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Comet McNaught in 2007
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Comet Machholz in LASCO C2 in 2002
Family ties: Meet the Machholz's -- possibly the most complex family in the solar system!Since its launch in 1995, the ESA/NASA SOHO satellite has seen over 2,300 previously unknown comets, most of which belong to the well-populated "Kreutz" comet family. You may have seen this term used a few times before but what do we mean when we say "family" in the context of comets? Well, we mean that these comets all share a common orbital path, and/or at one time (most likely millenia ago) were part of a much larger, single "parent" comet that broke up into smaller chunks. As time progressed, these chunks also broke up and the net result over very long timescales is a huge trail of comet crumbs stretching far out into space.
Kreutz-group comets have been pretty high-profile lately, particularly with the spectacular Comet Lovejoy in December and the release of our Science paper following the first ever direct observations of a comet completely evaporating in the solar atmosphere. This is great stuff, and clearly the Kreutz-group contains a lot of fascinating objects and amazing things for us to learn, but it is not the only comet group we know of. There's another population that is perhaps not so well understood, but personally I think is even more interesting than the Kreutz group...
Comet 96P/MachholzBefore I get into the meat of this article, I am going to introduce the central character to this story: Comet Machholz. Discovered in 1986, Comet 96P/Machholz is a fascinating comet that has passed through the SOHO/LASCO C3 images three times now, and is approaching its fourth apparition this year. It's a short-period comet, completing an orbit around the Sun every 5.24-years. It was also seen by our SECCHI HI1A instrument on the NASA STEREO satellite in 2007, and will also be seen again by instruments on STEREO when it reaches perihelion (closest point to the Sun) on July 14, 2012. It's not a huge comet but it is very photogenic, and puts on quite a display with its beautiful dusty tail. But Comet Machholz is also a bit of an odd-ball.
Back in 2007, astronomers at Lowell Observatory (Arizona) observed it and found that it contains significantly lower amounts of certain compounds (mainly carbon-based) than any other comets that have been studied in a similar way. This led some to ponder on whether it originates from within our solar system, or is actually an exile from another solar system that got flung far out into space and subsequently gravitationally trapped by our Sun. Either way it's a different kind of comet for sure but, per the title of this article, what have comet families got to do with it? Well, it turns out that comet Machholz has quite the extended family...
Image source: Chodas & Sekanina, 2002 (see credits) The image on the left introduces us to the extended family of Comet Machholz. The plot shows two of the orbital parameters of what we call "the Machholz Complex". I won't get into what these orbital elements actually mean and represent, suffice to say they are two of the three angles that define the orbit (or path) a comet travels through space. (You can read more about orbital elements here.) What we see is a plot of "orbital inclination" versus "longitude of ascending node" for a variety of objects that include the Quadrantid, Southern Delta Aquariid, and Daytime Arietids meteor showers, asteroid 2003 EH1, the hundred-or-so members of SOHO's Marsden and Kracht group of sungrazers, and then Comet Machholz itself. The "precursor" labels refer to theorized "ancestors" of this family that somehow evolved (orbitally) into the objects we now see. Cumulatively, these are what I'm going to herein refer to as the Machholz "complex" or "family".
Now, what this plot is showing us isn't quite as simplistic as it may (or may not) seem. The plot is the result of several calculations that again I don't want to get into here, but are certainly detailed in the paper from which it is extracted (see credits). Also, the plot is true specifically just for second half of the 20th Century -- a point that I'll be coming back to later. But what it basically shows is that with a bit of computational tweaking, there is a very clear relationship and lineage among a relatively large comet (Machholz), a bunch of cometary/asteroidal sungrazers (all discovered by the Sungrazer Project!), an asteroid, and a bunch of meteor showers. This raises lots of questions for scientists, and I'm going to cover a couple of them now.
OriginsNow, I want to take a step back here a minute and talk about origins. I have stated that the so-called Machholz Complex is a family, but what does that mean in terms of its origins? Basically, what it means is that some time ago -- at least a few millennia in this instance -- all of the above mentioned objects, but no doubt numerous others long-destroyed or as-yet undiscovered, were all part of the same parent, or progenitor, object. This would have been a very large comet, though not necessarily huge given that none of the Machholz family members are all that big. But at some point, this large object will have suffered a major fragmentation event, where it split into one or more chunks. These individual chunks would have also subsequently fragmented -- again on long timescales -- and the orbits of these individual members would have gradually been changed with successive gravitational interactions with Jupiter and the Sun (more on this in a minute). It is this successive fragmentation and orbital tweaking that has resulted in this large and complex extended family.
Jupiter's influenceFragmenting comets: Famously in 1994, Comet Shoemaker-Levy fragmented into many pieces before succumbing to Jupiter's gravitational pull and crashing into the gas giant. Image credit:NASA/JPL I mentioned earlier that the above plot refers just to the second half of the 20th Century, and also that Jupiter exerts a gravitational influence on the Machholz Complex. What does that mean, and how are these things related? Why does time, and a gas giant, make a difference? Surely a comet's orbit is whatever it is, and remains stable until it hits something or gets vaporized?
Orbits, particularly of small bodies such as comets and asteroids, change and evolve over the passage of their path through the solar system, primarily through gravitational interactions with planets, though to a lesser extent also from outgassing as dust/ice leaves the object (the latter only applies to comets, as asteroids are inert by definition). You see, as a comet or asteroid passes relatively close to a gravitionally strong object, it feels a strong pull from it. But it already has its own velocity and so, rather than just get sucked in by the strong gravitation field, it simply gets tugged in a certain direction and then flies off on a now modified orbital path by the larger object. The smaller the distance between the two, the greater this effect can be. And in our solar system, there is only one heavy-weight planet that does the bulk of the orbital tweaking of objects: Jupiter.
Jupiter is over 300 times more massive than the Earth, exerting a strong gravitational influence over passing comets and asteroids, and it turns out that Jupiter's gravitational pull is largely responsible for the variation we see in the Machholz complex. Over hundreds (thousands?) of years, as chunks of this family have passed close to Jupiter, their orbits have been tweaked, putting them on a slightly different trajectory. But, of course, Jupiter doesn't sit still, and so some fragments happen to pass close to it while others have rather fortuitously never come particularly close to Jupiter at all.
According to researchers, Comet Machholz itself "is remarkable in that, at least for two millennia, it has been very successful in avoiding the planet", and "integration of the comet’s motion back to AD 150 shows that it has never approached Jupiter to less than 0.5 AU".[Sekanina & Chodas, 2005] Therefore, during any given time period of, say, a century or so, these objects are going to get moved around quite a lot if they happen to pass near Jupiter during the gas giant's 12-year orbit around the Sun. Thus the plot that I showed earlier happens to be valid for the second half of the 20th Century because that's simply the time period the scientists used to create that plot. The values plotted there would change somewhat -- potentially by a lot -- if we considered a different time period.
So this explains how we have an extended population of objects in different orbits that share a common origin. But this leads us nicely to the next question, which to me is possibly the most puzzling...
The Quadrantid meteor shower: just one piece of the Machholz puzzle!
Image Credit: Pete Lawrence (UK) via spaceweather.com
Comet or Asteroid?If all these family members originated from the same parent comet, why aren't they all comets? How come at least one of them is an asteroid? These are great questions and ones to which we don't really know the answer! For me personaly, the Machholz complex, and arguably the Kracht and Marsden groups discovered as part of the Sungrazer project, bring in to question what actually defines a comet and what defines an asteroid. On the face of it, any astronomer can tell you that comets have signs of phyical activity (i.e. outgassing, dust/ice production, etc) and asteroids are essentially inert lumps of rock, and that is how you tell them apart. But one can also argue quite convincingly that no matter how inert a rock may be, if you get it hot enough then elements that comprise it will begin to sublimate, it will outgas, and hence be classified a comet.
The Marsden and Kracht-group comets that I mentioned, and that we see frequently in the SOHO data, have never shown any "classic" indications that they are cometary. They have no detectable diffuse dust/ice coma, and no tail. They are tiny objects and yet they seem to do a reasonable job of surviving multiple passages relatively close the Sun every five to six years. Thus you can make a strong argument that they are asteroidal. Yet the simple fact that they brighten up so dramatically as they approach the Sun, up to magnitude +6 or so in some cases, and then fade just as dramatically as they move away from the Sun, indicates that some kind of activity is occurring on their surface, as you would expect with a comet.
While there may be uncertainty around the nature of SOHO-observed members of this family, there's little doubt of the current nature of asteroid 2003 EH1, but yet questions still surround its past.
2003 EH1: An extinct comet?The Quadrantid meteor shower is extremely well-known, and is the brightest and most populous of the meteor showers that we see on Earth. A meteor shower is the result of Earth passing through a cloud of rocky debris left in the wake of some parent object either fragmenting or simply shedding large chunks of itself. All meteor streams have a parent if you go far enough back in their history, but it was not until relatively recently that we knew of the Quadrantid parent.
Prior to a discovery in 2003 and subsequent publication in 2004, there was no known candidate parent for the Quadrantids (and there was no obvious link for them to Comet Machholz). But in 2003, the LINEAR sky survey program discovered asteroid 2003 EH1, and in a paper by P.Jenniskens in 2004, it was demostrated that EH1 was very likely a "missing link", not only as a parent of the quadrantids, but also a connection between that meteor shower and Comet Machholz. So is EH1 the parent of the whole family? Or would that be Comet Machholz? Or neither of them?
C/1490 Y1: The Absent Parent?In this big soup we call the Maccholz Complex, we have another ingredient to add... maybe! Comet C/1490 Y1 was a fairly bright comet seen from Korea, Japan and China between December 1490 and February 1491. Of course, back then there were no complex optics and powerful telescopes that we have now, and so the positions in the sky recorded for that comet were a little rough to say the least. Poor observations don't lend themselves well to good orbit calculations, and thus when the orbit for the comet was calculated, nominal values had to be assumed for certain parameters (namely, the orbital eccentricity).
But in 2003, Williams & Wu published a work in which they demonstrated that if the orbital eccentricity for C/1490 Y1 was set to 0.77 instead of 1.0 (indicative of a periodic comet as opposed to a one-off visitor to the solar system) then it could actually be linked to the Quadrantids! Williams and Wu subsequently showed that a possible gravitation encounter with Jupiter in the mid-1600s could have flung C/1490 Y1 into a wildly different orbit while leaving its remnants (the Quadrantids) in place and thus explaining why we've never seen C/1490 Y1 again (that we know of!).
Where do Machholz and all the other family members fit into this scheme? Maybe C/1490 fragmented before, or after, being flung around by Jupiter? Maybe Comet Macchholz and C/1490 Y1 are twins; two halves of the same comet? There are really a very large number possibilities for this family.
Now Showing at a Website Near You...We know that families can get complicated, and this one is certainly no exception. Some of the relationships I have described above are more tentative than others, and there are actually several more that I haven't touched on at all (this includes the Areitids, Southern Delta Aquariids, and a couple of other SOHO-discovered objects). I could have made this article four times as long and still missed out details, but I hope the basic message came across. We also have so much more to discover about this and other comet families, and may never have all of our questions answered, but in time we will learn more as we continue to observe members of this intriguing family.
And talking of which, we don't have to wait to do a little more of that! I mentioned waaaay back at the start of this tome that Comet Machholz would be making its fourth appearance in the LASCO C3 data this year. Indeed that is so, and it's actually happening right now! Between July 12th (yesterday) and 17th, 96P/Machholz is passing from the lower-right to the upper-left of the SOHO/LASCO C3 images, reaching a peak brightness of around magnitude +2. This isn't quite the brightest we've ever seen it, but it's close to that (its brightness being a function of where Earth, and thus SOHO, happen to be at the time the comet nears the Sun).
I will be posting more movies, images, etc of this event as it evolves, so check back on this site, and follow me via @SungrazerComets on Twitter for more of those. And who knows, maybe Comet Machholz will do something completely and utterly unexpected like fragment into a swarm of Machholzlets ("Machholzlings?")?! Anyway, in the meantime you can keep up-to-date with all the latest SOHO images on the main SOHO website or on our very own LASCO website while we watch this fourth reappearance of one of our favorite short-period comets.
Much of the information I have presented can be found in far more (and far greater) detail in the following two published works:
i. Sekanina & Chodas (2005), "Origin of the Marsden and Kracht Groups of Sunskirting Comets. I. Association with Comet 96P/Machholz and its Interplanetary Complex, ApJ Suppl., 161:551-586
ii. Jenniskens (2004), "2003 EH1 is the Quadrantid Shower Parent Comet", ApJ 127:3018-3022
Finally, a very grateful hat-tip to Dr. Matthew Knight for his input on this article!
If there are any mistakes above (quite possible!) then they're almost certainly mine and not of the authors of the above publications. 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 email@example.com if you're not sure.
Karl Battams, NRL (July 13, 2012)
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