3I/ATLAS, the third interstellar comet ever detected in our solar system, continues to astonish astronomers with its unexpected behavior and changing characteristics. Discovered on July 1, 2025, by the Asteroid Terrestrial-impact Last Alert System (ATLAS), it is significant for being the largest of its kind, estimated to have a diameter between 5 and 11 kilometers, as reported by Live Science.
Typically, a comet”s tail forms as it approaches the Sun, where heat causes gases and dust to be released from its surface. These materials are then pushed away from the Sun by solar wind, resulting in a tail that always points away from the star. However, 3I/ATLAS has displayed an unusual feature: a “reverse tail,” or anticola, pointing directly toward the Sun, contrary to what is generally expected.
In July and August, the comet exhibited an elongated structure directed at the Sun, deviating from the norm. The European Space Agency (ESA) suggested that such phenomena might sometimes be an optical illusion based on the observer”s perspective from Earth. Nevertheless, a recent analysis by astrophysicists Avi Loeb and Eric Keto proposed that the anticola of 3I/ATLAS is indeed a genuine phenomenon, not merely a result of observational angle.
Astrophysicist Jason Wright noted that this occurrence is not entirely unprecedented. In certain cases, anticola are produced by larger, heavier fragments ejected from the comet”s nucleus that do not respond immediately to solar wind pressure, causing them to spread along the comet”s orbit.
As the months progressed, the appearance of 3I/ATLAS changed once again. By September, new observations from astronomers David Jewitt and Jane Luu, detailed in a preliminary study yet to undergo peer review, indicated that the comet had transformed its anticola into a conventional tail pointing away from the Sun. This shift is crucial for understanding the composition and evolution of the comet.
Data from the James Webb Space Telescope suggests that carbon dioxide is the primary driver of the comet”s activity, generating jets of gas and dust that take varying amounts of time to develop into a visible tail, depending on their size and ejection velocity. 3I/ATLAS is expected to reach its closest approach to the Sun around October 30.
According to astrophysicist Miquel Serra-Ricart, this behavior is typical, as jets generally direct toward the Sun while the tail points in the opposite direction. Similar behavior was observed in the comet NEOWISE in 2020. As 3I/ATLAS nears its perihelion, scientists are gearing up to study its evolution closely.
Between July and October, the comet is estimated to have lost nearly 2 million tons of mass, a minuscule fraction (0.00005%) of its total mass, which is over 33 billion tons, according to calculations by Loeb and colleagues. Speculations regarding extraterrestrial involvement have arisen, particularly from Loeb, who suggests that if the anticola were a form of propulsion, the transition to a traditional tail could indicate controlled maneuvers by a spacecraft. Other theorists, like Adam Hibberd, have proposed that this might suggest the object is attempting to enter a heliocentric orbit between Mars and Jupiter. However, the vast majority of astronomers maintain that 3I/ATLAS is a natural comet.
Upcoming observations, including those from the HiRISE camera on Mars taken on October 2, are anticipated to further clarify the nature of this intriguing celestial object once published.
