The third interstellar visitor confirmed to cross our Sistema Solar, named 3I/ATLAS, is defying astronomers’ expectations. Imagens captured in November 2025 revealed an unusual and fascinating feature: an elongated, teardrop-shaped structure that points directly at Sol. Essa formation, known as an anti-tail, is a rare phenomenon and its observation in an object from another star provides valuable data on the composition of other planetary systems.
The observations, conducted from an observatory at Alemanha, detail the anti-tail with an angular extent of approximately one arcminute across the sky. The phenomenon occurs simultaneously with a non-gravitational acceleration detected by NASA’s JPL Horizons system, which monitors the trajectory of celestial objects. Essa acceleration indicates that a force other than solar gravity is acting on 3I/ATLAS, pushing it away from Sol in a subtle but measurable way.
The main hypothesis to explain both the anti-tail and the anomalous acceleration points to a swarm of solid, non-evaporable particles that accompany the object’s main core. Previous Dados from Telescópio Espacial Hubble, collected in July 2025, already suggested that most of the light reflected by the coma (the “atmosphere” around the nucleus) came from this surrounding material, and not from the nucleus itself, reinforcing the theory that 3I/ATLAS is more complex than a simple comet or asteroid.
What defines non-gravitational acceleration
The non-gravitational acceleration observed in 3I/ATLAS is a subtle effect, but with significant consequences for its trajectory. Sua magnitude is about 0.0002 times the strength of the gravitational acceleration of Sol, which, in practice, equates to an effective reduction of 0.02% in the solar mass perceived by the object. Essa additional force pushes the core of 3I/ATLAS away from Sol, causing its orbit to deviate slightly from the path it would follow if influenced by gravity alone. Este phenomenon is crucial to understanding the dynamics behind the formation of the anti-tail.
Unlike comets of our Sistema Solar, where the non-gravitational acceleration is caused by the sublimation of ice that ejects gas as a propellant, the case of 3I/ATLAS appears to be different. The force maintains a constant proportion, varying inversely with the square of the distance to the Sol, which suggests a more stable mechanism, such as solar radiation pressure acting over a vast surface area. Partículas that do not lose mass through sublimation, such as dust grains or small rock fragments, follow a trajectory closer to pure gravity, creating a relative displacement between them and the accelerated nucleus, which results in the structure that points towards Sol.
The peculiar geometry of the anti-tail
The teardrop shape of the anti-tail is a direct consequence of the interaction between the acceleration of the nucleus and the trajectory of the fragments. Estando is currently about 270 million kilometers from Sol, the small difference in acceleration creates a physical separation of approximately 54 thousand kilometers between the main nucleus and the particles that follow it.
This physical separation, when viewed from Terra, translates into an angular elongation of 0.7 arcminutes in the sky, consistent with the images obtained. The stability of this configuration is one of the most intriguing points for scientists.
As long as non-gravitational acceleration follows the inverse-square law of distance, the teardrop shape should remain visible and proportionally constant, regardless of whether the object is approaching or moving away from perihelion, the point in its orbit closest to Sol.
The swarm hypothesis of solid objects
To explain the intense glow of the coma and the formation of the anti-tail, scientists propose that 3I/ATLAS is not a single body, but rather a main nucleus accompanied by a vast swarm of smaller particles. Estima This swarm can contain up to a trillion individual particles.
Although the total mass of these fragments is low, their combined surface area can be up to a hundred times greater than that of the nucleus. Essa’s huge surface area reflects much more sunlight, explaining why 99% of the brightness observed by Hubble came from the coma and not the core.
These particles, because they do not evaporate under intense solar radiation, do not form a conventional gas tail, which would be pushed in the opposite direction to the Sol by the solar wind. Instead, they remain on a purer orbital trajectory.
The subtle displacement of the main nucleus, caused by non-gravitational acceleration, makes this swarm of objects appear to extend in the direction of Sol, creating the appearance of an opposing tail, or anti-tail, which is a real structure and not just an optical illusion.
Details of observations and confirmations
Visual confirmation of the teardrop-shaped structure came from a series of observations carried out in the early hours of November 22, 2025. Using a 0.30-meter telescope at Sternwarte Feuerstein, at Alemanha, astronomers captured a sequence of images between 3am and 5am (UTC).
By combining the exposure of 106 individual images of 60 seconds each, it was possible to obtain a high-quality record that clearly reveals the extension of the anti-tail in the direction of Sol. Detailed analysis of the images made it possible to measure the elongation of one minute of arc and confirm that its orientation pointed precisely to the position of Sol in the sky at that moment.
Implications for the study of interstellar objects
The discovery at 3I/ATLAS adds a new layer of complexity to the study of interstellar visitors. Enquanto the first, ‘Oumuamua, was a rocky and elongated object, and the second, 2I/Borisov, behaved like a typical comet, 3I/ATLAS presents unprecedented and hybrid behavior. The presence of a swarm of non-volatile fragments suggests that it may be the remnant of a larger body that disintegrated or that formed differently than the objects in our Sistema Solar. Essa structure may indicate planetary formation processes in its home system where rock-rich and ice-poor bodies are common. Continued study of these objects is critical, as each offers a unique glimpse into the diversity of planetary building blocks that exist elsewhere in the galaxy, allowing scientists to test and refine models for how stellar systems, including our own, evolve over time.
The trajectory away from the object
Currently, 3I/ATLAS continues its journey out of Sistema Solar, moving at a hyperbolic speed that will ensure it never returns. Astrônomos will continue to monitor the object over the next few months to observe any changes in the structure of the coma and anti-tail as it moves away from the influence of Sol.
Crucial distinction between tail and anti-tail
It is important to differentiate the 3I/ATLAS anti-tail from other phenomena. The tail of a traditional comet is made up of gas and fine dust, pushed away from the Sol by the solar wind and radiation pressure, always pointing in the opposite direction to our star.
In some comets in our system, an “anti-tail” can appear as an optical illusion when Terra crosses the comet’s orbital plane, causing larger dust particles left behind in its orbit to appear to point toward Sol. In the case of 3I/ATLAS, however, the structure is a real physical formation, the result of the unique dynamics between its accelerated core and the swarm of particles that accompanies it.
