First Evidence of Non-Gravitational Acceleration in Interstellar Object 3I/ATLAS at Perihelion
A stack of 134 images captured by the PUNCH WFI3 satellite on 29 October 2025 reveals that the interstellar object designated 3I/ATLAS experienced a measurable non‑gravitational acceleration as it passed perihelion. The observations, obtained during a dedicated monitoring window, show a slight deviation from the trajectory predicted by purely gravitational models.
Analysis of the image series indicates that the object's speed increased by a fraction of a millimeter per second squared relative to the expected path. Such a perturbation is consistent with forces generated by outgassing, a phenomenon commonly observed in comets when volatile material sublimates under solar heating. The magnitude of the acceleration is comparable to that recorded for the earlier interstellar visitor ‘Oumuamua, though the data for 3I/ATLAS are more extensive, allowing a more robust statistical assessment.
The finding has prompted discussion among astronomers and planetary scientists. While some researchers, including those who have explored unconventional hypotheses for similar anomalies, note that the acceleration could hint at atypical composition or structure, the prevailing interpretation attributes it to conventional cometary activity. Experts emphasize that without direct spectroscopic confirmation of gas emission, the outgassing explanation remains provisional. The broader community views the result as an important data point for refining dynamical models of interstellar objects entering the solar system.
Future observations of 3I/ATLAS are planned with ground‑based telescopes and upcoming space missions to track its post‑perihelion evolution. Continued monitoring will help determine whether the acceleration persists, diminishes, or shows seasonal patterns. The episode underscores the need for rapid response capabilities to study transient interstellar visitors, which can offer unique insights into the composition and physical processes of bodies formed around other stars.
