No Clear Cometary Tail in Post-Perihelion Images of 3I/ATLAS
Avi Loeb
On November 5, 2025, two new images of the interstellar object 3I/ATLAS were released. They show a compact source of light without a clear cometary tail. The coma is not very different in morphology than its appearance in the Hubble Space Telescope on July 21, 2025 (accessible here).
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This is surprising in view of NASA’s JPL report here of a non-gravitational acceleration — normalized at a heliocentric value of 1 au:
1. A radial acceleration away from the Sun of 1.1×10^{-6} au per day squared.
2. A transverse acceleration relative to the Sun’s direction of 3.7×10^{-7} au per day squared.
Based on momentum conservation (as discussed here), I derived here that the mass fraction lost during the perihelion passage of 3I/ATLAS is larger than 13%. For a typical comet, this should have resulted in a massive coma with dust and gas that would have been pushed by the solar radiation pressure and the solar wind to the shape of a typical cometary tail pointing away from the Sun. No such tail is visible in the new images from November 5, 2025.
In comparison, I include below an image of the solar system’s comet Lemmon, taken by one of the telescopes that observed 3I/ATLAS two days ago. Lemmon features a clear cometary tail pointing away from the Sun, as expected.
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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).
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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 at least 13%of its mass (as calculated here), but preliminary post-perihelion images do not show evidence for it so far.