Interstellar Visitor 3I/ATLAS Shows Non‑Gravitational Acceleration — Mass Loss Likely, Speculation Remains

3I/ATLAS: An interstellar visitor with an unexpected push

In early July astronomers discovered a strange object passing through our solar system that was later confirmed as an interstellar visitor and designated 3I/ATLAS. The object reached perihelion last week, brightened far more rapidly than expected and appeared to become "distinctly bluer".

Analysis filed by navigation specialists at NASA's Jet Propulsion Laboratory, including work by Davide Farnocchia, reports measurable non‑gravitational acceleration — a small extra push not attributable to the Sun’s gravity alone. Such deviations are familiar in cometary science and are often caused by outgassing: when a body sheds gas and dust, the expelled material produces a reaction force that slightly alters its path.

Harvard astronomer Avi Loeb has proposed a detailed natural explanation in a recent blog post and media interviews. He argues that vigorous mass loss — shedding an elongated plume of dust and gas — could account for the observed acceleration. Loeb estimates the object might lose on the order of one‑tenth to one‑sixth of its mass over a short interval, producing a measurable recoil and an unusually brightening coma.

Loeb has also reported that ALMA (the Atacama Large Millimeter/submillimeter Array) data show a roughly four‑arcsecond deviation in right ascension from the expected path, a shift he describes as statistically significant. He predicts a large cloud of gas — potentially carrying billions of tons of material — could be visible when the object reemerges from behind the Sun in November–December 2025.

Observers emphasize that while mass loss is a straightforward, well‑understood mechanism for non‑gravitational acceleration, other hypotheses have been suggested. Loeb has previously noted anomalous features and briefly floated a speculative idea that an internal engine or artificial activity could produce both thrust and an atypical blue color. This suggestion is highly speculative and not supported by mainstream observations; most cometary scientists favor natural outgassing.

There are also plausible natural explanations for the blue tint: ionized carbon monoxide and certain gas emissions can make a cometary coma appear bluer than the Sun. Under ordinary conditions, dust typically reddens scattered sunlight, so determining the exact cause of the color change will require follow‑up spectroscopy and imaging.

Loeb has said his efforts to coordinate observations were hampered by a U.S. government shutdown, and he reported not receiving confirmations about whether NASA’s Mars Reconnaissance Orbiter collected relevant data during a recent Mars flyby. Access to all available observations will be important for testing the mass‑loss hypothesis and refining trajectory models.

Finally, scientists note that the detected non‑gravitational acceleration is modest and unlikely to alter 3I/ATLAS's trajectory enough to bring it significantly closer to any Solar System planet compared with its original gravitational path. The coming months, when the object becomes observable again, should provide crucial data to confirm whether a large plume appears and to resolve the cause of the brightening and color change.