Even interstellar comet 3I/ATLAS, at a distance of approximately 3.51 astronomical units (4.6) in relation to the Sun, was already generating water, and this is an energy regime in which a water ice on a nucleus is usually highly recalcitrant. The message came in an indirect manner with the faint glow of ultraviolet of hydroxyl (OH), a fragment that results when the sunlight disperses the molecules of water in the expanding atmosphere of a comet.
This was discovered by NASA Neil Gehrels Swift Observatory that can observe ultraviolet wavelengths, which are largely blocked prior to reaching ground telescopes. That vantage is relevant in that OH of approximately 3085 angstroms is a conventional tracer to convert thin gas of a distant comet into a water output that can be measured. In the case of 3I/ATLAS, the UV/Optical Telescope of Swift has logged OH happening when the comet was well above the orbit of Earth and was still inbound and July 31 and Aug. 1, 2025.
Based on that ultraviolet echo, scientists were able to estimate that the rate of water production is approximately 40 kilograms per second, which is equivalent to an open fire hose. In comet jargon, water provides the reference point: once the process of outgassing of H2O is known, the other species can be well-scaled against it, and activity may be judged using the same models used on comets borne around the Sun. This is why OH is important in this case: enabling 3I/ATLAS to be evaluated using familiar standards without necessarily behaving in a familiar manner.
The complication is distance. Sublimation of water ice becomes inefficient beyond about 3 AU, and OH detections at these distances are rare. The deduced production in the Swift analysis suggests a minimum active area of no less than 19 km 2 on the comet, which is hard to agree with a largely inert surface. With the help of the Hubble, the astronomers narrowed the possible diameter of the nucleus to 440 meters up to 5.6 kilometers, allowing them to leave a broad spectrum of the potential conditions of the surface, but still making it evident that the amount of water is large in the face of available sunlight.
One of the directions is suitable to both the physics and the imagery: the water does not have to be only flowing out of the exposed ice on the nucleus. Near-infrared images made at approximately the same time showed large icy grains in the coma, so that sunlight might be driving away particles already off the surface, making the coma itself a significant source of vapor. That type of distributed water release has precedents in a few long-period comets, and is of particular importance to an interstellar visitor the surface of which has been long exposed to interstellar radiation.
3I/ATLAS came with the dynamics of a real passer by. It orbiting the Sun on a hyperbolic orbit, it entered the solar system at approximately 221,000 km/h and accelerated towards perihelion to approximately 246,000 km/h, which is in agreement with an object not gravitationally bound to the Sun. It did not come any closer to Earth closer than 1.8 AU and so the encounter was scientifically worthwhile and did not pose a hazard.
Whenever we pick up the water of a comet in interstellar space, or even the slightest echo of it in ultraviolet, OH, we are reading a letter written by another star system, which is what Dennis Bodewits, the Professor of Physics at Auburn, has to say. It informs us that, the chemistry of life is not special to us only.
Another context layer is discovery statistics. Finding these three confirmed interstellar objects has required the sky to be searched, but modeling activities associated with the 2019 interstellar comet Borisov suggest that such objects might be more numerous than previously thought, possibly becoming mixed in with the far outer belt of comets surrounding the Sun. That possibility redefines 3I/ATLAS: not as an isolated anomaly but as a harbinger of a bigger populatedness, which better surveys will eventually be able to sample.
“All interstellar comets this far have been surprises,” said Zexi Xing, who was the postdoctoral researcher and principal author of the work. Oumuamua was now dry, and Borisov was now laden with carbon monoxide, and now ATLAS is betraying us by losing water at a place where we had not thought it would. They are all rewriting the history of the formation of planets and comets in the environs of the stars, as we thought we understood it.
The instruction on engineering is clear-cut as the chemistry. Swift used a 30-centimeter telescope which was above the atmospheric absorption and sky brightness to give an ultraviolet sensitivity comparable to large telescopes on the ground in the same band. With a high-speed, decaying target on a single trajectory, such an ability transforms a short window into a standardized reading- an all-important stage of making comparisons between planetary systems in the most distant ranges with the smallest, coolest debris they distribute into interstellar space.
