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Avi Loeb Medium
The latest image of a fading 3I/ATLAS on March 11, 2026 at 19:22:54 UTC, based on ten 120 second, R-band exposures with a 25-centimeter telescope and an angular resolution of 1.38 arcseconds per pixel. Jupiter is outside the field of view. (Image Credit: Toni Scarmato)The interstellar object 3I/ATLAS is fading away. It will get closest to Jupiter on March 16, 2026 and then head out of the Solar System in a nearly symmetric fashion to the way it came in. I say `nearly symmetric’ because 3I/ATLAS exhibited a small non-gravitational acceleration, owing to a remarkable system of jets.
The passage of 3I/ATLAS on a retrograde orbit within 5 degrees of the ecliptic plane provided an ideal opportunity for a spacecraft to intercept its path, take a close-up photograph, collect a sample from it, or even plant a capsule full of technology or life-as-we-know-it in its belly and hitchhike it to interstellar space at 60 kilometers per second — twice as fast as our fastest rockets. We missed all of this rare opportunity.
We should aim to do better in our next encounter with a mysterious interstellar object. But given the rare properties of 3I/ATLAS, it is unclear whether we will be given such an opportunity any time soon. So long, our interstellar friend.
The nature of 3I/ATLAS is intriguing because of the following 22 anomalies:
Mass Budget Discrepancy:
1. The inferred nucleus diameter of 2.6 kilometers and number density of its parent population (assuming a natural comet) exceed the mass reservoir of planetary disks around low-metallicity stars (as inferred here and here) by orders of magnitude. More details are available here.
Geometric Rarities:
2. The retrograde trajectory of 3I/ATLAS is aligned to within 5 degrees with the orbital plane of the planets around the Sun, with a probability of 0.2% (as discussed here). The Milky-Way disk is misaligned with the ecliptic plane by about 60 degrees. This suggests that the trajectory of 3I/ATLAS may have been planned.
3. The arrival time of 3I/ATLAS was fine-tuned to bring it to minimum distances of 29 and 54 million kilometers from Mars and Jupiter, respectively, and be unobservable from Earth at perihelion (as discussed here).
4. The perijove distance of 3I/ATLAS during its encounter with Jupiter on March 16, 2026 is 53.6 million kilometers, is very close to Jupiter’s Hill radius, 53.5 million kilometers (as discussed here).
5. Analysis of the Hubble Space Telescope image of 3I/ATLAS from July 21, 2025 (as discussed here) suggests that the anti-tail before perihelion must have been in the form of a collimated jet towards the Sun that is about ten times longer than it is wide. This is similar to the collimation observed in post-perihelion images out to several hundred thousand kilometers. No known comet exhibited a physical sunward jet of this length that is not a perspective effect. For a technological object, a beam of particles might be used to block the solar wind from impacting the nucleus surface at a relative speed of order 500 kilometers per second. In addition, the veil of dust around 3I/ATLAS is just of the right column needed to block sunlight from hitting the nucleus surface (as discussed here).
6. At large distances, the initial rotation axis of 3I/ATLAS was aligned to within 8 degrees with the sunward direction when it entered the solar system (as reported here). The probability for that is 0.5%.
7. The observed wobble of the pre-perihelion anti-tail jet in the direction of the Sun (as reported here during July and August 2025) requires the base of the jet to be within 8 degrees from the sun-facing pole, with a probability of 0.5%.
8. The existence of a prominent anti-tail jet towards the Sun on the way of 3I/ATLAS out of the solar system requires a similar coincidence near the opposite pole of the rotation axis (as discussed here). The fact that a collimated jet appears as the sunward anti-tail both before and after perihelion (while reversing direction at perihelion relative to the direction of motion), has a tiny probability of occurring at random, equal to the square of 0.5% or 0.000025.
9. The launch base of the post-perihelion anti-tail jet resided on the nightside of 3I/ATLAS before perihelion and the base of the pre-perihelion anti-tail jet is now on the nightside of 3I/ATLAS after perihelion. For these bases to be active only when facing the Sun, they must be well insulated on the nightside for a period longer than several months. Heat naturally flows by conduction throughout the body of a natural comet, making this insulation requirement difficult to satisfy.
10. The gravitational deflection of 3I/ATLAS by 16 degrees at perihelion (as discussed here), is exactly twice the opening angle of the anti-tail before perihelion. This coincidence allows the wobbling jet around the rotation axis to generate an anti-tail in the direction of the Sun before perihelion and a counter jet on the opposite pole after perihelion, with a jet opening angle of 8 degrees on both poles.
11. On January 22, 2026, 3I/ATLAS aligned with the Sun-Earth axis to within an extraordinarily small angle of 0.69 degrees (as discussed here). At that time its anti-tail pointed at Earth.
Possible techno-signatures:
12. 3I/ATLAS arrived from a direction coincident with the radio “Wow! Signal” to within 9 degrees, with a probability of 0.6% (as discussed here).
13. Processing of 40 Hubble Space Telescope Images from November 2025 to February 2026 by the Larson-Sekanina filter — which removes the circularly-symmetric glow round the nucleus, reveals a system of three mini-jets which are symmetrically separated by 120 degrees from each other (as discussed here and here). Are these symmetric jets coming the sublimation of pockets of ice on a rock or technological thrusters?
14. The non-gravitational acceleration of 3I/ATLAS was not directed away from the Sun but had a substantial sideways component (as discussed here).
Composition Anomalies:
15. Before perihelion, the gas plume surrounding 3I/ATLAS contained 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 for thousands of known comets, including 2I/Borisov (as reported here).
16. The anti-tail penetrated hundreds of thousands of kilometers through the solar wind and the solar radiation. In order not to be stopped, the dust particles must be much larger than common sub-micron particles of interstellar dust (as discussed here). However, if the particles are bigger than a millimeter, then they must carry an untenable amount of mass in order to account for 99% of the scattered sunlight around 3I/ATLAS, as observed in the Hubble images.
17. Data from the SPHEREx space observatory indicated the existence of icy fragments around 3I/ATLAS before perihelion (as reported here in August 2025). However, the spectral signature of ice disappeared in the SPHEREx data after perihelion, taken during December 2025 (as reported here), when abundant organic molecules in gas phase, such as CH3OH, H2CO, CH4, and C2H6, were discovered, along with an enhancement by a factor of ~20 in the water production rate. To survive cosmic-ray bombardment along an interstellar journey that lasted billions of years, these organic molecules must have been buried under a thick layer of material, at least 10-meters in depth.
18. The isotope abundances of hydrogen (D/H) and carbon (12C/13C) are markedly different from Solar System comets and suggest an rare birth environment with a freezing temperature of less than 30 degrees Kelvin and a low metallicity (as discussed here).
Possible Bio-Signature:
19. The volatility of methane (CH4) is between that of carbon dioxide (CO2) and carbon monoxide (CO), yet methane was detected in the gas plume around 3I/ATLAS only after perihelion, while CO2 and CO were detected long before perihelion. This puzzle (discussed here) suggest a strange layered composition of the nucleus. Methane and other organic molecules are known bio-markers. Does 3I/ATLAS carry life on it (as discussed here)?
Unusually High Mass, Speed, Polarization and Perihelion Brightening:
20. The nucleus of 3I/ATLAS is more massive than 1I/`Oumuamua and 2I/Borisov, while moving faster than both (as discussed here and here).
21. 3I/ATLAS shows extreme negative polarization, unprecedented for all known comets, including 2I/Borisov (as discussed here and here). This unusual polarization may be related to its unusual anti-tail.
22. Near perihelion, 3I/ATLAS brightened faster than any known comet and was bluer than the Sun (as discussed here).
***
Given these 22 anomalies, the nature of 3I/ATLAS remains mysterious. Even if 3I/ATLAS is a natural comet, there are some fundamental aspects of its physical properties and birth environment that we do not understand. Many of the above anomalies, like its near alignment with ecliptic plane or the “Wow! Signal”, will be treated by comet experts as chance coincidences. But others, like the mass budget discrepancy, the prominent anti-tail jet or the symmetric jet structure must be explained by physical models.
It is easy to insist that 3I/ATLAS is a natural comet while ignoring these anomalies. However, it is the responsibility of scientists and NASA officials to acknowledge the existence of unexplained puzzles rather than display the arrogance of expertise while ignoring them.
When 3I/ATLAS was discovered in July 2025, I gave it a rank of 4 on the Loeb Classification scale of interstellar objects (quantified here, here and here), where 0 means a natural comet and 10 means alien technology that poses a major threat to humanity. Given all that we have learned so far and assuming that nothing unusual will occur near Jupiter, I regard 3I/ATLAS — which showed cometary activity, to be only slightly less anomalous than 1I/`Oumuamua — which had no visible cometary activity and a larger non-gravitational acceleration.
Science is fun as long as we treat life as a learning experience.
ABOUT THE AUTHOR
(Image Credit: Chris Michel, National Academy of Sciences, 2023)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.
Professional website:
https://lweb.cfa.harvard.edu/~loeb/
Social media:
https://avi-loeb.medium.com/
https://www.youtube.com/@ProfessorAviLoeb
https://open.spotify.com/show/1zhndXkvSY2b8FdjspFpCd
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