Just a short note, prompted by the release of new imagery of the intersellar object 3I/ATLAS by the Gemini North telescope in Hawaii. It’s startling how quickly we’ve moved from the first pinpoint images of this comet to what we see below, which draws on Gemini North’s Multi-Object Spectrograph to show us the tight (thus far) coma of the object, the gas and dust cloud enshrouding its nucleus. Changes here as the comet nears perihelion will teach us much about the object’s composition and size. Some early estimates have the cometary nucleus as large as 20 kilometers, considerably larger than both ‘Oumuamua and 2I/Borisov, the first two such objects detected. This is a figure that will doubtless be adjusted with continued observation.
Image: Using the Gemini North telescope, astronomers have captured 3I/ATLAS as it makes its temporary passage through our cosmic neighborhood. These observations will help scientists study the characteristics of this rare object’s origin, orbit, and composition. Credit: NSF NOIRLab.
3I/ATLAS also shows a more eccentric orbit than its predecessors. Remember that an eccentricity of 0 means an orbit that is completely circular, while as we move from 0 to 1, the orbit becomes drawn out, to the point where an orbit with eccentricity values of 1 or above doesn’t return to the Sun, but continues into interstellar space. The new comet’s orbital eccentricity is 6.2, considerably higher than ‘Oumuamua (1.2) and Borisov (3.6). Perihelion will come at the end of October at a distance of 210 million kilometers, which will place the object just inside the orbit of Mars. Amateur astronomers with a good telescope may just be able to get a glimpse of it late in 2025.
I’ve read estimates from 0.8 to 20/25 km. A very large range. I gather, if on the large size, this would be very unusual and a very, very, rare event. I look forward mostly to the analysis of composition and whether the organics are mostly tholins or something more interesting. If, as predicted, we will now expect to detect a number of these ISOs every year, I will look forward to the theoretical analysis of what this means and what it implies, and not necessarily panspermia ;-)
Yes it would interesting to see it’s spectral lines to know it’s chemical composition.
They’re having trouble explaining the brightness relative to the lack of coma activity. From a paper today by Kareta et al : ‘Challenges in modeling the reflectivity of 3I may indicate that this comet has a complex grain size distribution, grain compositions unlike Solar System comets, or both’
Summarizing a recent summary (AAAS Science, 17 July 25):
“On 1 July an asteroid-hunting telescope in Chile spotted a white dot hurtling against background stars… Soon it was clear the dot was moving through the Solar System so fast—209,000 kilometers per hour—that it could not be orbiting the Sun.
“3I/ATLAS, named for the instrument that found it, the Asteroid Terrestrial-impact Last Alert System (ATLAS) survey telescope. The third object of its kind ever spotted. Predecessors, 1I/‘Oumuamua in 2017 and 2I/Borisov in 2019. The “nI” for interstellar object ). Characterized by Science article as “a stray leftover from the formation of exoplanets around other stars and will vanish back into interstellar space within months.”
Findings thus far:
“The object is conspicuously bright. ATLAS was discovered while it was still 670 million kilometers from the Sun, out near the orbit of Jupiter—more than five times farther than ‘Oumuamua was when it was first spotted. This suggests ATLAS might be as large as 20 kilometers across, much larger than ‘Oumuamua (thought to be a cigar-shaped chunk 400 meters long) and Borisov (975 meters wide). However, estimates of the object’s size come with an asterisk. If ATLAS is outgassing like a comet, it might be surrounded by a haze that makes it appear larger than it really is.
“What is certain is ATLAS’s unusually wide curve through the Solar System. To describe how much an object’s orbit deviates from a perfect circle, astronomers calculate a metric known as eccentricity. All planets, asteroids, and comets within the Solar System have orbits with an eccentricity between zero and one, meaning they trace out an ellipse around the Sun. ‘Oumuamua’s eccentricity was 1.2—a hyperbolic arc. Borisov’s was 3.3. With an eccentricity above six, ATLAS is blowing both out of the water.
“According to Auburn University astronomer John Noonan, ATLAS’s trajectory could mean the object has been drifting through the Galaxy for longer than ‘Oumuamua and Borisov. However, pinning down exactly where ATLAS came from remains challenging, as the gravity of dust clouds or stars that it passed could have bent its path through the Milky Way. At least one team has used high-precision data on the positions of our Galaxy’s stars to suggest ATLAS may have originated in the Milky Way’s sparse outer disk.
“…By the beginning of October, when ATLAS will have entered the inner Solar System and crossed Mars’s orbit, it will be too close to the Sun to be observed with ground-based telescopes. At that point scientists hope to enlist spacecraft like NASA’s Mars Reconnaissance Orbiter and the European Space Agency’s (ESA’s) Jupiter Icy Moons Explorer, which is inbound to the gas giant, to study the object. ATLAS then should reappear for Earth observation by early December and remain visible until May 2026.
…ATLAS’s composition:
“Comets and asteroids represent the leftovers of planet formation— meaning ATLAS is a discarded sample of alien worlds, flung out from the dusty disk surrounding a distant star. By comparing its ratios of things such as carbon monoxide, water, ammonia, and salts with those of our local comets and asteroids, researchers hope to learn whether other planetary systems have ingredients like our own. “If we see that this building block looks like all the building blocks in our Solar System, that’s good news,” says Karen Meech, an astronomer at the University of Hawaii at Manoa. “That’s encouraging for the potential for life” elsewhere in the Galaxy.
“Already ATLAS appears to have a reddish color, which means its surface probably contains organic materials such as methane, typical for both comets in our Solar System and ‘Oumuamua and Borisov. More detail is likely to come as larger instruments such as the Hubble Space Telescope and JWST swing into action. …
“…The powerful Vera C. Rubin Observatory on Cerro Pachón in Chile, which recently began to catalog the entire southern night sky every 3 days, is expected to find between six and 51 additional visitors over the coming decade. Upcoming telescopes designed to find near-Earth asteroids, such as NASA’s space-based NEO Surveyor satellite or ESA’s Flyeye instruments, should also detect a few interstellar objects once they become operational.”
https://astrobiology.com/2025/07/from-a-different-star-3i-atlas-in-the-context-of-the-otautahi-oxford-interstellar-object-population-model.html
From A Different Star: 3I/ATLAS In The Context Of The Ōtautahi-Oxford Interstellar Object Population Model
By Keith Cowing
Status Report
astro-ph.EP
July 15, 2025
The Otautahi–Oxford model’s predicted asymptotic speed, radiant, and Galactic velocity distribution for ¯ q < 5 au ISOs, overplotted with the three known ISOs. The blue and orange lines on the radiant plot mark the Galactic and ecliptic planes respectively, and the yellow square marks the solar apex/local standard of rest (Sch¨onrich et al. 2010). — astro-ph.EP
The discovery of the third interstellar object (ISO), 3I/ATLAS (`3I’), provides a rare chance to directly observe a small body from another Solar System.
Studying its chemistry and dynamics will add to our understanding of how the processes of planetesimal formation and evolution happen across the Milky Way’s disk, and how such objects respond to the Milky Way’s potential.
In this Letter, we present a first assessment of 3I in the context of the Ōtautahi-Oxford model, which uses data from Gaia in conjunction with models of protoplanetary disk chemistry and Galactic dynamics to predict the properties of the ISO population. The model shows that both the velocity and radiant of 3I are within the expected range.
Its velocity suggests an origin within the Milky Way’s thick disk, making it the first ISO from this population, and predicts a high water mass fraction, which may become observable shortly.
We also conclude that it is very unlikely that 3I shares an origin with either of the previous two interstellar object detections.
Matthew J. Hopkins, Rosemary C. Dorsey, John C. Forbes, Michele T. Bannister, Chris J. Lintott, Brayden Leicester
Comments: Submitted to ApJ Letters
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Astrophysics of Galaxies (astro-ph.GA)
Cite as: arXiv:2507.05318 [astro-ph.EP] (or arXiv:2507.05318v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2507.05318
Focus to learn more
Submission history
From: Matthew Hopkins
[v1] Mon, 7 Jul 2025 15:12:57 UTC (414 KB)
https://arxiv.org/abs/2507.05318
Astrobiology, Astronomy
Amid all the data coming in, we can see at least one initial controversy:
On one hand:
“At least one team has used high-precision data on the positions of our Galaxy’s stars to suggest ATLAS may have originated in the Milky Way’s sparse outer disk.”
OTOH:
“Its velocity suggests an origin within the Milky Way’s thick disk, making it the first ISO from this population, and predicts a high water mass fraction, which may become observable shortly.”
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I won’t place any bets on the two arguments, but this object should have a lot more to say about itself before the passage is over.