Could a comet traveling through the solar system at more than 150,000 miles per hour be something more than frozen rock and dust? That’s the question at the center of an emerging astronomical enigma on 3I/ATLAS, the third verified interstellar object ever seen and by far the largest.
Initially detected on 1 July 2025, by NASA’s Asteroid Terrestrial-impact Last Alert System (ATLAS) in Chile, 3I/ATLAS came from the Sagittarius direction and was soon confirmed to be an interstellar visitor. In contrast to its forerunners the cigar-like 1I/’Oumuamua in 2017 and the cometary 2I/Borisov in 2019 this object is much larger. Harvard astrophysicist Avi Loeb has estimated its mass to be more than 33 billion tons, and its nucleus at least 3.1 miles wide, making it three to five orders of magnitude heavier than either of the previous interstellar visitors.
High-resolution observations by the Hubble Space Telescope and the James Webb Space Telescope (JWST) have started to sharpen those estimates. Hubble’s picture indicates that the nucleus may be up to 3.5 miles across, although its actual size is hidden by the compact coma of gas and dust. The NIRSpec instrument on JWST has uncovered a carbon dioxide–rich coma with a CO₂-to-H₂O ratio of 8.0±1.0 one of the highest ever detected for a comet as well as faint traces of CO, OCS, water ice, and dust. This work implies either formation close to the CO₂ ice line in its original system or long exposure to high levels of radiation, as opposed to most comets in the solar system.
Its path is a further curiosity. Loeb points out that 3I/ATLAS has an exceptionally flat and straight trajectory, bringing it very close to Jupiter’s and Venus’ orbits and within 1.67 million miles of Mars’ orbit. Such accuracy, he suggests, might suggest “propulsion by a technologically manufactured engine” if any real maneuvers are found. The comet’s velocity at 130,000 mph measured by Hubble, and by other sensors up to 152,000 mph captures billions of years of gravitational acceleration within the dense disk of the Milky Way, the old stellar population from which it is thought to have originated. Monte Carlo simulations by Loeb and Shokhruz Kakharov date it back to about 4.6 billion years ago, so it is the oldest of the three known interstellar objects.
The comet’s physical motion is just as strange. Observations from the Keck Telescope and other facilities show it shedding large dust particles hundreds of microns across, ejected sunward rather than away from the Sun a reversal of the typical cometary tail orientation caused by solar radiation pressure. These particles’ slow departure speed, about one meter per second, suggests weak activity despite the prodigious CO₂ output. Spectroscopy has also identified cyanogen and carbon-chain molecules, byproducts of intricate chemical reactions within the coma as the ices sublimate as a result of solar heating.
The monitoring and observation of such transient callers call for the combined strengths of contemporary survey and space-based observatories. The ATLAS system, an asteroid impact early warning network designed to detect, was capable of detecting 3I/ATLAS at a distance of 420 million miles from the Sun, whereas archival observations from the Zwicky Transient Facility pushed its observation arc back to mid-June. Other instruments such as JWST and Hubble will then analyze its composition and morphology, whereas ideas to point NASA’s Mars Reconnaissance Orbiter’s HiRISE camera towards the comet may provide size limits from a single bright pixel.
To planetary scientists, 3I/ATLAS is an uncommon investigation from a foreign star system, bearing unchanged material from its natal environment. For Loeb, it is an open question whether 3I/ATLAS is “an unusually massive comet with an unusual chemical composition on an unusually rare trajectory or alien technology.” As he warns, “We should not decide about the nature of 3I/ATLAS based on the chemical composition of its skin, for the same reason that we should not judge a book by its cover.”
