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Alex – Yes, I should have plenty of material for the Christmas sky notes what with all this alien nonsense and the reports of spectacular meteor storms and auroral displays that appear regularly in the media.
Conventional modelling of the dust tail of a comet assumes that the dust grains are small and emitted from the nucleus in all directions at low velocity. Solar radiation pressure is then sufficient to accelerate the grains in the anti-Sun direction to form the dust tail. In the case of 3I one explanation for the “anti-tail” is that some of the dust grains are large and that they are emitted preferentially in the sunward direction at a higher velocity. It then takes more time for the solar radiation pressure to turn them around and push them back in the anti-solar direction.
This mechanism was postulated for the anti-tail of C/2014 UN271 in this paper by Farnham et al:
https://iopscience.iop.org/article/10.3847/PSJ/ac323d
The attached graphic is figure 7 of that paper. It shows modelled dust tail syndynes for different particle sizes and emission velocities and it demonstrates how an anti-tail can form in these circumstances.
Attachments:
Denis,
This is worth a read:
https://sites.psu.edu/astrowright/2025/09/29/3i-atlass-anti-tail-isnt-unique/
After debunking a load of Loeb junk it explains that the anti-tail in this case is related to relatively massive dust grains that don’t get accelerated backwards into the tail. It references the famous LSST paper that has a zillion (I exaggerate slightly) authors:
Loeb has linked to an image in our gallery:
See the second link in the first sentence. I’m not sure what point Loeb is trying to make in the subsequent text but Michael and Frank’s image of the tails of 3I is very impressive.
Isn’t it amazing enough that we have the opportunity to observe a comet from another star system? I imaged it this morning and it is where it should be which is absolutely no surprise.
https://britastro.org/observations/observation.php?id=20251109_060935_f9c3c09eb9bd6dc2
What will our nutty Harvard prof do when it just keeps on moving out of the Solar System and no alien attack ships arrive? What will Harvard do about him? That is a greater mystery than the comet.
There was a Falcon 9 launch from Florida at 16:35 UTC and the upper stage prop dump is usually around 2 hrs after launch so it may have been this.
Robin – I would agree with that but I don’t think that “science” is the main objective in these cases. Jager and Rhemann are very careful with what they do but there is always a concern that so many complex steps can lead to artefacts. I don’t think that this is the case here but I have seen plenty of examples online from less experienced imagers.
Personally, I think the whole idea of separating the comet from the background and then processing each one separately is a bit dodgy but I know that others will have a different opinion!
They certainly do produce extraordinary finished images and they are far better than anything that we have seen in the past so I think we can conclude that the “good” tech (sensors, mounts, optics and processing software) is currently beating the “bad” tech (LED lights, LEO mega-constellations). The comet imagers of old would have been astonished by the quality of images produced today.
We have to live in the world as it is rather than the world we would like so dealing with mega-constellations is, for now, just another hassle, like the weather or light pollution. If these images are processed by separating the comet from the star background, processing each one differently, possibly with some clever, statistical noise reduction and then putting them back together then using statistics or AI to remove the satellite trails is just another step.
Michael’s images are fantastic but I wonder why he doesn’t take more, shorter exposures although I’m not clear what his sub exposures are. It says 15×15 minutes in your description but that is almost four hours so that can’t be right. CMOS sensors have very low read noise and can be read out very fast so doing short exposures is not normally a problem and it allows you to stack statistically to remove sub frame pixels affected by satellite and aircraft trails. My image from here last night:
https://britastro.org/observations/observation.php?id=20251027_224004_df7ddf0b98656b8c
had multiple satellite trails per frame but none are visible on the final stack of 21x30s frames.
Jeager and Rhemann are undoubtedly the best comet images in the world so I must be missing something about their technique. It is unfortunately the case that comet imagers are badly affected by the mega-constellatons since we are imaging with wide-field, fast optics in twilight but there are solutions which don’t involve manually removing trails from each image.
This was discussed in my talk at the recent comet section meeting around 26 minutes in here:
https://britastro.org/video/2025-10-04_Comet_Section/03%20-%20Edinburgh_NDJ.mp4
I’ve uploaded the remaining three talks to Youtube and the meetings page here:
https://britastro.org/section_news_item/comet-section-meeting-edinburgh-2025-october-4
There was a very interesting panel discussion after the talks and I’ll upload that shortly.
Thanks. It was a very interesting meeting and great to see Martin back.
The videos are gradually being uploaded to our Youtube channel but I’ve also put the high quality video on our website so you can watch and/or download it without being pestered by annoying Youtube ads. The links are on the meeting report page here:
https://britastro.org/section_news_item/comet-section-meeting-edinburgh-2025-october-4
There are still a few videos to be edited but they should be available soon.
It’s a good programme and I would normally come along but it is near new moon and there is a certain comet in the sky so I’ll miss it this year. I hope it goes well.
Nick – Excellent image. I think there is a hint of tail there. It is not very often that we have two comets brighter than mag 6 in our skies.
We can do this manually although it is a bit fiddly to extract the metadata. Is there a query that I can run on the gallery database that would allow me to automatically download the full-res image and get the associated metadata in a standard format? Once I have the metadata I can reformat the filename and generate the data text file that goes into our section archive.
I got my first image of this comet from Chelmsford this evening when it was around 5 deg up in bright twilight. It is now moving north and the circumstances get better for the UK over the next week. It is currently around mag 6.
https://britastro.org/observations/observation.php?id=20251008_225616_f9ba3c0ba77b5e84
It was cloudy here in Chelmsford so I’ll I got was the double flash on the cloud. The FRIPON network of all-sky cameras picked it up at 19:10:54 UTC.
James – If you really are that lazy (and I’m sure you are not) then I’m surprised that you have the energy to make the observations in the first place! Each section will have its own way of accepting observations and the basic rules are pretty easy to follow. You don’t need to remember them, you can use the wonderful invention of writing to make notes and set up a procedure to follow for each object. It will then only take a few minutes of your time.
I have never dismissed people as “not serious observers” if they don’t want to do this. It is personal choice and the quality of the images in the gallery is fantastic whether or not you want to submit them to the section. I would like to encourage you to do so but you need to follow our rules. That means making sure that all of the key observation details are recorded and using a filename which is compatible with our requirements. One of the things that makes the BAA special is that we do try to encourage people to think of the science behind their observations. For me, that greatly increases my enjoyment of the hobby. It will be different for others.
As Dominic says, we could spend time trying to make the gallery into a source for the section archive but there are other priorities for our website developers at the moment and this would still mean that many images would not be imported since they lack critical details.
As a compromise, I’m currently discussing with Denis whether he is willing to manually collect data from the gallery that has appropriate metadata and reformat it so that it can be put in the archive. I’ll let you know the outcome of that discussion.
