Is the Observed Spin Reversal of the Comet 41P/Tuttle-Giacobini-Kresák, a Technological Signature?
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By Avi Loeb Medium
(Image credit: Futurism)Interstellar objects are identified by their positive energy relative to the Sun. This is not a metaphor but rather a physical characteristic, formulated as: E > 0. It means that interstellar objects move faster than the escape speed from the Solar System, which is dominated by the Sun’s gravity.
Far from the Sun, their energy is purely in the form of kinetic energy (per unit mass):
E = (1/2) U²
where U is their velocity in interstellar space. Since energy is conserved under the Sun’s gravity, their local velocity v evolves as a function of their changing distance r from the Sun according to the relation:
E = -GM/r + (1/2) v²
Altogether, in the presence of gravity:
v² = U² + 2GM/r
The local escape speed v_e at a distance r from the Sun is defined as the value of v for U=0, namely:
v_e² = 2GM/r
The Solar escape speed v_e represents the minimum speed needed to carry an object out of the Solar System. Any object observed moving faster than v_e at a heliocentric distance r is flagged as interstellar in origin. At the orbital radius of Earth, the escape speed is 42.1 kilometers per second.
However, the situation changes under the action of a non-gravitational force, such as the rocket effect from outgassing. Let us restrict our attention to the simple case of a non-gravitational acceleration, A[r], that scales similarly to gravity as 1/r² (as was the case for 1I/`Oumuamua or 3I/ATLAS) and always points opposite to the object’s velocity, namely away from the Sun before perihelion. In this case, the energy E of the interstellar object will be reduced by A*r as the object arrives from interstellar space to a distance r from the Sun. The reduction in energy results from the fact that this non-gravitational acceleration allows down the object and reduces its positive kinetic energy. The effect is equivalent to pumping the breaks on a vehicle.
If the energy change exceeds the positive value E that the interstellar object possessed to start with, then the net energy value will turn negative and the object will become gravitationally bound to the Sun.
The condition for an interstellar object to be trapped by the Sun owing to its non-gravitational acceleration is A*r > (1/2) U², or equivalently:
A > U²/2r
This can be compared to the gravitational acceleration at a distance r from the Sun, g=(GM/r²) = (v_e^/2r).
The above requirement for trapping an interstellar object in the Solar System is therefore:
A/g > (U/v_e)²
Sublimation of ice by sunlight typically results in outgassing limited by the thermal speed of a few hundred meters per second, which is a hundred times slower than v_e at the Earth-Sun separation. This means that the resulting non-gravitational acceleration of natural icebergs near Earth can only reach values as small as: A/g < (0.01)²=0.0001.
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Consider the example of the interstellar object 3I/ATLAS which entered the Solar System with an interstellar speed of U = 58 kilometers per second. The escape speed at its perihelion distance of 1.36 times the Earth-Sun separation is v_e = 36 kilometers per second. In order for 3I/ATLAS to slow down enough and stay in the Solar System, the object had to break with a non-gravitational acceleration that is larger than the gravitational acceleration by a factor of
A/g > (58/36)² = 2.6
The actual non-gravitational acceleration that was measured for 3I/ATLAS, as discussed in the recent paper I co-authored with Valentin Thoss and Andi Burkert here, is merely:
A/g ~ 0.0001
Clearly, 3I/ATLAS did not slow down at the level needed for it to stay in the Solar System. The required threshold for staying of A/g > 2.6 applies to any fragments released by 3I/ATLAS, since the acceleration condition for any object to stay in the Solar System does not depend on the object’s mass.
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Today, the NSF-DOE Rubin Observatory released its data preview here. The full Rubin database over the coming decade is expected to reveal dozens of new interstellar objects. If any of them appears to slow down enough to become gravitational bound to the Solar System, this breaking should be regarded as a strong enough technological signature to elevate its rank close to 10 on the Loeb Classification Scale of interstellar objects (as discussed here, here and here).
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
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Are the Three Mini-Jets, Coming Out of 3I/ATLAS at 120 Degree Separation, a Technological Signature?By Avi Loeb Medium
Brightness maps of the projected jet structure within a distance of 25,000 kilometers from 3I/ATLAS, observed on November 30, December 4, 12, 27 and January 7, 14, 2026 by the Hubble Space Telescope. The image was processed through a Larson-Sekanina Rotational Gradient filter, which removes the circularly symmetric glow around the nucleus. On all these dates, 3I/ATLAS displayed a rotating system of three mini-jets which are equally separated by an angle of 120 degrees from each other. The bottom panel shows the 3 mini-jets along with the 10-times longer anti-tail jet in the sunward direction, as observed on January 14, 2026. (Image credit: Toni Scarmato, based on data released by NASA/ESA/STScI here)Shortly after its perihelion passage on October 29, 2025, the interstellar object 3I/ATLAS was imaged by amateur astronomers to have multiple jets coming out of it (as I discussed here).
The jet structure is best revealed in projected images by applying the Larson-Sekanina Rotational Gradient filter that removes the circularly symmetric glowing halo around the nucleus of 3I/ATLAS.
Applying this filter to 6 images of 3I/ATLAS taken by the Hubble Space Telescope on January 14, 2026 (available here), revealed a symmetric system of 3 mini-jets out to 25,000 kilometers. The 3 mini-jets are separated by 120 degrees from each other in sky projection, and are supplemented by a 4th anti-tail jet extending 10 times farther in the sunward direction.
The 3 symmetric jets appear persistently in the Larson-Sekanina filtering of 24 other images taken by the Hubble Space Telescopes on November 30, December 4, 12, 27, 2025 and January 7, 14, 2026 (as listed here).
Analysis of these images was presented in a new paper that I co-authored with Toni Scarmato , which was posted today here. It demonstrates that the jet structure around 3I/ATLAS wobbles periodically by +/-20 degrees over a period of 7.1 hours. The wobble is likely the result of rotation, with each jet behaving like a misaligned beam from a rotating lighthouse. The rotation axis is oriented surprisingly close (within 10–20 degrees) to the sunward direction, as already inferred back in August 2025 (and reported here).
In another paper that I co-authored with Mauro Barbieri last week (available here), we had shown that on January 22, 2026, the observing direction of 3I/ATLAS from Earth will align to within 0.69 degrees with the anti-sunward direction. During that rare alignment, the rotating configuration of 3 mini-jets might trace a circle in the sky.
My brilliant colleague, Dr. Frank Laukien, noted: “The symmetric 120-degrees angular spacing between the 3 mini-jets is astounding! It is hard to imagine such a symmetry in a natural macroscopic object. A key question is: are three symmetric mini-jets the minimum viable configuration of propulsion devices to re-orient an object in three-dimensional space? Human-made satellite thrusters usually come in pairs on opposing directions. Could the mini-jet geometric arrangement be an indication of active technology? For orthogonal thruster systems, one would need 6 thrusters, with two opposing thrusters along each of the three orthogonal spatial directions. If 3I/ATLAS rotates around its major axis every 7.1 hours or so, then thrusters in the direction of the major axis would not be very useful, and perhaps just (3+1)=4 thrusters in the plane orthogonal to the major rotation axis are sufficient for some sort of technical manipulation like attitude adjustments?”
Indeed, we observe a system of 3 symmetric mini-jets plus 1 major jet. Additional data might help to address Frank’s question. The symmetric jet structure is just one out of 18 anomalies of 3I/ATLAS, as listed here. Identifying one of them as a clear technological signature would boost the rank of 3I/ATLAS high on the Loeb Classification Scale, formulated here, here and here.
Scientific work requires patience, since new knowledge comes one day at a time. Life brings joy when each day teaches us something new. My morning jog at sunrise today was decorated by symmetric fresh snowflakes. There is nothing more invigorating than the beauty of symmetry in nature, both on Earth and in the sky.
A snow-decorated tree along a jogging path at sunrise. (Image credit: Avi Loeb, January 19, 2026)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.
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By Avi Loeb Medium
If 3I/ATLAS is a Comet, Why Would the CIA “Neither Deny, Nor Confirm” the Existence of Records on It?
An image of 3I/ATLAS at 3:34 UTC on December 30, 2025 from nine 120-second exposures, taken with the Virtual Telescope Project facility in Manciano, Italy. A prominent anti-tail jet is clearly observed in the direction of the Sun, towards the lower left. (Image credit: Virtual Telescope)So far, 3I/ATLAS displayed a number of unexplained features relative to familiar comets — as I listed here. One could have hoped that these puzzles would trigger a healthy scientific debate about the nature of 3I/ATLAS. Instead, the case was officially closed by NASA officials without any discussion on the puzzling nature of these anomalies.
The anomalous features include the existence of a prominent anti-tail jet directed at the Sun both before and after perihelion, the geometric alignment to within 8 degrees between the rotation axis of 3I/ATLAS at large distances and the sunward direction, the alignment to within 5 degrees of the orbital plane of 3I/ATLAS with the ecliptic plane, as well as the prominence of nickel relative to iron in the gas that 3I/ATLAS sheds — reminiscent of industrially-produced nickel alloys.
On December 31, 2025, the Central Intelligence Agency (CIA) replied to a Freedom of Information Act (FOIA) inquiry by John Greenewald Jr., that it can “neither deny nor confirm the existence or nonexistence of records” related to 3I/ATLAS. The response was tweeted here.
That this information is treated as sensitive enough to be classified by the CIA is surprising, given that NASA officials stated decisively at a press conference on November 19, 2025 (posted here), that 3I/ATLAS is definitely a comet of natural origin. If this conclusion was clear all along to everyone within government and academia — as NASA officials presented the case, then why would the CIA treat the possible existence of records dealing with a natural comet as sensitive enough to be classified?
The simplest interpretation of the CIA response to the FOIA request is that some government officials wished to verify that 3I/ATLAS is not a black swan event, posing a potential threat to society even as comet experts regard such a possibility as highly unlikely. By multiplying a small probability for the existence of a threat with its high impact factor on society, the sober conclusion is that such events must be taken seriously and monitored. This lesson, akin to Pascal’s Wager, was learned the hard way by the residents of the City of Troy after they welcomed the Trojan Horse, as well as by intelligence agencies like the CIA after their miscalculations in gauging the risk from the events that unfolded on September 11, 2001 and October 7, 2023.
Under such circumstances, NASA officials were encouraged to deliver the likely scientific interpretation, while at the same time — the serious consideration of a black swan event by the CIA was hidden from public view in order to prevent panic from taking hold for no good reason. This is a wise policy for mitigating societal unrest or instability of financial markets at a time when the reality of a black swan event is still regarded as highly unlikely. In order to maintain the public’s trust, the CIA would prefer not to raise a false alarm — like the shepherd who repeatedly shouted “Wolf!” — so when a real wolf came, no one believed him. The “neither deny nor confirm” is the best way to keep the investigation of black swan events hidden from view.
If the above interpretation is correct, 3I/ATLAS represents the first case of an astronomical object for which the “neither deny nor confirm” response was adopted purposely by the intelligence agencies.
During the summer months of 2025, I proposed the “Loeb Classification Scale” for the threat level of interstellar objects as alien tech. The scale was quantified in three peer-reviewed papers that I co-authored with Omer Eldadi, Gershon Tennenbaum and Oem Trivedi, which were first posted here, here and here. During the same time, I alerted policy makers to the possibility that 3I/ATLAS might be a black swan event, as mentioned here. Perhaps someone was listening to my request.
Today, several reporters asked me for comments on the latest report (accessible here) on limits of technological radio transmission by 3I/ATLAS, based on data from the Green Bank Telescope.
I clarified to the reporters that I encouraged the research team of the Breakthrough Listen Initiative to check for radio signals from 3I/ATLAS, because the arrival direction of 3I/ATLAS into the solar system aligned with the direction of the “Wow! Signal” from 1977 to within 9 degrees — with a chance probability of 0.6% as discussed here. The new data rules out a technological signal in the radio frequency band of 1–12 GHz towards Earth over a period of 5 hours between 04:15–09:15 UTC on December 18, 2025.
It is unclear whether one should expect a technological object to transmit radio signals to its senders. Such signals would take several tens of thousands of years to cross the Milky-Way galaxy, whereas the time that 3I/ATLAS spends inside the solar system — all the way from and to the outer edge of the Oort Cloud, is only 16,000 years. Interstellar objects are unlikely to continuously transmit to their senders, because they do not have enough time to benefit from a two-way dialog. Given that their journey takes billions of years, the chance of a transmission on any particular 5-hour interval is miniscule. Moreover, such a signal may not be transmitted in the direction of Earth or at the frequency band that was monitored. To save energy, an efficient signaling would be beamed and compressed into a short burst with a small duty cycle. A proper observing program would have monitored 3I/ATLAS from many directions for a long period of time and not just over a 5-hour window.
The data collection on 3I/ATLAS will not be over until it passes near the Hill radius of Jupiter. Hopefully, the curious community of government officials will be swayed by the dogmatic community of comet experts so as to check whether 3I/ATLAS deploys any small probes as new satellites of Jupiter. Unless we check, we might never know if this swan is white or black.
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.
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By Avi Loeb Medium
If 3I/ATLAS is a Comet, then Its Anti-Tail Jet Should Not Include Streaming Gas Beyond ~5,000 kilometers
False-color image of 3I/ATLAS (top panel), taken on December 27, 2025 by a 0.2-meter telescope in Belgium. The field of view is 14.4 by 23.3 arcminutes, corresponding to 1.1 by 1.8 million kilometers. Another image from the same telescope on December 19, 2025 which was processed through the Larson-Sekanina rotational gradient filter (bottom panel), shows a prominent anti-tail jet towards the Sun, as indicated by the yellow line in the inset. (Image credit: Alfons Diepvens)During the past two months after perihelion, the anti-tail jet from the interstellar object 3I/ATLAS was observed to extend out to a distance of several hundred million kilometers towards the Sun. In a previous calculation reported here and here, I showed that dust particles with a radius of order 10 micron could be dragged to a velocity of hundreds of meters per second by gas near their launch base and then reach a scale of several hundred million kilometers before being decelerated by solar radiation pressure. Will the outflowing gas accompany these dust particles out to the same distance or get pushed back more forcefully?
Whereas the main pushback on dust stems from radiation pressure by sunlight, the main pushback on gas originates from the solar wind. Before being slowed-down, the mass density of the outflowing gas in the jet, D_j, declines inversely with the square of distance, d:
D_j=Mdot/(2*pi*V*d²),
where Mdot and V are the mass loss rate and velocity of the sunward jet. The total mass loss was estimated from data obtained by the Webb telescope before perihelion (as reported here) at a value of Mdot~150 kg/s and most likely grew by a factor of a few near perihelion. I therefore adopt an enhanced value of Mdot~500 kg/s for the post-perihelion anti-tail.
In the context of 3I/ATLAS being a natural comet, volatiles from ices heated by sunlight yield a maximum outflow speed of the gas that is comparable to the thermal speed of the dominant constituent of CO2 molecules, V=0.2 km/s, as dictated by the surface temperature of ~200 degrees Kelvin at the current heliocentric distance of 3I/ATLAS of about twice the Earth-Sun separation (AU).
This gives the gas mass density of the anti-tail as a function of distance from 3I/ATLAS:
D_j~(1.6x10^{-17} g/cm³)/(d/5,000 km)²
The solar wind carries about 3x10^{-14} solar masses per year per 4*pi steradian and flows at a speed of about v~500 km/s, providing a wind mass density at a heliocentric distance of ~2 AU:
D_w~3x10^{-24} g/cm³.
The jetted gas is expected to be stopped at a distance from 3I/ATLAS where its ram pressure (1/2)D_j*V² is balanced by the ram-pressure of the solar wind, (1/2)D_w*v². This occurs at a stopping distance:
d_s~5,000 km.
Interestingly, this is roughly the traverse radius of the glowing halo (coma) around the nucleus of 3I/ATLAS in the images of 3I/ATLAS (including the Hubble Space Telescope images listed here). As the solar wind sweeps up the gas, it carries it together with sub-micron dust particles away from the Sun along the tail of 3I/ATLAS. However, the large dust particles above 10-microns continue to stream along the anti-tail out to a scale that is ~10 times longer in the direction of the Sun. On that scale, the jet is expected to be gas-free if 3I/ATLAS is a natural comet where gas is launched at a speed dictated by the sublimation of CO2 ice on the surface of a natural rock and limited to ~200 meters per second.
This result presents a clean test for the nature of 3I/ATLAS: if it is a natural comet, then the anti-tail jet should not include streaming gas beyond a distance of 5,000 kilometers from the nucleus. At distances much larger than 5,000 kilometers from 3I/ATLAS as a comet, the anti-tail should be composed primarily of a stream of 10-micron dust particles with no streaming gas.
However, if the launch speed of the anti-tail jet is set by a technological thruster, then the extent of the streaming gas could reach much larger scales towards the Sun:
1. For chemical-propellant thrusters with an exhaust speed of V=5 km/s, the streaming gas could extend out to d=25,000 kilometers.
2. For ion thrusters with an ejection speed of V=90 km/s, the streaming gas could extend out to d=100,000 kilometers.
The existence of streaming gas along the anti-tail can be tested through tagging a molecular tracer like CO2 or CO along the axis of the anti-tail jet and plotting the spatial profile of the tracer relative to scattered sunlight from dust inside the jet.
Here’s hoping that such data will be collected by ground-based telescopes, like Keck, VLT or ALMA, or by space observatories like SPHEREx or the Webb telescope.
Science is a learning experience. The best way to learn is by observing nature rather than forcing it to a popular narrative.
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.
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By Avi Loeb Medium
A composite image of the interstellar object 3I/ATLAS, captured on November 6, 2025 by the Europa Ultraviolet Spectrograph instrument (UVS) on NASA’s Europa Clipper spacecraft, from a distance of 164 million kilometers. (Image credit: NASA/JPL-Caltech/SWRI)NASA’s Europa Clipper spacecraft observed the interstellar object 3I/ATLAS on November 6, 2025, from a distance of 164 million kilometers. During an observing period of seven hours, the Europa Ultraviolet Spectrograph (UVS) instrument recorded data on the abundances of various elements in the gas plume surrounding the nucleus of 3I/ATLAS.
A false-color image was obtained by stacking the brightness maps at multiple UV wavelengths. The image is extremely valuable in constraining the anti-tail and tail geometry of 3I/ATLAS near perihelion. Europa Clipper observed 3I/ATLAS when Earth-based observations were largely blocked by the Sun’s position.
With 3I/ATLAS passing between Europa Clipper and the Sun, the vantage point of Clipper’s observations provided a unique perspective. The sunward viewpoint provided a downstream view of the anti-tail and tail stemming out of the nucleus of 3I/ATLAS when it went close to the Sun.
Europa-UVS detected the spectral fingerprints of oxygen, hydrogen and dust from the outgassing of 3I/ATLAS near perihelion.
Europa Clipper was launched from Earth in October 2024, and is expected to arrive near Jupiter in April 2030, where it will study Jupiter’s frozen moon Europa — inside of which a subsurface liquid ocean might support life. Clipper will orbit Jupiter and perform 49 close flybys of Europa.
Following the rendezvous with Europa Clipper, 3I/ATLAS will arrive closest to Jupiter as early as March 16, 2026, demonstrating how fast it is relative to a human-made spacecraft. In a TV interview today, I expressed my wish to board a fast interstellar traveler like 3I/ATLAS so that it would carry me out of the solar system faster than human-made spacecraft. Ad Astra!
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.
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