The Anti-Tail of 3i/atlas Turned to a Tail!

The Evolution of 3I/ATLAS: From Anti-tail to Tail

In recent months, the interstellar object known as 3I/ATLAS has undergone a significant transformation in its appearance. As detailed by Avi Loeb and Eric Keto, new images from the Nordic Optical Telescope on October 22, 2025, show that the anti-tail observed during July and August of the same year has evolved into a tail by September 2025.

The transition is particularly noteworthy as it provides insight into the physical processes governing this unique celestial body. The anti-tail initially pointed towards the Sun but reversed direction in September, indicating changes in its composition or behavior due to increasing solar radiation.

Scientific Observations and Theoretical Models

The authors note that the mass loss rate from 3I/ATLAS scales roughly with the solar radiation flux impinging on its surface. This dependence is consistent with carbon dioxide (CO2) being the main driver of activity, as previously inferred by spectroscopic data obtained through the SPHEREx space observatory and the Webb Space Telescope.

The mass loss rate was estimated to be about 150 kilograms per second at a distance of 3.32 times the Earth-Sun separation (au). The high-resolution imaging from the Hubble Space Telescope revealed a pronounced sunward anti-tail, which was explained as being dominated by scattering of sunlight by H2O ice fragments ejected from the nucleus due to CO2 sublimation.

Theoretical Explanations and Future Observations

Avi Loeb and Eric Keto have posted a detailed theoretical model that explains this transition. Their work shows how, as 3I/ATLAS approaches the Sun, the exponential temperature dependence of the sublimation rate causes an increase in the production rate of ice fragments and a decrease in their residence time in the outflow.

At closer heliocentric distances, the scattering becomes dominated by longer-lived refractory dust particles and larger volatile grains with survival times long enough to form a tail that stretches away from the Sun. Observations with other telescopes, such as the Keck telescope and Gemini South observatory, confirm this transition.

Further observations are expected when 3I/ATLAS reaches its closest approach to Earth on December 19, 2025, where it will be subjected to intense solar heating. These post-perihelion observations could provide crucial insights into the nature of the object and its potential origins.

Significance for Extraterrestrial Hypotheses

The transition from an anti-tail to a tail is significant in the context of Avi Loeb’s broader research, which includes the possibility that 3I/ATLAS might be an alien spacecraft. If such a scenario were true, this change could indicate controlled maneuvering near perihelion.

Loeb suggests that if 3I/ATLAS is indeed an alien spacecraft slowing down and using anti-tail braking thrust, the transition would be expected as it approaches the Sun. Post-perihelion observations will be crucial in determining whether this object displays characteristics of a natural comet or something more unusual.

Regardless of its true nature, 3I/ATLAS continues to captivate scientists and the public alike, offering a unique opportunity to study an interstellar visitor from beyond our solar system.