Over seven billion years of cosmic history are careening through our solar system at this very moment, riding the frozen body of a comet known as 3I/ATLAS. This extraordinary interstellar object, discovered on July 1, 2025, by the ATLAS survey, marks the third confirmed visitor to our solar system outside of our planetary family, with a size of about 11 kilometers in diameter and a speed of over 210,000 kilometers an hour, predating our solar system by billions of years.
Its path has pulled an unprecedented team of observatories and satellites into joint observations. In the first week of October, the NASA Mars Reconnaissance Orbiter, equipped with the High Resolution Imaging Science Experiment camera, swung its gaze from the Martian surface at a distance of 30 million kilometers and took images of the comet. Although the camera is meant for mapping the Martian surface, its images were expected to have resolution up to three times better than that of the Hubble Space Telescope in July observations, according to Avi Loeb, an astrophysicist from Harvard University, who added that the resolution could be sufficient for estimating the comet’s diameter, even if its nucleus is not resolved.
However, other instruments have shown very distinctive signatures. Data collected with the NIRSpec aboard the James Webb Space Telescope and the SPHEREx mission for NASA have shown that there is an extraordinarily high ratio of CO₂/H₂O ratio of 7.6 ± 0.3, much higher than that for comets in our solar system. These results, combined with high levels of CO and a reddish spectrum, are indicative of galactic cosmic-ray irradiation of the outer layers of the comet on a gigayear timescale. Laboratory analyses have shown that this leads to the production of CO2 from CO, along with organic crusts, so that current outgassing is only sampling the surface layers that have been so affected.
The origin of the comet was traced by kinematic modeling. By using the astrometry provided by ESA’s Gaia Data Release 3, its orbit was inverted over a period of 10 million years, yielding 93 stellar encounters, of which 62 were classified as high-confidence, all of them being main-sequence stars. However, the most notable close encounter was when it was near HD 187760, some 72,000 years ago, and it had little effect on its velocity. Its stability over 4.27 million years supports that it indeed originated in the Milky Way’s thick disk, an ancient, metal-poor stellar population, fitting its exceptional age.
Confirmation of its cometary origin came from the field of radio astronomy. The MeerKAT radio telescope in South Africa discovered the hydroxyl radical absorption lines at 1665 and 1667 MHz, which is the expected result of water ice sublimation as a result of solar heating. Follow-up observations confirmed the expected transition from absorption to emission as it changed geometry with the Sun. The Breakthrough Listen experiment also running at the time found no narrowband signals suggesting artificial origin, establishing power flux limits below a cell phone at ranges of hundreds of millions of kilometers.
From a visual standpoint, 3I/ATLAS has shown a coma, a tail, and even an “anti-tail” which points toward the Sun. Although these are unusual features, they are not unheard of when it comes to comets, especially when larger particles of dust are observed to be thrown toward the Sun, resisting the effects of solar pressure. These types of occurrences have been observed in previous comets, supporting the idea that 3I/ATLAS is acting in accordance with known physics when it comes to comets.
The findings from such studies have many other applications beyond this particular object. The interstellar comets provide scientists with an opportunity to study matter from other star systems without having to send an interstellar mission. The study of interstellar comets can enable scientists to develop models for the formation and ejection of ice bodies from other star systems. With regard to 3I/ATLAS, the origin from the thick disk and the irradiated crust can offer clues on the chemical evolution of ancient star systems. With its closest approach to our planet occurring on December 19, 3I/ATLAS remains a passing but precious messenger with continued observations from space missions like Juno and JUICE as well as ground-based telescopes around the world. Every observation contributes to a developing portrait of interstellar objects’ roles in maintaining and modifying their tales over billions of years.
