Gravity tends to leave the neatest path in the inner solar system. Comet 3I/ATLAS, which has come into the field outside the region of the Sun, introduced a slight, but significant complication: a constant “additional push,” which cannot be properly taken into account by standard comet models at the accuracy now possible.
Only the third object to enter our solar system externally has been known to be 3I/ATLAS. It was detected by ATLAS survey which is NASA-funded on July 1, 2025, on a hyperbolic orbit, too rapid to orbit the Sun, and reached perihelion on Oct. 30, 2025, near the orbit of Mars at 1.4 AU. It moved inward at a speed of approximately 137,000 miles per hour, which increased to approximately 153,000 miles per hour nearest approach, which is in accordance with the pull of the solar gravity.
It came as a surprise in the small print of the motion of the comet. Substantially high-precision tracking near perihelion gave a non-gravitational acceleration larger than would be expected using a simple estimate of a “rocket effect” due to mere outgassing. The anomaly was described at a perihelion distance of approximately 1.36 AU and a radial acceleration of 135 kilometers per day 2 and a transverse component of 60 kilometers per day 2. The common physics of comets, such a thrust is provided by the momentum of escaping gases and dust, and its direction is determined by volatiles outgassing where, how the nucleus spins, and how the jets are turned on and off as the light varies. In the case of 3I/ATLAS, it is not the cause that is of interest, but rather the requirement to reconcile the three components of astrometry, coma morphology, and volatile production into a single consistent momentum budget.
That budget is more detailed since the perihelion. Hubble limited the size of the nucleus prior to perihelion, and subsequent studies based on post-perihelion data suggested an effective diameter of 2.6 ± 0.4 kilometers based on the simplest dark-comet reflectivity model, having the nucleus at the large end of early estimates. A heavier body is able to sustain the same acceleration with a lesser acceleration change, which narrows the association between the acceleration measurements and the amount of mass that needs to be emitted in the form of gas and dust.
The chemistry, in the meantime, will not appear mundane.
Prior to perihelion, infrared spectroscopy had revealed a species of excessively high carbon dioxide-to-water ratio, and optical observations had revealed that the blue hue of the comet did not fade away even after the closest approach. Farther away at solar distances, where volatility of metals is not likely to occur easily, spectra also revealed the presence of nickel vapor, emitted at irregularly high abundance with respect to iron, about 3.88 AU. Perihelion was then followed by the MIRI spectroscopy of JWST, revealing water and carbon dioxide once again, detecting nickel brightness, and a strong signal of methane (CH4) developing in the 7.6-micrometer spectrum on mid-and late-December of 2025 observations. The methane was observed as the comet faded at heliocentric distances of approximately 2.20–2.54 AU together with indication that general outgassing decreased in a brief period with water falling at a steeper rate than certain other species.
A natural cause of the acceleration consists in localized jets of a very small fraction of the active surface area, with which even small patches can produce significant thrust, in case venting is directional. Themophysical and Monte Carlo simulations indicate that the observed magnitude and direction can be reproduced by realistic jets driven by volatiles with sub-percent active coverage, which is consistent with the “extra push” being tied to normal physics in unfamiliar initial conditions.
An additional issue that has been brought out in the discussion of the public is the ease with which measurements may be over-interpreted at the time when the target is faint, fast and in a changing coma. According to planetary scientists, comets have regular idiosyncrasy, and that measurement of position of a fuzzy object can have veiled systematic error. Practically, the crucial test is not that of rhetoric, but that the independent instruments agree in the same acceleration-vector, and the observed production of comas is sufficient to carry out the necessary momentum-flow.
To readers of Modern Engineering Marvels, 3I/ATLAS is an instance of a tracking stack used nowadays at its capacity: Hubble unraveling the inner coma, JWST detecting volatile fingerprints in thermal infrared, and radio facilities tracking the products of dissociation of water. The interstellar comet in this regime is not so much an interstellar curiosity, but rather a standard of gauge of the accuracy with which the solar system can measure – and indeed explain physically – small forces upon an object constructed in a different star system.
