New analysis in Research Notes of the AAS finds that interstellar object 3I/ATLAS shows clear signs of cometary outgassing and behaves like active comets in our solar system. Long-baseline astrometry from NASA's Psyche and ESA's Mars Trace Gas Orbiter measured a non-gravitational acceleration near 5 × 10^-7 m/s² and helped estimate a mass around 44 million metric tons with a radius of 260–370 meters. Unlike 'Oumuamua, which lacked obvious outgassing, 3I/ATLAS displays jets and a bright, green coma. Its closest approach—about 167 million miles—poses no threat and provides a rare chance to study extrasolar material.
New Study Confirms 3I/ATLAS Is a Comet — Not an Alien Spacecraft
An ultraviolet image of hydrogen surrounding comet 3I/ATLAS, the third known interstellar obejct, taken Sept. 28, 2025 by NASA's MAVEN spacecraft shortly before its close pass by Mars. (NASA/Goddard/LASP/CU Boulder)
New observations and analysis published in Research Notes of the AAS show that the interstellar visitor 3I/ATLAS behaves like an ordinary active comet rather than engineered technology. While some observers highlighted its unusual motion as possible evidence of alien propulsion, the new study attributes those deviations to natural outgassing — the same effect seen in many small comets in our solar system.
The research team measured tiny non-gravitational accelerations using long-baseline astrometry from NASA's Psyche spacecraft and ESA's Mars Trace Gas Orbiter. Those measurements indicate an acceleration of roughly 5 × 10^-7 m/s², consistent with gas and dust jets producing small rocket-like thrusts as the object warmed near the Sun.
“The results are pretty typical of ordinary comets, and certainly not record-breaking,”lead author Marshall Eubanks told SpaceWeather.com. Using the measured acceleration together with carbon dioxide observations, the team estimates 3I/ATLAS has a mass near 44 million metric tons and a radius between about 260 and 370 meters — values well within comet norms.
NASA's Hubble Space Telescope reobserved interstellar comet 3I/ATLAS Nov. 30, with its Wide Field Camera 3 instrument. (NASA, ESA, STScI, D. Jewitt (UCLA), M.-T. Hui (Shanghai Astronomical Observatory). Image Processing: J. DePasquale (STScI))
The earlier interstellar visitor 'Oumuamua (2017) also showed non-gravitational acceleration (on the order of 10^-6 m/s²) but lacked clear signs of outgassing, a puzzle that prompted alternative hypotheses such as a light sail. By contrast, 3I/ATLAS displays visible outgassing, distinct jets and a bright, greenish coma since its perihelion passage in late October — all hallmarks of an active comet.
3I/ATLAS will make its closest approach to Earth this Friday at roughly 167 million miles (nearly twice the Earth–Sun distance). It poses no danger, but its passage offers astronomers a rare, safe opportunity to study material from another star system. Ongoing observations with ground- and space-based telescopes will continue to monitor its coma, jets and composition while refining size and mass estimates.
Why this matters: Measuring non-gravitational accelerations in a single flythrough using interplanetary spacecraft demonstrates a powerful new method for studying interstellar visitors and helps distinguish natural cometary activity from exotic explanations.
