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The top panel shows an image of 3I/ATLAS, taken on December 15, 2025 at 02:28:12 UTC on a 0.25-meter telescope in Calabria, Italy. The bottom panel includes the brightness map from a Larson-Sekanina rotational gradient filter on a fraction of the entire field of view, spanning 0.86 by 0.39 million kilometers and featuring an anti-tail going out to half a million kilometers from 3I/ATLAS in the sunward direction towards the lower left. (Image credit: Toni Scarmato)

As of December 15, 2025, the interstellar object 3I/ATLAS arrived at a distance of about 270 million kilometers from Earth. By December 19, 2025, it will get to a perigee distance of 268.9 million kilometers, only 0.4% closer than it is today. The data collected by numerous observatories on Earth and in space will inform us in the coming weeks on the nature of 3I/ATLAS.

The latest images from December 14 and 15, 2025 show a prominent anti-tail that extends out to half a million kilometers away from the nucleus of 3I/ATLAS towards the Sun. This length is larger than the average distance to the Moon: 384,400 kilometers. An anti-tail of this size had never been observed before for a comet.

To reach a scale of 500,000 kilometers over the past 45 days after perihelion, the sunward speed of the material in the anti-tail must be at least 130 meters per second relative to the nucleus of 3I/ATLAS. Whether this speed can be maintained by sublimated dust or gas from pockets of ice facing the solar wind and solar radiation pressure remains to be studied. The alternative is a jet from a technological thruster.

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An image of 3I/ATLAS, taken on December 14, 2025, with a total integration time of 2 hours and 22 minutes and no filter on a 14-inch (0.356-meter) telescope in June Lake, California. The field of view displayed in the bottom panel spans 4.24 million kilometers on a side. A green-blue anti-tail is pointing in the sunward direction towards the lower left. (Image credit: Dan Bartlett)

How unusual are the properties of the anti-tail displayed by 3I/ATLAS?

We do not have a good census of the population of interstellar objects, including any traffic of technological objects through the inner solar system. Our state-of-the-art survey telescopes, such as PanSTARRS, ATLAS or Rubin, can only detect reflection of sunlight from objects that are larger than 100-meters in diameter, roughly the size of a football field, within a distance comparable to the Earth-Sun separation. This diameter is an order of magnitude bigger than the largest spacecraft that humanity launched so far. In addition, existing astronomical surveys would miss near-Earth objects that travel much faster than tens of kilometers per second, the typical speed of asteroids or comets.

In order to assess whether an interstellar object is an outlier that deserves a high rank on the Loeb Classification Scale — discussed here, here and here, we need to know the probability distribution functions for its properties. Our assessment will be more reliable as the sample of interstellar objects gets larger. The challenge is familiar for those who seek a partner from a small pool of dating candidates, as it is difficult to decide how exceptional any of the early dating candidates are until we sample a large number of them. Our interstellar dating pool might go beyond what Science Fiction writers imagine. Enrico Fermi’s question: “Where is everybody?” is often asked by lonely people. Our best advice to them is to engage in the search.

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Position of 3I/ATLAS relative to Earth and Jupiter on December 15, 2025. (Image credit: JPL Horizons)

Earlier today, I received the following uplifting message from a young person who indeed wishes to engage in the search for interstellar technological objects:

“Dear Professor Loeb,

My name is Rúben, and I am writing to you today from Portugal to express my sincere admiration for your work and your courage in pushing scientific curiosity beyond conventional limits. Your approach to open, evidence driven inquiry has been truly inspiring to me.

Since I was young, I was fascinated by space and even though I have never got a degree as a scientist or similar, I grew up seeking cosmic awareness and knowledge just because I was so interested in it all the time.

And so, I am writing to you today from Portugal hoping that I could somehow be working on something that can make a difference for humanity in the future, as I want to be a part of that movement and work. That being said, I would be honored to contribute to your mission in any way possible. While I am not an academic researcher, I bring strong technical, analytical and creative skills, including software development, data handling, and digital communication, and I am fully willing to dedicate my time and effort to support your work where I may be useful, as my love for the greater purpose it has, is immense.

If there is any opportunity to assist, collaborate, or contribute directly or indirectly I would be very grateful to learn how I could be of help.

Thank you for your time and for the impact your work continues to have.

Kind regards,
Rúben
”

ABOUT THE AUTHOR

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(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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