Could a frozen wanderer older than the Sun rewrite what we know about the galaxy’s past? Comet 3I/ATLAS, the third known confirmed interstellar object to enter our solar system, is offering scientists a chance rarely seen: to study matter forged billions of years before the birth of our star. Discovered on July 1, 2025, by the ATLAS survey, this huge, approximately 11-kilometer-wide body is moving at over 130,000 mph (210,000 km/h) and is likely more than 7 billion years old, putting it at least 3 billion years ahead of the clock on the solar system.
With the HiRISE camera, NASA’s Mars Reconnaissance Orbiter has taken what are expected to be the most detailed shots of 3I/ATLAS to date. The orbiter flew within 30 million kilometers of the Red Planet in early October. These observations have a promised spatial resolution up to three times sharper than that from the Hubble Space Telescope in its July 21 imagery. HiRISE was designed to map Mars’s surface in exquisite detail, but the ability to lock onto a fast-moving target millions of kilometers away was a demonstration of the versatility of orbital imaging systems in cometary and planetary science. “The HiRISE image would give us a side-view as well as a spatial resolution that is three times better than that of the Hubble Space Telescope,” explained Avi Loeb, an astrophysicist at Harvard, and without resolving the solid nucleus itself, brightness data could tightly constrain its diameter.
Its anomalous chemistry has already generated great interest. Observations with JWST’s NIRSpec and NASA’s SPHEREx mission revealed an extreme CO₂/H₂O ratio of 7.6 ± 0.3, placing it 4.5σ above trends seen in solar system comets. Linked to high absolute CO and a red spectral slope, such enrichment points to the long-term galactic cosmic ray processing of its outer layers. Laboratory experiments show that GCR irradiation efficiently transforms CO to CO₂ while forming organic-rich crusts, thereby modifying the first 15–20 meters of a comet’s surface on gigayear timescales. This implies that current outgassing only samples the irradiated zone and not the pristine interior-a perception that reinvents interstellar comets as depositories of cosmic ray exposure rather than pristine messengers from their native environments.
Accurate kinematic tracing is required for the determination of 3I/ATLAS’s origin. Making use of the ESA’s Gaia DR3 astrometry, scientists integrated its orbit backwards over 10 million years and found 93 stellar encounters, out of which 62 were high-confidence ones with main sequence stars. The strongest one was with HD 187760 around 72,000 years ago, and it changed its velocity negligibly; hence, it has not suffered any important perturbations during the past 4.27 million years. A steep trajectory for the comet, together with its high velocity, points toward a source within the thick disk of the Milky Way, which is a region populated by ancient stars and in agreement with its very old age.
Spectroscopy also confirms its natural cometary nature. Familiar volatile molecules have been detected, for example, by the Very Large Telescope, while South Africa’s MeerKAT radio array detected hydroxyl radical absorption at 1665 and 1667 MHz, a standard signature of cometary activity near the Sun. The active state of the object, with a coma and tail, with even an “anti-tail” pointing toward the Sun, gives rise to the sublimation of water, carbon dioxide, and other ices due to solar heating. But the scientific stakes could hardly be higher.
Study of the irradiated crust of 3I/ATLAS may show how cosmic rays shape icy bodies over billions of years and yield vital insights into the chemical evolution of planetary systems throughout the galaxy. At the same time, its trajectory and composition may finally enable astronomers to refine models of comet formation in ancient stellar environments that may distinguish between origins in primordial planetesimal disks or exo-Oort clouds.
Coming closest to Earth on December 19 but continuing in view of NASA’s Juno, ESA’s JUICE, and ground-based observatories well into next year, 3I/ATLAS is much more than a fleeting spectacle; rather, it is a deep-time emissary carrying scars from its journey through interstellar space. The following HiRISE images will be crucial for constraining its physical structure, validating compositional models, and taking the next step in understanding how such ancient wanderers record the history of the galaxy.
