In a new analysis (accessible here ) of the Hubble Space Telescope image of the interstellar object 3I/ATLAS that I co-authored with my brilliant colleague, Eric Keto, the glow of scattered sunlight appears twice as long from the object towards the Sun than it is in the directions perpendicular or opposite to the Sun — where the extent of the glow appears to be the same.
This sunward extension of the glow around 3I/ATLAS is opposite in direction to the tail of scattered sunlight that is commonly observed for comets. The physics behind these cometary tails is simple: dust particles with a size comparable to the characteristic wavelength of sunlight, ~0.5 micrometer, scatter that light and hence are pushed by solar radiation pressure away from the Sun. As a result, comets display tails of reflected sunlight that point away from the Sun. If so, why does 3I/ATLAS exhibit an anti-tail?
An even more remarkable fact about the anti-tail of 3I/ATLAS is that it is noticeable as a factor of 2 in a very unfavorable projection. The line-of-sight connecting 3I/ATLAS to Earth was misaligned with the line connecting 3I/ATLAS to the Sun by only 10 degrees on July 21, 2025, when the Hubble image was taken. This means that we are seeing the extension of the glow nearly edge-on, smaller by a geometric factor of the sine of 10 degrees. The geometric correction factor for this projection is 1/sin(10 degrees)=5.76. This implies that if we were to board a spacecraft and hover vertically to the sunward direction above the elongated glow of scattered sunlight around 3I/ATLAS, we would see it extending ten times farther towards the Sun than its width or tail. This is a remarkable elongation that was not discussed in the scientific literature prior to our new paper.
We explain the scattered sunlight around 3I/ATLAS as a result of ice fragments and not the commonly assumed refractory dust. These ice fragments are evaporated by sunlight, but since most of them are launched from the sun-facing side of 3I/ATLAS, the largest among these ice fragments survive a distance that is ten times longer towards the Sun than sideways or backwards.
There might be other explanations for this 10-fold extension of the glow around 3I/ATLAS towards the Sun, but one thing is clear: its existence is unprecedented.
During an interview titled “Question Everything” which was featured yesterday on CBS News (with the video available here and the full report here), I explained that 3I/ATLAS deserves special attention because of the following five anomalies:
· The retrograde trajectory of 3I/ATLAS is aligned with the ecliptic plane of the planets around the Sun to within 5 degrees, with a chance probability of 1 in 500 (as discussed here). This coincidence brings 3I/ATLAS to pass within several tens of millions of kilometers from Mars, Venus and Jupiter.
· The diameter of the nucleus of 3I/ATLAS has an upper limit of 46 kilometers (as derived here), which would make it a million times more massive than the interstellar comet 2I/Borisov. On October 3, the HiRISE camera on the Mars Reconnaissance orbiter will be able to image 3I/ATLAS with a pixel resolution of 30 kilometers, providing the best limit on the diameter of its nucleus.
· For a couple of months after its discovery — when 3I/ATLAS was at a heliocentric distance of 3–4.5 times the Earth-Sun separation, it featured a glow which extended towards the Sun and not in the opposite direction — as discussed above. This feature of an anti-tail was never observed before for comets (as analyzed here).
· The plume of gas around 3I/ATLAS is composed primarily of carbon dioxide — CO2 (87% by mass) with traces of carbon monoxide — CO (9%), and water — H2O (most of the remaining 4%), as derived here. It also features nickel without iron (as known to exist in industrially-produced nickel alloys) and cyanide, both rising sharply with decreasing heliocentric distance (as reported here).
· 3I/ATLAS is characterized by an extremely deep and narrow negative polarization of −2.77 percent with a low inversion angle of 6.41 degrees (as reported here). This polarimetric behavior is significantly different from all known comets, either interstellar or bound to the Solar System. 3I/ATLAS is the first object known with this combination of low inversion angle and extreme negative polarization. Its anomalous polarization might be the result of the very elongated configuration of scattered sunlight around it with a 10:1 aspect ratio.
In the existing scientific literature on 3I/ATLAS, each of these anomalies is shoved under the carpet of traditional thinking in the context of a dusty water-rich comet. This is not surprising as it follows the mindset of Ecclesiastes 1:9,
“What has been will be again,
what has been done will be done again;
there is nothing new under the sun.”
However, the combination of the above five anomalies argues for something more interesting. Let us harvest as much data as possible about 3I/ATLAS from all available telescopes on Earth and in space, since we might discover something new when 3I/ATLAS passes under the Sun. It is conceivable that Ecclesiastes, like any other human, was wrong. We should question everything.
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.
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