able to catch fragment after fragment of these Kreutz sungrazers and other sungrazers that come from the outer parts of the solar system.
Fraser Cain: So, if a thing just crashes into the sun, is that a sungrazer?
Dr. Pamela Gay: Yes. It’s one that kind of failed to graze, but yes, it’s a sungrazer.
Fraser Cain: So, how many of these sungrazers are there? How many has SOHO found?
Dr. Pamela Gay: Well, since it launched in 1995, it’s found over 2,400. So, there’s a lot. And, they aren’t all part of the Kreutz sungrazer family, but many of them are. So, again, that starts to give you an idea of how a comet fragments.
And, these things can start as several kilometer across cores. Ison was estimated to be, by some estimates, two kilometers across. And so you start with something that’s several kilometers across and just progressively break it up over time until you have chunks that are meters across. And, the forces that fragment the comet also cause slight differences in the velocity of each of the fragments, which causes them, over time, to spread out in their orbits. And, that’s why we see them year after year.
And what’s kind of neat to think is it’s estimated that we’re due for a bigger chunk. So maybe we’re due for another great comet that would be visible during daylight.
Fraser Cain: We’re totally due. We’re absolutely due.
Dr. Pamela Gay: You were so certain it was going to be Ison.
Fraser Cain: I was so certain. I was so ready to enjoy this comet, even though I’d have to get up early in the morning to go watch it. I was ready. Because, I mean, Hyakutake and Hale-Bopp were so great.
Dr. Pamela Gay: Well, they had the advantage. They were older. This means that they had organics built up on their surfaces which kind of forms a sludgy crust. A lot of these Kreutz comets, they’re also older fragments. And when you have an older fragment that’s pretty solid, survived several times, it’s had time for more of its volatiles on its surface to go away. It builds up that crust. That’s just a little bit more stable.
Now what’s interesting, though, is Hale-Bopp may become the next Kreutz. It may someday, there’s a 15 percent probability that over the eons, orbital perturbations are going to cause it to get closer and closer to the sun at perihelion. It’ll probably fragment. It’s actually thought that maybe it’s already a fragment of a past great comet. So, we could end up with a whole new family of sungrazers someday in the future.
Fraser Cain: So, where do these objects come from in the first place? I mean, we talked about the Keurtz comet that are like some object that it’s the fragmented particles of some comet, and they’re just going around and around the sun. Where did that object originally come from?
Dr. Pamela Gay: Well, comets in general come either from the Oort Cloud, which is a not yet really detected sphere of icy bodies far out in the outer edges of our solar system, or they come from the Kuiper Belt, which starts out around the orbit of Neptune and extends out to we’re not quite sure, but somewhere in the ADAU range.
Comets in general, due to orbital perturbations, have a strong probability of becoming sungrazers. It’s unclear exactly how many are likely to become sungrazers, but any individual comet has a 10s of percent chance of, over time, should it not die on its early orbits, becoming a sungrazer.
And, comets can die any number of ways. They can dive into Jupiter, they can dive into Saturn. We’ve seen them dive into Saturn in the past. Comet Shoemaker-Levy 9 did that in, I believe, ’94. But, should they survive other encounters in the solar system, over time, their perihelion distance, their closest approach to the sun, is going to get moved closer and closer in on the sun. And, eventually they graze the sun and eventually they die.
Fraser Cain: Right, right. And so, you get the situation where originally it’s perfectly fine, some perturbation hits it, it ends up on one of these orbits, crosses the sun, and either like Ison, that’s that, it was a nice ride and then it’s dead, or it survives and then can break apart and come back around multiple times.
But eventually, once it starts that sun grazing process, it’s a goner, right?
Dr. Pamela Gay: Yeah, it’s thought that maybe all the comets that come within two AU of the sun, so come within twice the Earth distance, have a 15 percent or greater chance of becoming sungrazers. So, it just depends on what’s the fate, and we need to see more of these to really understand what’s going on, and we need more data. It’s always a matter of more data.
So, hopefully we’ll get more chances like comet Ison to watch fresh bodies come in and die brutal deaths.
Fraser Cain: Now, we had a great sort of view, this time with Ison because there’s a bunch of spacecraft out. It’s not just SOHO. There’s SDO. There’s Hinode. So, all of these have different sort of fields of view on the sun. And, it was great to watch the comet come into one field of view, and then into the next one. So, do you think our detections are going to go up?
Dr. Pamela Gay: So, the way we were able to observe it was actually pretty interesting. Because, we had, as it went in from one angle, we followed it with SOHO. As it wrapped around the back, we were able to see it with Steria, which is currently aligned such that it can look behind the sun for us. SDO wasn’t actually able to see it as it got too close to the sun. But, this fleet of sun watching satellites, each of them have slightly different jobs, each of them have slightly different sets of detectors. And once we find a comet, each one of them can play a different role.
The reality is, SOHO’s field of view, with its corona graphs, its L2 and L3 systems, those are perfectly set up for finding comets. They offer a nice, wide field of view, and it’s that larger field of view with that blocked out sun that really makes them perfect for finding sungrazers.
Many of the other instruments – well, you see the entire disk of the sun. And, well, if you have your detector adjusted to not die a nasty death from sunlight, it’s not going to be able to see the fainter objects.
Fraser Cain: Yeah. And, so if you’re just a regular person and you want to participate in this, you can go to SOHO’s website, live website, and find these comets, right?
Dr. Pamela Gay: Right. So, all of the data from SOHO is live pretty much in real time. Data comes down, they process it, it goes on the web. It’s an automated process. And, you can download the images. And, there are individuals who have found 10s to over 100 of these different objects. And, they do it by looking at image after image after image, sometimes in automated ways, using software like Maxim DL or Apes or whatever it is that’s out there, they want to download to process images. And, they look for things that are moving in a set way, and they can calculate orbits and submit what they discover to the Minor Planet Center out in Boston.
Fraser Cain: And, what does the Minor Planet Center do?
Dr. Pamela Gay: Well they log, “Is this new?” They log, “Is this accurate?” And, if you have enough measurements on your orbital elements, it ends up named after you. That’s the way comets work.
Fraser Cain: Right. That’s the great part. And so, there aren’t many objects. You can’t name a star after yourself, and you can’t name a planet after yourself, or galaxy, and if you want to name an asteroid, it’s a bit of a fight. But, if you want to name a comet –
Dr. Pamela Gay: You just have to discover it.
Fraser Cain: You just have to discover it, yeah.
Dr. Pamela Gay: Which isn’t always easy. You’re out there competing against all the other amateurs doing it, and there’s a chance you’ll be the winner who gets the comet on its way to death. So, you get to name something in the process of dying after yourself.
Fraser Cain: That’s why there’s been multiple Comet Lovejoys because Terry Lovejoy in Australia is a machine at finding comets, and he just keeps getting them named after himself.
Dr. Pamela Gay: Yeah, and that’s why you have the entire nomenclature which is C for comet slash name of the human, and then the year. And then, there’s also usually a type. So you’ll have for instance C/2011W3 Lovejoy. W3 tells what type of comet it is, and then Lovejoy says who it was that found it.
Fraser Cain: And so as we stand right now, we’ve lost Ison. But, there’s actually four other comets that are visible in the morning sky with binoculars. So, we did a round up of it in our most recent weekly space hangout. So there’s still comets, you just need binoculars to see them. And there are more being discovered all the time.
Dr. Pamela Gay: And, you know, if you don’t keep looking up, eventually the universe is going to throw something at us to get our attention. So keep looking up, discover everything there is to discover, and a great pair of binoculars is all you need, along with an internet connection to keep track of things, so that you can go out and find all of these faint fuzzies of the night sky for yourself.
Fraser Cain: Very cool. Well, Ison disappointed, but I still am holding out hope that in the next couple of years we will have a great comet, something with one of those big, long tails that you can see without binoculars, just the unaided eye. I can’t wait.
Dr. Pamela Gay: We need another Hyakutake.
Fraser Cain: Or a Hale-Bopp. That’s the class. That’s what I’m waiting for. The universe owes me a comet.
Dr. Pamela Gay: The universe owes you nothing. It’s just trying to kill you.
Fraser Cain: Of course. With comets. Thank you very much, Pamela.
Thanks for listening to Astronomy Cast, a non-profit resource provided by Astro Sphere New Media Association, Fraser Cain, and Dr. Pamela Gay. You can find show notes and transcripts for every episode at astronomycast.com. You can email us at firstname.lastname@example.org . Tweet us @astronomycast. Like us on Facebook, or circle us on Google Plus.
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Duration: 28 minutes