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Comet 3I/ATLAS is a rare guest: it came from interstellar space, from another star. As it passed near the Sun, ice on its surface began to sublimate, and gas jets acted like tiny rocket engines. This force, called non-gravitational acceleration, nudges the comet off a purely gravitational trajectory.

Astronomers gathered all available measurements of the comet's position on the sky — before and after its closest approach to the Sun. They tested different models of the comet's 'breathing': uniform sublimation, sublimation with a delay after peak heating, and asymmetric sublimation where the comet is more active 'exhaling' after passing the Sun. It turned out that without accounting for non-gravitational force, the observations cannot be explained. But here's the interesting part: if you build the orbit using only pre-encounter or only post-encounter observations, you get incompatible parameters. This means either there are hidden systematic errors in the measurements, or a simple model with constant parameters does not describe the comet's actual behavior.

For the full dataset, the acceleration came very close to the latest solution from NASA's Jet Propulsion Laboratory. This confirms that the acceleration is reliably determined and can be used to estimate the comet's nucleus size. Models with delayed or asymmetric sublimation fit the observations better than a symmetric one — meaning the comet indeed 'breathes' unevenly. But the authors warn: systematic measurement errors and simplifications in sublimation models exceed the formal fitting errors. So precise nucleus size numbers should be taken with caution.

Comet 3I/ATLAS is only the second discovered interstellar object after asteroid 'Oumuamua. Studying it helps us understand what planetary systems around other stars are made of and how often such 'travelers' enter our Solar System.