Comets are rarely able to go against expectations over a span of centuries; however, the case of interstellar comet 3I/ATLAS is one where this very thing has occurred: this comet has developed a tail pointed directly toward the Sun. In this unusual case, the so-called “anti-tail” is more than just a trick of the light; rather, it is a physical feature reaching lengths of up to 1 million kilometers, laden with periodic jets of matter.
Cometary tails are generally produced by solar radiation pressure and the solar wind, extending away from the Sun, with the sublimation of ice volatiles. On the other hand, the antitail of the comet, 3I/ATLAS, has been attributed to the physics of sublimation anisotropy. The sublimation timescales of ice grains within the coma, depending on the angle of illumination, have been shown to be strongly dependent on the angle of illumination, with the sublimation mass flux, proportional to CO2 ice, producing more massive grains with greater sublimation timescales, extending the solar “snow line” much beyond the usual angles. These studies have employed the Haser equations for radial flux, with the destruction rate dependent on the exponential expression. Fortunately, the curvature of the surface brightness has also been explained by these calculations.
The comet jets proved to be another fascinating aspect. The two-meter twin telescope at Teide Observatory photographed the inner coma undergoing a changeover from a dust fan-like feature pointing towards the Sun to a more typical antisolar tail during 37 nights stretching between July and September 2025. Also, within the antitail, a faint jet could be detected on seven different nights. It was possible to determine the anisotropic morphology of the jet, with its bright coma-like background removed using image processing techniques like Laplacian filtering. This indicated a periodicity of 7.74 ± 0.35 hours, in its position angle, corresponding to its nucleus rotational period of 15.48 ± 0.70 hours if it is near a rotational pole.
This rotational dynamic is more than a curiosity rather, it is a constraint on the geometry of active regions and the orientation of the spin axis, and provides insight into the internal structure of the comet. The geometry of the high-latitude jet is indicative of a +75° source, suggesting a process associated with CO₂ sublimation. Comet jets can provide a nongravitational source for torques, potentially allowing the rotation periods to change with time.
In terms of composition, the 3I/ATLAS is unique compared with other solar system comets. Spectroscopic results suggest that the comet has a volatiles content dominated by CO₂, with a CO₂/H₂O ratio highly in excess of traditional values. This is consistent with the polarimetric observation that the coma is dominated by large, compact grains of mixed water ice and Mg-rich silicates. The real part of the refractive index, Re(m) ∼ 1.387 ± 0.085, indicates that water ice is present in proportions several times higher than silicates. Larger grains are less affected by solar radiation pressure, and so the lackluster antisolar tail and the survival of the sunward fan.
The formation of anti-tails is also related to thermal physics processes of comets. The process of CO₂ sublimation cools surfaces sufficiently to inhibit direct H₂O gas release from the nuclei, and H₂O vapor emerges as a result of ice grain sublimation accelerated into comas. It reverses the typical ratio of gaseous ingredients, and comas may have a H₂O:CO₂ ratio of 1:8, with a greater content of H₂O ice in nuclei.
Dynamically, the hyperbolic orbit of 3I/ATLAS solidifies the interstellar nature of the object, ranking it only the third confirmed interstellar discovery after ‘Oumuamua and 2I/Borisov. While its antecedents provided limited pathways toward understanding the spin state of the comets, the favorable viewing conditions of this comet enabled direct measurements of the spin state based on jet behavior. The constant viewing conditions enabled the accurate measurement of jet oscillations, which then facilitated the spin state based on morphology rather than light curve alone.
The significance of this result to cometary science is extensive. Anti-tails have been observed on solar system comets before, but never before on an interstellar visitor, where it is a real feature rather than a projection artifact. Together, anisotropic snow line extension, CO2-driven jetting activity, and rotation modulation create a unique experimental environment to test models of volatile sublimation, dust transport, and anisotropic comae evolution against a surrounding stellar climate that differs from our sun. Now, as the 3I/ATLAS approaches the distance of 270 million kilometers from Earth and fades from sight, this temporary encounter with the comet results in a rich harvest not only for astrophotographers, who have been treated to the sight of the comet’s wobbling anticoma, but also for astrophysicists, who sense in this anticoma the secrets of the physical properties of cometary bodies formed in distant star systems.
