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Breaking News: Comet Machholz had babies!!This is a very unexpected update of the article I wrote last week about the so-called "Machholz Complex". The tldr; of that story is that the well-known short period comet 96P/Machholz is believed to be related to a couple of meteor streams, an asteroid, two entire groups of comets that SOHO observes/discovers, and likely a few individual SOHO objects as well. This relationship is the result of centuries of repeated fragmentations and gravitational interactions with Jupiter and the Sun.
At the end of that article, I made a rather tongue-in-cheek comment: "And who knows, maybe Comet Machholz will do something completely and utterly unexpected like fragment into a swarm of Machholzlets ("Machholzlings?")?!". I really was not very serious about that, and I was simply trying to convey that comets are not "static" things like asteroids; they are dynamic, active and constantly evolving, and every passage past the Sun is an intense and stressful experience (thermally and gravitationally!) for these very low-density "dirty snowballs".
Nonetheless, I did not expect this!
It seems that Comet Machholz has had babies! Twins!!
OK, what's really going on here? Well, as expected, Comet Machholz appeared in LASCO C3 on July 12, 2012, and entered LASCO C2 on July 14 (leaving that small field of view the same day). The ever-diligent SOHO Comet Hunters were watching the images closely, as they always do, when they noticed not one but two small, faint objects a few hours ahead of Comet Machholz. At first they thought they were members of the "Meyer-group", which are unrelated to Machholz but at this time of year they do appear to follow a similar path through the LASCO C2 images as Machholz was on. But the motion of this new pair of objects is not consistent with Meyer-group comets, and is essentially identical to that of 96P/Machholz. Thus, while we can't be certain until I make some precise measurements, it seems extremely likely that these are direct children of Comet Machholz!
A couple of things of interest here. Well, several actually, but there's a couple I'll focus on. First point to note is that these "fragments" are actually ahead of Comet Machholz, and not behind it. "Huh?! How does that work??!" What that means is that these are not "new" fragments in the sense that they happened within the past few days. For these little guys to have been pushed out ahead of the parent, the separation must have occurred quite some time ago -- almost certainly at the last perihelion passage in 2007. Also, the fragments are closer to one another than they are to Machholz, so I would imagine that they probably broke off as one lump that subsequently split in half shortly after, though that's a bit more speculative on my part. Regardless, there would have been some separation velocity between the parent and the fragment(s), and over the course of the past ~5.2yrs, this small difference in velocity has resulted in the distance we see between them right now, which is actually very substantial.
How substantial? Well, the leading fragment is ~05hrs 12mins ahead of Machholz, and let's assume they're moving at 220km/s (a reasonable velocity for Machholz at this point in its orbit). So that's 312mins = 18720 seconds = 4,118,400km. Thus at 220km/s, Comet Machholz has to travel ~4.12million kilometers to reach that same point as the fragment that leads it. So what would have been the separation velocity? Well the orbit of Machholz is approximately 2 billion kilometers long, and it takes 5.28yrs to travel that. Assuming that after one orbit, they are 4.12 million kilometers apart, it means a steady separation velocity of very roughly 20m/s (meters per second), or about 45mph, which really is not a lot for a solar system object. So it only takes a relatively minimal difference in velocity to make a substantial difference in separation after a complete orbit. [*Please see footnote on these calculations.]
Second point of interest: despite how many different objects in the solar system we know are related to Comet Machholz, this is the very first time that we have witnessed a fragmentation of Machholz with our own eyes. Given the volatile nature of this population I suppose it's not surprising it happened, but to actually witness it is really awesome!
Finally, I want to make a shameless plug for the Sungrazer program and its diligent army of volunteers. Discoveries like this are really crucial to help us understand the dynamic nature of comet populations, and can help us infer the properties of comets -- the left-over debris from the building of our solar system. It is because amateur "armchair" astronomers and enthusiasts volunteer so much of their time to scour through our data, that we are able to make these discoveries. There is simply too much data, and it takes too much time (= money, sadly...), for scientists to do this. These Comet Hunters do not get to name the comets, they get no monetary rewards or compensation, and they do not get anywhere near as much credit as they deserve. Yet they do it for the pleasure of doing it and the thrill of being the first to discover a new object in our solar system. The two Comet Hunters that found these objects -- Prafull Sharma (India) and Liang Liu (China) -- only joined the Sungrazer project in January of this year, and
Congratulations indeed to both of them, and it is because of enthusiasts like them that the Sungrazer project has remained so tremendously successful since its inception in 2002. Long live Citizen Science!
Karl Battams, NRL (July 16, 2012)
*Please note: I used very rough numbers and basic assumptions (square cows...) in my calculations and did them in all of 5-minutes. The biggest source of error is assuming that the comets split exactly one orbit (5.2yrs) ago... that's just a straight-up guess. Also, a comet's velocity depends where it is in its orbit, as they go much faster near to the Sun. But regardless, I think the numbers should be good to within an order of magnitude or so, as it's pretty simple math with a reasonable margin for error without impacting the result too much. Please don't quote them as "true values" though!
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