Avi Loeb
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As of today, December 14, 2025, the interstellar object 3I/ATLAS is at a distance of about 270.5 million kilometers from Earth. Its images are delayed by a quarter of an hour, as it takes light from 3I/ATLAS 15 minutes to reach us. On December 19, 2025, 3I/ATLAS will get closest to Earth at a perigee distance of 268.9097 (+/-0.0060) million kilometers.
The latest image of 3I/ATLAS was taken by Teerasak Thaluang on December 13, 2025 at 21:30:26 UTC with a 0.26-meter telescope in Rayong, Thailand (as reported here). The rotational-gradient brightness map shows a prominent anti-tail, uncommon for comets, pointing in the direction of the Sun.
Whereas an anti-tail had been seen for solar system comets as a temporary perspective effect when the Earth crossed the comets’ orbital plane, this is clearly not the case with 3I/ATLAS. The anti-tail was apparent in the first Hubble Space Telescope image, taken on July 21, 2025, when 3I/ATLAS was approaching the Sun from a geocentric distance of 2.98 times the Earth-Sun separation (AU) — as reported here and analyzed here, and was also apparent in the second Hubble image taken on November 30, 2025, when 3I/ATLAS was receding away from the Sun at a distance of 1.91 AU from Earth — as reported here. The anti-tail was also apparent in thousands of images taken in between these dates.
The anti-tail of 3I/ATLAS is therefore not a perspective effect. It is a real physical jet, with a glow extending from 3I/ATLAS towards the Sun. Its nature is a mystery because gas and micrometer-dust particles are expected to be pushed away from the Sun by solar radiation pressure and the solar wind, creating the appearance of a tail — as routinely seen in solar-system comets. There was no mention of this mystery at the NASA press conference about 3I/ATLAS on November 19, 2025 (accessible here).
To explain the physics of the anti-tail of 3I/ATLAS, I have written three scientific papers. The first two of these peer-reviewed papers, co-authored with Eric Keto (accessible here and here), associate the anti-tail with scattering of sunlight by fragments of ice shed from the sun-facing side of 3I/ATLAS. These tiny ice particles evaporate before they get pushed back significantly by the solar radiation pressure and so they never appear as a conventional cometary tail. The third (single-authored) paper, published on December 8, 2025 here, associates the anti-tail with a swarm of objects that lag behind 3I/ATLAS because of its non-gravitational acceleration away from the Sun (as reported by JPL Horizons here). Analysis of the latest Hubble image could potentially favor one of these explanations.
By recognizing anomalies, we can learn something new. By ignoring them, we remain ignorant.