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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
The detection of cometary activity is really exciting. It looks as if this object will be brighter than mag 20 for the next year or so and it will be well placed for us in the northern hemisphere after perihelion. A great opportunity to study something from another star system.
Good luck Nick. Yes, 18.5 should be relatively easy even that low down. I have a tree in that direction that blocks my main telescope so I’ll have to go remote.
Who’s going to be the first person to get an interstellar object with a Seestar?
Nick.
In case anyone needs an ephemeris for the next few days I’ve just generated this from Findorb. It is for Rio Hurtado, Chile but the parallax is small so it will work for anywhere.
The current elements from Findorb using all of the astrometry on the NEOCP are:
<plaintext>Orbital elements: A11pl3Z Perihelion 2025 Oct 29.67795 +/- 0.216 TT = 16:16:14 (JD 2460978.17795) Epoch 2025 Jul 2.0 TT = JDT 2460858.5 Earth MOID: 0.3549 Ju: 0.2479 q 1.34626730 +/- 0.0133 Ma: 0.0201 Sa: 0.4082 Find_Orb H 11.91 G 0.15 Peri. 128.11453 +/- 0.14 z -3.7575161799 +/- 0.0375 Node 322.06864 +/- 0.10 e 6.0586211 +/- 0.1 Incl. 175.10933 +/- 0.0044 111 of 113 observations 2025 June 14-July 2; mean residual 0".37</plaintext>Yes, this is a very interesting object that appears to be brightening quite rapidly so it could be cometary (I hope). We’ll have plenty of time to observe it but not when it is at perihelion since it is at inferior conjunction then. I’m preparing a news note and will upload it later. Astrometry will be very useful but the arc (with precoveries) is already long enough to give us a pretty stable orbit solution. It is definitely interstellar with a rather high approach velocity.
For now, beware the most planetarium programs will break with elements that have e=6!
Here’s a link to the real-time video clip from UK004E. The cloud starts in northern Capricornus and then moves up into Aquila. You can see a number of the deployed spacecraft ahead of the cloud and a brighter object in the cloud. The brightest dot, towards the end, is the venting second stage itself. Mel’s video linked above shows the individual events in far higher resolution and I assume that the significant brightening of the second stage around 00:12:45 is the large venting event shown on Mel’s video.
https://nickdjames.com/Spacecraft/20250524_propvent/UK004E_20250624_001126_ndj.mp4
The final spacecraft deployment was the Mission Possible re-entry experiment at T+2:43:53 which is just before 00:09 UTC and the first sign of the cloud is at around 00:11:30 so only around 2.5 mins later.
Stan,
According to the Space-X video here:
https://www.spacex.com/launches/mission/?missionId=transporter14
The second stage de-orbit burn occurred over north Africa starting at T+2:40:16 and it lasted a few seconds. Launch was at 21:25:00 UTC so that means that the burn was over by 00:06 UTC, before we saw the stage come over the UK. Three minutes later it deployed a re-entry demonstrator called Mission Possible and then started the passivation with thruster firings and venting residual propellant which is what we saw.
I have video from my SE pointing camera (UK004E) showing the stage rising out of the Earth’s shadow in the SSE with with the venting starting around 00:11:46 UTC.
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Mel – That’s a brilliant video and it shows the various thruster pulses and the main burn very well. What equipment did you use?
Steve – You were in bed? So was I. I thought you had an army of Seestars permanently monitoring the sky…
Cees Bassa confirms that it was Transporter 14. Presumably the upper stage deorbit and passivation.
I don’t yet know what it was but it certainly looks like propellant venting. Prompted by your post I checked my cameras and did pick it up. The screengrab here shows it and the video at the link below shows it slowly moving downwards towards the northern horizon.
https://nickdjames.com/Spacecraft/20250524_propvent/propvent2_20250624_001530_ndj.mp4
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That is very sad news. I remember seeing Terry and one of his early cameras at an Exhibition Meeting in the early nineties. I bought an early SX and was astonished by its capabilities. Terry was always incredibly helpful.
My first light with the SX was on 1994 September 1. The image of M57 doesn’t look much now but it was a revelation then. The chip was 510×256 pixels, the pixels weren’t square and the field of view was tiny but the sensor was much more sensitive than film and there was no messing around with horrible chemicals afterwards.
Terry led the way in the UK and changed my (astronomical) life completely.
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