Harvard Astrophysicist Avi Loeb Explains the Persistent Anti-Tail of Exocomet 3I/ATLAS as It Nears Earth

The interstellar comet 3I/ATLAS is on its way to making its closest approach to Earth, the only known planet teeming with life. Scientists around the globe plan to focus their observatories on the exocomet during its flyby on December 19, 2025. Their goal is to finally resolve the ongoing debate surrounding its origins.

Since its discovery on July 1, 2025, by the Asteroid Terrestrial-Impact Last Alert System (ATLAS), this interstellar object has captivated both the scientific community and amateur stargazers. Meanwhile, online, many social media users have developed various speculative theories about it.

The exocomet's unusual characteristics are further highlighted by the somewhat unclear images captured by NASA, ESA, and other observatories. This is not due to any fault of the observatories themselves, though many internet users often expect high-definition, Hollywood-quality space visuals.

Could this interstellar comet be of extraterrestrial origin, perhaps carrying probes or even alien life? While its cometary behavior makes this unlikely, Harvard astrophysicist Professor Avi Loeb points out that a space probe could exhibit similar behavior after traveling through space for an extended period. Typically, comets display a tail and sometimes a temporary anti-tail. That said, the reality is a bit more complicated. 3I/ATLAS continues to show a persistent anti-tail, which was initially thought to be multiple jets emanating from the object.

According to Professor Loeb, recent images taken on December 13, 2025, by Teerasak Thaluang using a 0.26-meter telescope in Rayong, Thailand, reveal a prominent anti-tail in the rotational-gradient brightness map. This is quite unusual for comets, as the anti-tail points directly towards the Sun.

Loeb further explains that while anti-tails have been observed in solar system comets as a temporary effect when Earth crosses the comets’ orbital plane, this is not the case with 3I/ATLAS. The anti-tail was visible in the initial Hubble Space Telescope image from July 21, 2025, when 3I/ATLAS was approaching the Sun at a distance of 2.98 AU from Earth. It was also present in the second Hubble image taken on November 30, 2025, when 3I/ATLAS was moving away from the Sun at a distance of 1.91 AU from Earth.

Professor Loeb argues that the anti-tail of 3I/ATLAS is not simply a trick of perspective. Instead, it represents a genuine physical jet extending from 3I/ATLAS towards the Sun. The nature of this jet is puzzling because gas and micrometer-sized dust particles are normally pushed away from the Sun by solar radiation pressure and the solar wind. This process typically creates the appearance of a tail, as commonly seen in solar system comets.

Loeb also criticizes NASA for not addressing the mystery of the anti-tail during a press conference about 3I/ATLAS held on November 19, 2025.

In one of his published papers, Avi Loeb suggests that the anti-tail of 3I/ATLAS might be caused by a swarm of objects trailing behind the comet due to its non-gravitational acceleration away from the Sun.