3I/ATLAS' anti-tail longer than Earth–Moon distance, Harvard's Avi Loeb reiterates technological possibility

The average distance between Earth and the Moon is 384,400 kilometers (about 238,855 miles).

PUBLISHED DEC 16, 2025

The High Resolution Imaging Science Experiment (HiRISE) camera aboard NASA’s Mars Reconnaissance Orbiter captured this image of interstellar comet 3I/ATLAS on Oct. 2, 2025. (Cover Image Source: NASA/JPL-Caltech/University of Arizona)

Despite considerable backlash from the scientific community studying 3I/ATLAS, Harvard astronomer Avi Loeb continues to explore the possibility that the interstellar visitor is an object of technological origin. His latest discussion (posted on his Medium blog) is based on the comet's anti-tail as observed in the images taken on December 14 and December 15, 2025.

A black space background with white streaks and a single fuzzy white dot. The streaks are stars and the dot is comet 3I/ATLAS (Image Source: ESA/TGO/CaSSIS)

More specifically, it's the size of the anti-tail that Loeb is concerned about. Extending to half a million kilometers away from the nucleus of the comet, the sunward tail is the longest that has ever been observed. For the sake of providing perspective, Loeb notes that the length dwarfs even the average distance to the Moon, which is 384,400 kilometers.

A view of comet 3I/ATLAS, as seen by NASA’s SPHEREx, which observed the interstellar object from 7–15 August 2025. (Image Source: NASA | Photo by NASA / SPHEREx)

According to the Harvard astronomer's calculations, for the length of the anti-tail to reach a length of half a million kilometers within 45 days of perihelion, the sunward speed of its constituent materials should be no less than 130 meters per second relative to the comet's nucleus. The uncertainty for Loeb lies in whether the dust and gas evaporating from the icy nucleus alone can generate that kind of speed. That is something that still needs to be examined. "The alternative is a jet from a technological thruster," Loeb wrote.

Comet 3I/ATLAS streaks across a dense star field in this image captured by the Gemini Multi-Object Spectrograph (GMOS) on Gemini South at Cerro Pachón in Chile (Cover Image Source: International Gemini Observatory/NOIRLab) — Comet 3I/ATLAS streaks across a dense star field in this image captured by the Gemini Multi-Object Spectrograph (GMOS) on Gemini South at Cerro Pachón in Chile (Image Source: International Gemini Observatory/NOIRLab)

Back in October, in a Q&A on his Medium blog, Loeb had claimed that there was a 30-40% chance that 3I/ATLAS could have technological origins. "This low-probability scenario includes the possibility of a black swan event akin to a Trojan Horse, where a technological object masquerades as a natural comet," he had written. The claim was based on a list of anomalies (one of them was the anti-tail) that has only grown since then. However, as controversial as his assessment was, it was not set in stone. In the same blog, he had noted that his views could change based on the data that was to come as the comet journeyed through our solar system and made its closest approach to our planet.

This image, taken by the MAVEN spacecraft on Oct. 9, shows the halo of gas and dust, or coma, surrounding comet 3I/ATLAS. (Image Credit: NASA/Goddard/LASP/CU Boulder)

In fact, even as he claimed that the unusual length of the tail could have technological factors behind it, he also issued a reminder that our pool of interstellar objects is fairly small. After all, 3I/ATLAS is only the third known interstellar object to enter our solar system. Likening the challenge to that of someone seeking a partner after dating a small number of people, Loeb assured, "Our assessment will be more reliable as the sample of interstellar objects gets larger."

Hubble Space Telescope reobserved 3I/ATLAS on November 30, with its Wide Field Camera 3 instrument. (Cover Image Source: NASA, ESA, STScI, D. Jewitt (UCLA), M.-T. Hui (Shanghai Astronomical Observatory)

Loeb, like everyone else following the developments in and around the comet, is hoping that the data yet to come reveals more about the comet's nature. Currently in the constellation Leo, 3I/ATLAS will make its closest approach to our planet later this week on December 19, 2025. On the day, it will come as close as 167 million miles from Earth.