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My image using a remote telescope in Namibia (FSQ106 + IMX455) shows a 4 deg tail and the total magnitude using comphot is 7.4. A nice surprise comet even though it is not currently visible from the UK.
This comet has been formally designated C/2025 R2 (SWAN) on CBET 5606. The attached chart is based on the latest orbit but the comet won’t be observable from the UK until early October by which time it is likely to have faded considerably.
These plots are based on the current, rather uncertain orbit and the predicted magnitude is just a guess. The observability plot shows the elevation of the comet at evening nautical twilight for various latitudes. At 50N we don’t get a chance until early October but it will be a good target for observers with access to telescopes further south.
The elongation plot shows that it has been within 30 deg of the Sun since late July. That fact that it wasn’t picked up by surveys earlier this year when it was at a large elongation implies that it has brightened rapidly and so it will possibly fade rapidly as well.
I’ll update the plots when we have a bit more astrometry.
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This comet was first noted in SWAN data by Vladimir Bezugly on September 11. We now have a 40 hour arc of decent astrometry from southern hemisphere sites and, forcing e=1, I get a perihelion of 0.50 au on Sept 10 (see the elements attached from FINDORB). This orbit is still pretty unreliable but it indicates that the comet will make a close approach to the Earth on October 17 when it is at a decent elongation (76 deg). How bright it will be at that time is anybody’s guess.
I’ve also just updated the item on C/2025 A6 and that is looking promising now, so October could be an exciting month for comet observers:
https://britastro.org/section_news_item/c-2025-a6-lemmon
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Nick – For your 30 min stack the length of the trails in your image is around 50 arcsec. The Hubble image that Denis linked shows a short (approx 4 arcsec) and very faint extension in PA 300 or so. I doubt that you are detecting the coma yet but your image is definitely of high quality.
The HST constraints on nucleus diameter are interesting. Details are in the ArXiv paper here: https://arxiv.org/pdf/2508.02934.
Ian,
It is worth submitting magnitude estimates to COBS if you can. There is a lot of scatter on the current estimates but they suggest that the comet might reach mag 14 in November as it comes out from solar conjunction.
James,
The maths is simple but probably not very useful for your application.
Tensile strength is measured in Pascals and a Pascal is a Newton per metre squared. If a rod has tensile strength T [N/m^2] and diameter d [m] then it will support a weight which is a product of its tensile strength and area, i.e. W = T * pi * d^2 / 4 [N]. The material will permanently deform long before it breaks.
Taking your example of T = 600MPa and d = 0.004m then W is around 7500 N which corresponds to a mass of 750 kg at the Earth’s surface.
I got the comet with a RedCat 51 last night. This has the same aperture as a Seestar 50 so it should be in range of those now although I had the advantage of a cooled camera and a dark sky at 2100 m on La Palma!
https://britastro.org/observations/observation.php?id=20250724_150857_253c99e511be2b41
The magnitude estimate on that image is probably rubbish since it is contaminated by stars but it is brighter than I expected.
Nick – It is definitely worth trying to get some photometry if you can. You could use Sloan r’ mags for your long-pass data and you would at least be able to see if you are consistent from night to night.
Paul – I’m pretty sure that the scatter is due to faint stars in the aperture since this is a very crowded field. I think it is far to early to say anything about the lightcurve of this object but the more observations the better. I have used DAOPHOT and it can be very good but I’ve not been able to get it to work reliably with moving objects.
I’ve been doing some astrometry on 3I/ATLAS and have been surprised how much scatter in the magnitude that I’m getting day to day. The plot of magnitudes from MPC astrometry data also shows a huge scatter. This data doesn’t have a consistent photometry aperture or filtering but, for most observers at the moment 3I/ATLAS is a point source so it is surprising. The scatter may be due to measurement errors in the crowded star field but it is worth making estimates of the magnitude if you can.
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Astrometrica will work for stacking but you will need to enter the offsets manually since the internal ephemeris generator doesn’t work for significantly hyperbolic orbits. I’m hoping Herbert Raab will get that fixed sometime.
It has been interesting to see how many common programs can’t cope with e=6. This will be a good opportunity to get them fixed! I use my own code for stacking and it uses the Laguerre algorithm for solving Kepler’s equation to find the eccentric anomaly as documented by Bruce Conway in 1986 here:
https://adsabs.harvard.edu/full/1986CeMec..39..199C
It is very fast and appears to be stable at least up to the eccentricity of 3I/ATLAS.
Looking at the ATEL it would be interesting to see an image of an equivalent magnitude star processed and displayed in the same way. I also note that the sub-exposure length was 50s, during which the comet would have moved around an arcsecond so that probably explains the elliptical shape of the bright core.
I don’t think the coma is currently detectable with “small”, i.e. sub metre class, telescopes but the image from the ATEL reference above looks reasonably convincing.
It is interesting that 2I/Borisov was definitely cometary and it looks like 3I/ATLAS is as well. 1I/Oumuamua was not but it did have anomalous nongravs which implies it must have been comet-like. This paper suggests that the thrust in that case was from sublimation of molecular Hydrogen:
Dan Bartlett asked this question on comets-ml and I had convinced myself that the expected probability distribution of inclinations for objects arriving from random directions was uniform. I was wrong. It is actually proportional to sin(i) so the most probable inclination is 90 degrees and the least probable is 0/180 degrees. You can see this sinusoidal distribution in the inclinations of long-period comets wince these come from random directions.
I think interstellar comets should be similar with a slight bias towards our direction of travel. 3I/ATLAS has an inclination of 175 deg which is quite unlikely, 1I and 2I were 123 and 44 deg respectively. We need a lot more interstellar comets to get some good statistics though. Vera-Rubin may help with that.
I think I detected it using a 90mm refractor from Chelmsford last night:
https://britastro.org/observations/observation.php?id=20250703_210130_832805f398624544
It is right on the limit of detection using a stack of 88x60s frames but I’m pleased to have got it from the UK given that my main telescope can’t get that low.
Nick – Great image.
Here’s mine. I can’t get that far south with my main scope due to a neighbour’s tree. This is from Chile:
https://britastro.org/observations/observation.php?id=20250703_055356_fc54046de537751b
