Seven hours and forty-five minutes – that was the length of time that the jets in the anti-tail of comet 3I/ATLAS took to wobble as the comet was drawn towards the Sun. For astrophysicists, the wobble of the jets was more than just an interesting phenomenon – it was the first observation of its kind in the history of interstellar comets.
The tails in comets are generally produced by the relentless pressure from solar radiations and the solar wind, which pushes the tail away from the Sun. In most comets, the dust tail is gently curved, due to radiation pressure, while the ion tail is directly away from the Sun because it is formed by ionized particles in the solar wind. An anti-tail is, on the other hand, an optical illusion in which smaller particles that are left behind in the orbit of the comet appear to point towards the Sun when observed from Earth. In 3I/ATLAS, an anti-tail extended 620,000 miles (1 million kilometers), in which a peculiar gas and dust jet existed that showed no symmetry in cometary activity.
The key to this discovery was the Two-meter Twin Telescope (TTT) at Teide Observatory in the Canary Island of Tenerife. Over 37 nights from the 2nd of July to the 5th of September 2025, the team of researchers utilized Laplacian image filtering to better discern faint features in the coma interior. This particular image filtering helped identify a high-latitude jet in the shape of a wide dust plume at a position angle of about 280°. However, during seven nights of observation in August, there existed a subtle but systematic periodic variation in the position angle of the jet, a precessional behavior about the projected spin axis of the nucleus, which indicated a nuclear rotation period of 15 hours 30 minutes, shorter than previous estimates and well matched with photometric observations.
The physics underlying such wobbling jets has been linked to cometary outgasing processes. As the Sun heats volatile ices on the surface of the nucleus, processes of sublimation lead to the release of gases and dust. In a rotating nucleus, active regions could be expected to create jets with changing orientation. Such processes could be simulated in a triaxial shape model of a comet, such as 67P/Churyumov-Gerasimenko, where the consequences of restricted sublimation on the dayside could lead to a change in the orientation of the spin axis or a periodically changing jet orientation. In 3I/ATLAS, the periodicity corresponds to a single source around a pole, with changing orientation due to the rotor.
Moreover, this detection is all the more important because 3I/ATLAS is just the third confirmed interstellar object to have entered our Solar System so far, after the cometary objects 1I/‘Oumuamua in 2017 and 2I/Borisov in 2019.
Research has also indicated the origins of these three cometary objects to be generated from different stellar populations: 3I/ATLAS is thought to have formed ∼4.6 billion years ago in the thick disk of the Milky Way galaxy in association with older, metal-poor stars. Every entry of such an object is a source of fresh material brought by it to our Solar System without any Solar System processes acting upon it; therefore, it is a goldmine for comparative comet
Observationally, the resolution of such a detailed pattern in the coma of an interstellar comet would be very difficult. The wavy jet in 3I/ATLAS was discovered at a projected distance of 6,000 km from the optocenter. The robotic aspect of the TTT enables observations with high resolution, whereas image filtering enabled the detection of anisotropic components in the bright coma. This compares with missions such as Rosetta in which in situ measurements showed accelerations due to surface activity pattern changes.
The changes seen in coma shape of comet 3I/ATLAS from a sunward-oriented dust plume in July to a prominent antisolar tail late August occurred as a result of stronger solar radiation pressure on dust particles as the comet moved closer to perihelion on October 30, 2025, at a distance of 130 million miles (210 million kilometers) from the Sun. Radiation pressure is stronger on smaller particles that trail to form the tail of comets while larger particles form the anti-tail seen from Earth.
For comet scientists, the periodic change in the jet position angle corresponds to a direct test of the nucleus’s rotational dynamics. For the 3I/ATLAS, the identification of spin-axis orientation and the rotational period allow the modelers the chance to constrain the nucleus’s shape and possible triaxial nature and the locations of the active zones. This information serves as the starting point for the modelers to simulate the sublimation-driven torques that could change the rotational state of the nucleus over time. For the interstellar comets, all these phenomena would occur under diverse evolutionary paths that could provide clues on the nature of the activity patterns based on the composition and shape from foreign planetary environments.
While 3I/ATLAS fades into the distance towards the outer Solar System, with its closest approach to our planet on December 19, 2025, at 168 million miles (270 million km), the chance to observe it up close may never come again. However, the techniques developed in this mission, ranging from baseline observation to advanced coma imaging, point towards an exciting future. The Vera C. Rubin Observatory’s LSST, a upcoming observatory that promises to spot tens of interstellar objects every year, may provide scientists with enough guests to allow them to better interpret their unique messages of distant planetary bodies.
