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An interstellar asteroid with an orbital eccentricity of around 6 has recently been discovered. At the moment it is around magnitude 18.2 and getting brighter. More information at https://cneos.jpl.nasa.gov/scout/#/object/A11pl3Z
The sky may be a bit bright for observers up here in the sub-arctic but observers further south, and those using remote telescopes, may wish to take images of the asteroid, measure them, and submit the results to the MPC. The orbit still needs refining. I wish I was in La Palma …
Paul
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!
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>I got an ephemeris from a link on cloudy nights earlier and it seems to be perfectly placed, if a bit south at -18deg. Elongation 170degrees, near the galactic centre. Blue sky now and I’m going to image this tonight if its the last thing I do. At mag 18.5 should be a walk in the park, an hour should burn it into the sensor. So excited!
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.
This object has now been officially designated as an interstellar comet
Now officially announced as 3I/ATLAS = C/2025 N1 (ATLAS)
https://minorplanetcenter.net/mpec/K25/K25N12.htmlThe 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.
We’re getting our Comet Chasers on it. It is well timed as we’ve got a photo shoot for a new space magazine lined up for Friday, so that will add some special interest 😎
Image attached! – about 90 mins of 10sec unfiltered subs with the 12″ newt and ASI294MM camera. Looks diffuse in one direction, but i dont quite trust my collimation, so im not calling it just yet!!!
Image is 3.84′ wide – 0.8″ pixels.
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
Great images everyone.
…and here’s mine from my currently very hot remote observatory in Extremadura, Spain.
https://britastro.org/observations/observation.php?id=20250703_091533_1ef0a496903087dbI 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.
There’s a really good high SNR image of 3I/ATLAS available here that shows it to be cometary with a slightly offset nucleus:
https://lightbridges.es/wp-content/uploads/2025/07/3I_stacked_20250702.png
Richard,
Worth giving the Astronomer’s Telegram reference for the image above for additional information: Astronomer’s Telegram #17264Interesting trajectory for this object, almost in the plane of the ecliptic. Odd for an interstellar object given the solar system is tilted about 60 degrees from the galactic equator. How does it compare to the other two known interstellar objects?
Mike
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.
Infrared image from night of 3rd-4th. No sign of coma.
Attachments:
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:
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.
It is clearly an alien space ship. 😉
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.
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