A recent observation of the interstellar object 3I/ATLAS at its perihelion on October 29, 2025, has provided the first evidence of non-gravitational acceleration, as reported by navigation engineer Davide Farnoccia from NASA’s Jet Propulsion Laboratory. The measurements indicate an intriguing dual-component acceleration at a distance of 1.36 astronomical units from the Sun, or approximately 203 million kilometers.
The findings showed a radial acceleration of 135 kilometers per day squared away from the Sun, along with a transverse acceleration of 60 kilometers per day squared. This suggests a possible propulsion mechanism, potentially due to the release of gas from the object. If this hypothesis holds, it implies that 3I/ATLAS could lose about half its mass over a short timescale, specifically approximately one-tenth of its mass over the month surrounding its perihelion. Such significant mass loss is expected to be observable as a plume of gas forming around 3I/ATLAS in November and December of 2025.
European Space Agency’s juice spacecraft is anticipated to be the first to witness this substantial mass loss early in November. Additionally, on December 19, 2025, 3I/ATLAS will make its closest approach to Earth, passing at a distance of 269 million kilometers, offering a prime opportunity for observations from hundreds of ground-based telescopes, as well as the Hubble and Webb space telescopes. Scientists hope that this period will clarify whether the brightening of 3I/ATLAS has resulted from the sublimation of volatile ices due to heating from sunlight.
These brightening observations have been documented through various instruments, including STEREO and SOHO, showing a significant increase in brightness that correlates with its proximity to the Sun, indicating a potential relationship between sunlight interaction and non-gravitational effects. However, there is also speculation that the non-gravitational acceleration may hint at an artificial origin, such as an internal engine, which would explain its unexpected blue coloration. Typically, dust on a comet’s surface would lead to a redder appearance, making 3I/ATLAS’s bright blue observation yet another anomaly in its behavior.
Despite the observed non-gravitational acceleration, the deviation from its gravitational path is relatively modest, suggesting that 3I/ATLAS will not significantly approach any planets in the Solar System from its expected trajectory.
Avi Loeb, a prominent astrophysicist and director at Harvard’s Institute for Theory and Computation, recently shared insights from a luncheon event he hosts for astrophysics professionals, emphasizing the importance of being open to anomalies in data rather than dismissing them. This perspective resonates with the current situation surrounding 3I/ATLAS, as scientists continue to explore its mysterious properties without jumping to conclusions.
Loeb’s vast expertise in the field, along with his leadership of initiatives like the Galileo Project, positions him at the forefront of discussing these remarkable observations as the astronomical community awaits further insights from upcoming observations.