Three remarkable facts about 3I/ATLAS are that it passed closest to the Sun on October 29, 2025, just 8 days after solar conjunction relative to Earth on October 21 and 26 days after crossing the ecliptic plane on October 3, 2025 — when it was closest to Mars. Whether this confluence of orbital coincidences might be a signature of design or not, it offers a remarkable opportunity to observe the effect of gravitational lensing of 3I/ATLAS by the Sun. Here are the details.
According to Einstein’s theory of gravity, which I am about to teach in my class at the Harvard Astronomy department tomorrow, the light from 3I/ATLAS is deflected by an angle:
(4GM/c²)*[D_ds/(A*D_s*D_d)]
where G is Newton’s constant, M is the mass of the Sun, c is the speed of light, A is the angular separation of the image of 3I/ATLAS from the center of the Sun and D_ds, D_s, D_d are the distances between the Sun and 3I/ATLAS (deflector-source), the Earth and 3I/ATLAS (source), and the Sun from Earth (deflector). Substituting the values of D_d=1 au (where `au’=astronomical unit), D_ds=1.36 au and D_s=2.36 au at the perihelion of 3I/ATLAS, I get that the deflection angle equals:
0.27 arcsecond/(A/1 degree)
The derived value for the deflection angle is measurable by astronomical telescopes.
On October 29, 2025, a non-gravitational acceleration of 3I/ATLAS near perihelion was reported here and discussed here. The related data was obtained by the Atacama Large Millimeter Array (ALMA), which indicated that 3I/ATLAS was 4 arcseconds away in Right Ascension from where it was supposed to be near perihelion according to its forecasted trajectory. The gravitational lensing by the Sun amounts to a small fraction of the reported deviation. However, it would be exciting to analyze the ALMA data and check whether it shows evidence for the small effect of gravitational lensing by the Sun.
Observations of 3I/ATLAS close to perihelion by the Solar observatories STEREO, SOHO and GOES-19 (as discussed here and reported here), revealed unprecedented brightening and a color bluer than the Sun. Adding that to the list of anomalies displayed by 3I/ATLAS brings the total tally to 9, as follows:
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).
If these anomalies are all associated with a natural comet, then the non-gravitational acceleration of 3I/ATLAS must have resulted from it losing at least 15% of its mass, as I calculated here. This should have resulted in a massive debris cloud around 3I/ATLAS, carrying over 5 billion tons of gas. It would be impossible to miss this massive cloud in upcoming observations during November and December, as 3I/ATLAS comes closest to Earth on December 19, 2025. If such a massive cloud is not observed, then the 10th anomaly of 3I/ATLAS would be the display of non-gravitational acceleration without the required massive coma.
Out of the hundreds of emails I received from around the world yesterday, one carried a particularly uplifting message:
“Dr. Loeb,
My name is Dustin Collier. I’m a civil rights trial attorney based out of the San Francisco Bay Area, representing victims of workplace discrimination, harassment, and retaliation all up and down the West coast.
I also hold a bachelor’s degree in philosophy. I heard you mention on JRE (here) that you originally wanted to be a philosopher, before getting drawn into astrophysics through a program you were selected for at Princeton, and now you attempt to apply your scientific prowess to philosophical inquiries.
The word philosophy derives from “philos” and “sophos,” meaning “love” and “knowledge,” respectively. Any true philosopher has a thirst for knowledge across all fields, and they really encourage philosophy students to broaden their horizons with interests in other fields. Towards that end, during my philosophy studies, I also took upper division courses in political science, macroeconomics, and psychology, to name a few.
But one of my oldest passions has always been science. Every single day, I wind down by watching an hour or two of PBS Space Time on YouTube, trying to wrap my head around quantum mechanics, astrophysics, general and special relativity, and the like. It is also so fascinating and, for the genuine “lover of knowledge,” it is an endless stream of wonder, mystery, and pioneering research. At times I fantasize about an alternate timeline where I became a physicist myself.
One of my favorite undergraduate classes was “Philosophy of Science,” where we discussed the various paradigm shifts that have occurred throughout the history of science (often initiated by philosophers dabbling in science on the side). Suffice to say, those shifts have never occurred as a result of a person refusing to acknowledge anomalies and insisting on dogmatic existing explanations for new phenomena that warrant reconsideration of that very same dogma.
All of this is background to tell you just how much I enjoy your work, your writing, and your fearless approach to scientific inquiry. These last several weeks, I have found myself engaged in more than one fruitless debate with your detractors online — purported scientists who brutally attack your integrity and your hypotheses without bother to test them, analyze them, or provide natural explanations for the phenomena you are so actively attempting to resolve. There’s also this clear pattern of mischaracterizing what you say to make it seem as though you have bet your entire professional reputation on us making first contact, when in truth you have clearly stated time and again that more data is needed and you merely think the possibility is substantial enough and the risks potentially catastrophic enough (“black swan”) to warrant an open mind and continued investigation.
In philosophy, we learn that appeals to authority, popular opinion, and ad hominem attacks are all logical fallacies to be disregarded. What matters is data, logic, and experimentation by those willing to be proven wrong. You so thoroughly exemplify the proper approach to the scientific method, and your critics so thoroughly exemplify the opposite, that I have found it discouraging and something of a metaphor for the state of our crumbling societal discourse writ large.
But you give me hope. I am writing today because I want you to know what an inspiration you are to so many people who you no doubt have never thought about and don’t even realize they are following your work, including but by no means limited to myself. Please don’t let the naysayers slow you down one bit, though I know from your public statements that this isn’t in your DNA anyways.
And if you ever need a lawyer to threaten your detractors who are bordering more on defamation than legitimate criticism or feedback, I would be happy to assist you with that on a pro bono basis in my spare time. I sincerely hope you never need that assistance.
Whatever happens over the next three months, thank you for reminding me that logical and good-hearted people still exist. I share your optimism that any non-human intelligence we may discover in the future is likely to have resolved many of the problems presently facing humanity (e.g., war, climate crisis, etc.) and we will have much to learn from them. For that reason alone, your work is vitally important in a way your critics don’t appear to appreciate, but I do.”
ABOUT THE AUTHOR
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.