“The radio signature is exactly what we’d expect from a water-rich comet,” said one of the researchers, which captures the essence of the dualism of discovery and explanation that characterizes the comet 3I/ATLAS. This third confirmed interstellar object has found prominent attention within the astronomical community because of the anomaly that its orbit introduces to standard expectations. When 3I/ATLAS passed through its perihelion on 30 October 2025, at 1.36 astronomical units away from the Sun, its radial acceleration, at 135 km/day² and 60 km/day² for the transverse acceleration, exceeded the standard outgassing model estimates.
The physics of this acceleration relies upon the presence of non-gravitational forces resulting from sublimation volatilities. When solar heat is applied to the surface of a comet, gases release with a resultant recoil. This process is concentration-dependent on volatiles, surface activity patterns, and rotation rates. With regard to 3I/ATLAS, a measured rotation period of 16.16 hours and a dust mass-loss rate of between 0.3 and 4.2 kg/s indicates greater-than-expected activity levels for an object originally formed in a distant, cold region. Coma dust particles with high dimensions, resistant to solar-pressure acceleration, add to the intricacies of the velocity vector.
Spectroscopy has uncovered dramatic chemical anomalies. Data taken with the VLT’s X-shooter and UVES instruments showed the presence of nickel vapor at 3.88 AU, well beyond the temperatures needed for the sublimation of metallic compounds, and at much greater abundance relative to iron. This peculiarity, together with the JWST discovery of the CO₂-to-water ratio of roughly 8:1, is indicative of an origin close to the CO2 ice line in its native environment. The extraordinary blue color, probably due to ionized CO or exotic volatiles, remained after perihelion passage. The nickel material could be the result of the decomposition of Ni(CO)4 or similar carbonyl compounds, which release metals at low temperatures, which is consistent with the abundance of CO and CO2 detected within the coma.
The MeerKAT radio telescope offered crucial evidence of natural cometary activity with the detection of hydroxyl radicals at 1665 and 1667 MHz, which resulted from the sublimation of water ice in the presence of solar UV radiation. The Ultraviolet/Optical Telescope on the Swift satellite verified the activity of water at a heliocentric distance three times that of the Earth and measured a water loss of 40 kg/s the high level characteristic of a few remote comets.
Leiden Institute of Advanced Computer Science thermophysical models have shown that CO-dominant outgassing is capable of reproducing the measured values without the need for exotic propulsion. By matching the absorbed solar radiation with the thermal re-emission and the latent heat flux, and using vapor pressure equations for CO, CO₂, NH₃, and CH₄, the model finds that CO is the only volatiles species that can provide force levels comparable with the measured values at 1-2 AU. The Monte Carlo modeling of the jet morphologies has suggested that slightly collimated CO jets with active areas amounting to several tens of percent of the surface are capable of producing force levels comparable with the measured values, while CO₂ with its low vapor pressure at these temperatures is again dynamically irrelevant, and NH₃ and CH₄ are negligible.
The NASA multi-platform observing campaign has utilized the resources available for Martian orbiters to heliophysics missions such as SOHO/STEREO, as well as deep space probes Psyche and Lucy, to monitor the comet’s orbit and shape. The Hubble Space Telescope’s high-resolution observations have placed constraints on the nucleus size between 320 meters to 5.6 km, showing dust plumes commensurate with active solar system comets. The James Webb Space Telescope’s IR spectroscopy has contributed ratios of volatiles, with MeerKAT & Swift analyses contributing to the radio & UV information on water activity.
The coming Vera Rubin Observatory looks forward to improving this list of visiting aliens, with their sensitivity to fainter objects and their earlier and more refined analysis being invaluable. Until then, 3I/ATLAS provides a unique opportunity to examine cometary physics with foreign chemistry. Irrespective of whether this acceleration is of pure CO-based jets or goes beyond, this combination of astrometry, spectroscopic, and thermophysical observations is changing the paradigm of what happens to ice bodies from outside the Sun as they pass through our solar system.
