Once Around Comet Holmes

Added: 2026-05-31

[00:00:02]Once around, comet Holmes.

[00:00:06]So, comet Holmes was discovered by Edwin Holmes during an observation session of the Andromeda galaxy back on the 6th of November of 1892.

[00:00:19]So, it's been known for over a hundred years.

[00:00:24]And in this uh image that we have here, you can see the Andromeda galaxy with its satellite galaxies up to the top right of the image there. And down at the bottom, the very bright blob, which is the comet.

[00:00:38]And we think that he probably discovered Comet Holmes because it had suddenly become rather brighter with some sort of outburst. And that's very interesting because we've seen it do it again.

[00:00:53]Now the orbit of the comet was soon worked out by Croitz and S to be an elliptical orbit taking it from just beyond the orbit of Mars at two astronomical units twice the Earth's sun distance out to 5.2 which is nearly the orbit of Jupiter and taking just under seven years 6.9 years for the period for the orbit. Now that means that the next time it comes in close to the sun, the perihelion point will be the 13th of June of 2028.

[00:01:29]So just a couple of years and we should see this comet brightening again. Here's a little animation of the orbit slightly tilted to that of the plane of the solar system going around the sun very much under the influence of the planet Jupiter. a Jupiter family comet. It's the gravity of Jupiter that is corelling its orbit to arrive round about that five astronomical unit point at the uh outside there and uh then swing into the inner solar system taking a period that uh is almost half that of Jupiter.

[00:02:12]Now, what really brought this to my attention was that I saw it back in 2007.

[00:02:19]On the night of the 23rd and 24th of October, the comet suddenly brightened up from dim obscurity at magnitude 17, so faint that you had no chance of seeing it, all the way up to magnitude three, becoming one of the brightest objects in the constellation of Perseus uh in a fairly short period of time, just 48 hours to make this enormous increase in brightness. It became easily visible to the naked eye and with binoculars it was just stunning. I was actually on holiday in the Caribbean at the time uh under some very dark skies and able to wield my trusty binoculars which I always take with me and uh get some fantastic views and uh even entertain quite a few of the other hotel guests to a look at this amazing suddenly brightened comet.

[00:03:23]Now, it took a path through Perseus, looping around, as you can see in the little animation there, gradually changing its brightness, expanding and getting larger but fainter through the constellation over the period of weeks that we were able to observe it. And I've got another diagram of that here.

[00:03:44]So showing that uh the brightening occurred at the beginning there around about the 23rd, 4th, 5th of October and then it gradually increased in uh apparent size, brightening and becoming quite spectacular right through until the size had increased so much that it began to fade away in the spring of that year in February and March as the light became more and more diffuse and lost into the background.

[00:04:14]Quite an amazing thing to watch. Now, it's estimated that it's got a modestsized nucleus, 3.4 kilometers in diameter, the little tiny dirty snowball in the center. but that during the outburst the coma the envelope around the comet expanded to over a million kilometers and in fact became larger than the sun.

[00:04:43]So the one of the largest extended objects in the solar system although there are several structures that uh people claim like the magnetic field of Jupiter as being larger but I'm not sure that a magnetic field is really uh a physical structure in the same way.

[00:05:02]Anyway, truly amazing and you can see in the picture there 10 arc minutes across uh that is a sixth of a degree. So compare that to the size of the full moon which is 30 ark minutes and uh you will see that this was really quite a big object on the sky.

[00:05:22]Now this attracted enormous amounts of astronomical attention and some telescopes zoomed in to try to work out what was going on. And perhaps the best picture is this one. This is the core of the comet deep inside there with the uh shock wave and the envelope around it calculated by looking at a number of images and doing what's called a LLAS transform on them which has brought out the detailed structure of what was going on very very much into the central region there showing that the comet had in fact fragmented into quite a number of separate comet fragments that were breaking away from the main core as well as the enormous eruption of dust and gas forming the coma around it. And nowadays when we look back and trace comet homes, we can see the main comet and fragments from it and the dust trail reconverging as all of their individual orbits which are now drifting apart under the influence of the gravitational pertubations of the planets and the effect of the solar wind pushing on them. They're gradually separating and moving offline, but their orbits have a a crossing at the original eruption site. Now, the image at the top there is a recent image of comet homes itself. The bright point with the black shadow there, that is the comet. And the black shadow is the dust trail leading off to the right on the image. And then at the bottom we can see a highly enhanced picture showing many of the small comet fragments and a faint belt running diagonally which is the comet dust.

[00:07:29]So this was an amazing thing. The question before us really is what happened?

[00:07:36]Can we identify the reason why this comet suddenly had this enormous outburst and indeed created this shower of separate fragments. And I love the animation on the right again built with that LLAS transform of a series of images showing how the coma expanded and how the fragments internally uh were beginning to separate but not as fast as the expansion of the main envelope. So was this a collision between objects in the asteroid belt?

[00:08:13]Perhaps comet Holmes crashed into a small asteroid. Perhaps a large meteor traveling at high speed in a different direction. This could have been a slow speed impact of two large bodies or a head-on smash with something much smaller. Other theories are that it was just the warming of the material caused by the heat from the sun generating a buildup of gas pressure in a deep pocket inside the comet that suddenly managed to crack the surface and explode out in a huge uh outburst.

[00:08:56]And some have suggested that this was water being melted internally uh forming at a high pressure water vapor inside the comet that then burst forth into space. And there certainly was a lot of water visible. But that's not really a surprise because we think that comets are essentially dirty snowballs. So what really happened? I think it's likely that this was a collision. Collisions are entirely possible in the asteroid belt. Fairly rare and we haven't seen other objects like this. But who knows?

[00:09:36]Unless we are able to send a space probe and understand this better by getting up close and personal to the main fragments, maybe we'll be able to learn a bit more about what caused this. But it is interesting that the discovery of the image by Edward Holmes back in 1892 was probably at a magnitude that suggested a previous outburst.

[00:10:04]Two collisions seems unlikely. So maybe it really was the gas pressure or a water explosion idea. Um it's just uh fascinating. And I hope you've enjoyed that little look at comet homes, an event which I witnessed and uh was very very uh much keen on looking at through my binoculars uh under the dark skies out there in the Caribbean when it was at its peak. Uh so thanks very much for that one.