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Reports from the Minor Planet Center (here) and NASA’s Jet Propulsion Laboratory (here) just released new data on the interstellar object 3I/ATLAS following its perihelion passage on October 29, 2025. The new data covers the period between October 31 and November 4.
When compared to previous data obtained on October 5–8, it appears that 3I/ATLAS brightened by a factor of ~5 in the Green-band which is centered at a wavelength of 0.464 micrometers.
The value of the non-gravitational acceleration was reduced by a third relative to the value reported on October 29. This reduction is within the uncertainties. The net detection is now more robust, standing at a level of 3.7 standard deviations.
The non-gravitational acceleration was measured at the current perihelion distance of 1.38 times the Earth-Sun separation (defined as an astronomical unit or `au’), equivalent to 206 million kilometers from the Sun. It has two components in the orbital plane of 3I/ATLAS but no detectable component vertical to that plane. The measured components — parametrized to follow an inverse square law as a function of distance from the Sun, have the following values — normalized at a heliocentric value of 1 au:
1. A radial acceleration away from the Sun of 1.1x10^{-6} au per day squared.
2. A transverse acceleration relative to the Sun’s direction of 3.7x10^{-7} au per day squared.
Based on momentum conservation (as discussed here), it is straightforward to show that 3I/ATLAS must have lost a measurable fraction of its mass in order to gain this non-gravitational acceleration through the rocket effect. As I derived here, the mass fraction lost during the perihelion passage time t equals: t*(a/v), where v is the ejection speed of gas from the nucleus surface and a is the non-gravitational-acceleration displayed by 3I/ATLAS.
The perihelion passage time is the ratio of the perihelion distance of 203 million kilometers and the perihelion speed of 68 kilometers per second, yielding t~1 month. The reported non-gravitational acceleration amounts to 94 kilometers per day squared at perihelion. These values combine to imply that 3I/ATLAS lost a fraction of its mass equal to:
~13% divided by v in units of 300 meters per second,
where a value of v~300 meters per second corresponds to the characteristic thermal speed of molecules at the surface temperature of 3I/ATLAS near perihelion. This ejection speed would be the maximum expected value for a natural comet, thus implying that 3I/ATLAS must have lost more than 13% of its mass near perihelion in the natural scenario. However, a technological rocket engine could expel gas through its exhaust at a much higher speed, thus reducing the required fraction of the mass loss. The considerable brightening and blue color of 3I/ATLAS near perihelion (as reported here) could either be a signature of cometary mass loss and CO+ emission (as discussed here) or a hot engine with a much smaller mass loss.
This contrast offers a clean test of the nature of 3I/ATLAS in the coming weeks. If 3I/ATLAS is a natural comet, it should be surrounded by a massive cloud of gas that carries at least 13% of the original nucleus mass. This cloud must be much more than evident during July through September, when 3I/ATLAS did not display any non-gravitational acceleration (as derived by the analysis of 4,022 data points from 227 observatories here). Spectroscopic observations of this new massive coma by the Webb telescope in December would allow us to infer the composition of the interior of 3I/ATLAS and not just its skin.
However, if 3I/ATLAS is not enshrouded in a much more massive gas cloud after perihelion than it had in the months preceding perihelion, then its recent non-gravitational acceleration must have resulted from a different cause than cometary evaporation.
The anomalies displayed so far by 3I/ATLAS include:
1. Its retrograde trajectory is aligned to within 5 degrees with the ecliptic plane of the planets around the Sun, with a likelihood of 0.2% (see here).
2. During July and August 2025, it displayed a sunward jet (anti-tail) that is not an optical illusion from geometric perspective, unlike familiar comets (see here).
3. Its nucleus is about a million times more massive than 1I/`Oumuamua and a thousand times more massive than 2I/Borisov, while moving faster than both, altogether with a likelihood of less than 0.1% (see here and here).
4. Its arrival time was fine-tuned to bring it within tens of millions of kilometers from Mars, Venus and Jupiter and be unobservable from Earth at perihelion, with a likelihood of 0.005% (see here).
5. Its gas plume contains much more nickel than iron (as found in industrially-produced nickel alloys) and a nickel to cyanide ratio that is orders of magnitude larger than that of all known comets, including 2I/Borisov, with a likelihood below 1% (see here).
6. Its gas plume contains only 4% water by mass, a primary constituent of familiar comets (see here).
7. It shows extreme negative polarization, unprecedented for all known comets, including 2I/Borisov, with a likelihood below 1% (see here).
8. It arrived from a direction coincident with the radio “Wow! Signal” to within 9 degrees, with a likelihood of 0.6% (see here).
9. Near perihelion, it brightened faster than any known comet and was bluer than the Sun (see here).
10. It exhibits non-gravitational acceleration which requires massive evaporation of a sixth of its mass (as calculated here), but preliminary post-perihelion images do not show evidence for it so far.
This morning, I received the following email all the way from Australia, down under:
“Dear Professor Loeb,
I am writing to you to say I have been really enjoying your writings and posts concerning 3I/ATLAS! But I am not writing to you about 3I/ATLAS or any of its several anomalies observed thus far — I am sure you must be regularly receiving a plethora of communications from various quarters on that topic! Rather I am writing to you to express my strong solidarity with your position on the current state of scientific thought, which I must say is completely aligned with mine. I am neither an astronomer nor a physicist, but I do like to study about and ponder upon all developments in science, cutting across discipline boundaries including my own, which happens to be soft computing applications in managerial decision-making. This spirit of wondrous curiosity is what has drawn me to know more about 3I/ATLAS and the astronomical observations/conjectures that have been snowballing around this event over the past few months.
What I find in your views on the topic, of course only as far as I can follow given my rather rudimentary understanding of the background theories and models, is a very refreshing and bold move to rise above the imposed walls within which current scientific thinking seems to relish staying confined. I think these walls have unfortunately been imposed, and keep getting imposed, not necessarily via any consensus of the global scientific community at large but rather by some well-endowed core groups and lobbies within that community, often with interests that are perhaps not exclusively of an intellectual kind. In my upcoming book titled ‘Brainmaker — coevolution of human and synthetic intelligence’, I have drawn sort of an analogical parallel between the ‘scientific power lobbies’ of today and the church of the Middle Ages. It is of course well known that scientific thinkers and astronomers like Galileo who propounded alternative viewpoints (eventually proved correct) were severely persecuted as their viewpoints flew in the face of those held by the church back at the time. I think (and fear) that science is increasingly being positioned as kind of an organized religion and scientific power lobbies have started to resemble the high priesthoods of ancient temples. If dissenting ideas are to sprout outside of what is deemed mainstream by those lobbies, they tend to be met with a resistance ironically comparable in their destructive ferocity to what proponents of scientific knowledge like Galileo had faced from the church. Perhaps the only real difference is the substitution of physical persecution with intellectual persecution — systematically belittling radical ideas (and their propagators) often via ad hominem and straw man attacks and calling for an intellectual banishment from the so-called mainstream.
That you seem to be always able to thwart all such attacks, veiled or direct, with remarkable poise and grace as well as rigorous intellectual rebuttal, is what I have come to greatly respect and admire. While I do not of course consider myself at par with your intellectual stature (not even close), it will be a matter of great honor to hear from you and perhaps even have a chance someday to exchange views with you on this rather unfortunate current state of science and its potential impact on the future of our species.
I thank you for your valuable time in reading my mail (among the hundreds that you must be receiving and reading on a regular basis) and will be elated and honored if you kindly do reply.
Sincerely,
Sukanto
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Dr Sukanto Bhattacharya
Deakin University
Victoria 3220, Australia
ABOUT THE AUTHOR
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Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021. The paperback edition of his new book, titled “Interstellar”, was published in August 2024.
