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.”
====
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.
These objects come from other solar systems: we know how to determine their trajectory: could we envisage using them as messengers to transmit a message ?
What would be the value? The message travels very slowly compared to light. Yet to read the message requires a rendezvous with the ISO on a fast hyperbolic orbit.
I get the reference to the idea that an object moving relatively slowly can carry a lot more information than a transmitted signal, but I don’t see the value here. Now, if the ISO dropped off a copy of the message in each system it visited to be detected sometime in the future…
We can retrace their orbits and determine which direction they immediately came from, but there’s no way of knowing how long they have been flying around the galaxy. Their place of origin may now be lost. They (and we) may have already made 40 orbits around the Milky Way before finally encountering us. Their star of origin may have disappeared long ago, or been ejected out of the galaxy before our sun even formed. Its also possible that their original orbit may have been perturbed by some previous encounter, perhaps even several times. They may be visitors from other galaxies like the Magellanic Clouds or remnants of past galactic collisions or encounters.
However, if we can learn about their chemical composition, and compare it to their probable origin (disk, halo, nucleus, extra-galactic) as deduced from their orbital characteristics, we may be able to get some insight into our own galaxy’s history and evolution, or about conditions in other galaxies where they might have originated. For example, if one of these objects exhibits an extremely low fraction of metals, it would suggest it was formed early in the universe’s history when the universe was deficient in elements heavier than H or He.
What is highly unlikely is that we can “hitch a ride” on one of these guys and use them to transport us to distant destinations. Matching speeds with one of these objects and gently landing on it would require our being able to accelerate (and then slow down) to the same velocity they currently have, which means we would have no need to hitch a ride in the first place. With our current propulsive technology, I’m afraid only a collision or flyby mission is the only encounter that makes sense.
This reasoning is what leads me to believe these objects are extremely unlikely to be artifacts from other civilizations. Alien spacecraft would be traveling several orders of magnitude faster than the general galactic orbital drift (compared to the Local Standard of Rest) at our location. Our sun already is orbiting the galactic nucleus at about 250 km/sec. Any ETI probe would likely be going much faster than that.
Perhaps not so much a message such as a two-way conversation, but rather as a transmission of information preserving knowledge about a culture from one to another:
https://www.centauri-dreams.org/2008/08/22/rosetta-and-the-language-of-hope/
https://www.centauri-dreams.org/2013/01/18/the-last-pictures-contemporary-pessimism-and-hope-for-the-future/
Unlike an artifact that is easily discoverable over time, like a buried monolith, the presence of an ISO is extremely transient. If there is a message accompanying 3I/ATLAS, we will not be able to acquire it. Similarly, if Rama entered our system, we would not be able to explore it like the astronauts in Clarke’s “Rendezvous with Rama”. To be useful, ISOs as message carriers need to leave their messages in locations that can be found by a civilization sometime in its lifetime. Unless, like the lunar monolith/tetrahedron, the whole point is to only allow discovery by a sufficiently advanced spacefaring civilization.
My sense is that whether a benevolent galactic civilization or a predatory one, the optimal solution is to leave messages/technology in a place that can be discovered once that civilization becomes sufficiently advanced. Either the message helps the civilization, or it destroys them. (Like “Nuking them from orbit”, it is the only way to be sure, either way.)
It would be exciting if there are potential occultations predicted. With an object that small, you’d have to have amateurs all over half the planet looking up at the same instant, in the hope of nailing down the orbit with extraordinary precision for the benefit of a potential flyby mission.
Like TNG’s episode “Masks?”
Avi would like that :)
My thinking is that this thing may have plowed through one or two analogs of the Coalsack…with quite the crust—perhaps explaining a less than spectacular appearance?
For those wondering about the Star Trek: The Next Generation episode titled Masks, see here…
https://memory-alpha.fandom.com/wiki/Masks_(episode)
Of course there is Avi Loeb’s tentative analysis and commentary – https://arxiv.org/pdf/2507.05881. He concludes currently that 3I/Atlas is likely a comet but with interesting constitution and projected path through the solar system. At least if Avi’s existential concerns are accurate, we will know by Christmas.
It bears watching just how close it gets to Mars.
I wonder if Mars Reconnaissance Orbiter (MRO) or some of the other operational Mars orbiters could get images of our newest interstellar visitor and if they would be better than those from Earth?
There is a precedence for this…
https://mars.nasa.gov/comets/sidingspring/
Also see here:
https://www.universetoday.com/articles/spotting-new-interstellar-comet-c2025-n1-atlas
To quote:
A close pass near Mars on October 3rd is certainly intriguing, and assets like the Mars Reconnaissance Orbiter (MRO) could in principle be re-tasked to image the comet. For planetary and spacecraft prospects versus 3I/ATLAS, we see:
-Passes 0.2 AU from Mars on October 3rd. -Passes 1.8 AU from Earth on December 19th. -Passes 0.33 AU from Jupiter on March 26, 2026.
For spacecraft across the solar system, we see:
-Passes 0.3 AU from Psyche on September 4th. -Passes 0.4 AU from JUICE on November 4th.
https://astrobiology.com/2025/07/x-shooter-spectrum-of-comet-c-2025-n1-3i-atlas-insights-into-a-distant-interstellar-visitor.html
X-SHOOTER Spectrum Of Comet C/2025 N1 (3I/ATLAS): Insights Into A Distant Interstellar Visitor
By Keith Cowing
Status Report
astro-ph.EP
July 20, 2025
Spectrum of C/2025 N1 (3I/ATLAS) showing the wavelength ranges where the emissions should appear above the noise level. — astro-ph.EP
Comets are primitive remnants of the early Solar System whose composition offers fundamental clues to their formation and evolution. High-resolution, broad-wavelength spectroscopy is crucial for identifying volatile species and constraining the physical conditions within the coma.
We aim to characterize the gas composition and physical environment of the newly discovered comet C/2025 N1 through optical and near-infrared spectroscopy. We used a medium-resolution spectrum of comet C/2025 N1 with X-shooter at the ESO Very Large Telescope (VLT), covering the 300-2500 nm wavelength range. Standard data reduction and flux calibration were applied.
Although the object clearly shows activity, only upper limits to the production rates of OH and CN can be estimated: 8.0×1024 s−1 and 4.9×1023 s−1, respectively. We obtained red spectral slopes consistent with those of typical D-type asteroids and outer Solar System objects.
A. Alvarez-Candal, J. L. Rizos, L. M. Lara, P. Santos-Sanz, P. J. Gutierrez, J. L. Ortiz, N. Morales
Comments: 4 pages, 2 figures, submitted as a Letter to the Editor
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Astrophysics of Galaxies (astro-ph.GA)
Cite as: arXiv:2507.07312 [astro-ph.EP] (or arXiv:2507.07312v2 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2507.07312
Focus to learn more
Submission history
From: Alvaro Alvarez-Candal
[v1] Wed, 9 Jul 2025 22:18:50 UTC (249 KB)
[v2] Sat, 12 Jul 2025 20:58:05 UTC (249 KB)
https://arxiv.org/abs/2507.07312
Astrobiology
So at this point, nothing. We wait until it gets nearer the sun.
NSF-DOE Vera C. Rubin Observatory Observations of Interstellar Comet 3I/ATLAS (C/2025 N1)
We report on the observation and measurement of astrometry, photometry, morphology, and activity of the interstellar object 3I/ATLAS, also designated C/2025 N1 (ATLAS), with the NSF-DOE Vera C. Rubin Observatory. The third interstellar object, comet 3I/ATLAS, was first discovered on UT 2025 July 1. Serendipitously, the Rubin Observatory collected imaging in the area of the sky inhabited by the object during regular commissioning activities. We successfully recovered object detections from Rubin visits spanning UT 2025 June 21 (10 days before discovery) to UT 2025 July 7. Facilitated by Rubin’s high resolution and large aperture, we report on the detection of cometary activity as early as June 21st, and observe it throughout. We measure the location and magnitude of the object on 37 Rubin images in r, i, and z bands, with typical precision of about 20 mas (100 mas, systematic) and about 10 mmag, respectively. We use these to derive improved orbit solutions, and to show there is no detectable photometric variability on hourly timescales. We derive a V-band absolute magnitude of H_V = (13.7 +/- 0.2) mag, and an equivalent effective nucleus radius of around (5.6 +/- 0.7) km. These data represent the earliest observations of this object by a large (8-meter class) telescope reported to date, and illustrate the type of measurements (and discoveries) Rubin’s Legacy Survey of Space and Time (LSST) will begin to provide once operational later this year.
https://arxiv.org/abs/2507.13409
Godier’s YT channel (the universe in which we liiiiive guy) talked about an arxiv paper by A Yaginuma which claims it may be possible to get a Mars orbiter out to this comet.
Could we aim Juno at our third known interstellar visitor?
https://www.arxiv.org/pdf/2507.21402
https://astrobiology.com/2025/07/precovery-observations-of-3i-atlas-from-tess-suggests-possible-distant-activity.html
Precovery Observations of 3I/ATLAS from TESS Suggests Possible Distant Activity
By Keith Cowing
Status Report
astro-ph.GA
July 31, 2025
3I/ATLAS is the third macroscopic interstellar object detected traversing the Solar System. Since its initial discovery on UT 01 July 2025, hundreds of hours on a range of observational facilities have been dedicated to measure the physical properties of this object.
These observations have provided astrometry to refine the orbital solution, photometry to measure the color, a rotation period and secular light curve, and spectroscopy to characterize the composition of the coma. Here, we report precovery photometry of 3I/ATLAS as observed with NASA’s Transiting Exoplanet Survey Satellite (TESS).
3I/ATLAS was observed nearly continuously by TESS from UT 07 May 2025 to 02 June 2025. We use the shift-stack method to create deep stack images to recover the object. These composite images reveal that 3I/ATLAS has an average TESS magnitude of Tmag=19.6±0.1 and an absolute visual magnitude of HV=12.5±0.3, consistent with magnitudes reported in July 2025, suggesting that 3I/ATLAS may have been active out at ∼6.4 au. Additionally, we extract a ∼20 day light curve and find no statistically significant evidence of a nucleus rotation period.
Nevertheless, the data presented here are some of the earliest precovery images of 3I/ATLAS and may be used in conjunction with future observations to constrain the properties of our third interstellar interloper.
Adina D. Feinstein, John W. Noonan, Darryl Z. Seligman
Comments: 10 pages, 7 figures, submitted to AAS Journals. Data behind the figures can be found here:this https URL
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Astrophysics of Galaxies (astro-ph.GA)
Cite as: arXiv:2507.21967 [astro-ph.EP] (or arXiv:2507.21967v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2507.21967
Focus to learn more
Submission history
From: Adina Feinstein
[v1] Tue, 29 Jul 2025 16:16:07 UTC (3,936 KB)
https://arxiv.org/abs/2507.21967
Astrobiology