It is not often that each decade a comet older than the Sun passes by, leaving behind a chemical footprint never witnessed before. But that is exactly what 3I/ATLAS just the third confirmed interstellar visitor has left in the solar system, and scientists are scrambling to unlock its secrets before it disappears back into the black.
1. Discovery at High Speed
On July 1, 2025, NASA’s Asteroid Terrestrial-impact Last Alert System (ATLAS) flagged a faint, fast-moving point of light from its 20-inch telescope in Chile. Within 24 hours, follow-up observations from facilities including the European Southern Observatory’s Very Large Telescope confirmed the object’s hyperbolic trajectory proof it was unbound to the Sun. Its speed of approximately 60 km/s (some 130,000 mph) surpasses that of earlier visitors 1I/’Oumuamua and 2I/Borisov, implying it has been accelerated for billions of years by gravitational encounters within the Milky Way galaxy.
2. The ATLAS Survey System
ATLAS has four cloud-watching telescopes two in Hawaii, one in Chile, and one in South Africa that survey the full sky multiple times each night. Built to find dangerous near-Earth objects, its automated image differencing and motion detection algorithms are equally effective at identifying unusual interstellar guests. The speed of the system’s cloud alert permitted astronomers globally to mobilize within hours, obtaining early spectra and astrometry prior to the comet reaching solar conjunction.
3. Size, Shape, and Orbit
Initial brightness suggested a nucleus as large as 10 kilometers, but Hubble imaging has constrained it to no more than 5.6 km across. Its orbital eccentricity of 6.2 is extreme far exceeding Borisov’s 3.6 ensuring it will never return. Perihelion will occur October 30 at 1.4 AU, just inside Mars’ orbit, before it slingshots back into interstellar space.
4. Anomalous Chemistry
Spectroscopy by the James Webb Space Telescope’s NIRSpec instrument on Aug. 6 showed an unparalleled CO₂-to-H₂O ratio in the coma. “Its ratio of CO₂ to water in the coma is one of the highest ever observed in any comet,” stated Martin Cordiner, lead investigator of the JWST observations. In addition to CO₂, Webb observed water vapor, water ice, carbon monoxide, and carbonyl sulfide. The predominance of CO₂ can suggest formation within the vicinity of the carbon dioxide ice line in its birth protoplanetary disk, or due to radiation-driven chemistry during its multibillion-year journey.
5. Detection of Early Water
Between July 31 and August 1, ultraviolet imaging by the Neil Gehrels-Swift Observatory detected OH emission a photodissociation fragment of water, at 3.51 AU from the Sun. The estimated rate of water production of ~40 kg/s is exceptionally high for so far out, suggesting that more than 20% of the surface is active. The large icy grains within the coma could be providing water vapour far away from the Sun, an unusual behaviour for most comets of the solar system.
6. Nickel Without Iron
The Very Large Telescope’s X-shooter spectrograph found nickel in the coma but no iron, a curious chemical imbalance. One theory is nickel tetracarbonyl a volatile compound that breaks down in ultraviolet light releasing nickel atoms and carbon monoxide. That might explain the comet’s high activity while yet far from the Sun.
7. Clues to Galactic Origins
Modeling of its incoming velocity vector indicates a source in the thick disk of the Milky Way, an ancient population of stars and planetary systems. If true, 3I/ATLAS would be more than 7 billion years old, as the oldest macroscopic object ever seen in the solar system. Its reddish color, which is typical of organic-rich surfaces, and potential high water mass fraction potentially provide a direct sample of planetesimal chemistry from a low-metallicity environment.
8. Coordinated Global and Space-Based Campaign
Dozens of observatories, ranging from Gemini North’s Multi-Object Spectrograph to commissioning Vera C. Rubin Observatory, have provided data. Precovery images from Rubin on June 21 caught early cometary activity, tightening the orbit and rotation constraints. Spacecraft teams are investigating opportunistic flybys: Mars Reconnaissance Orbiter might photograph the comet during its close pass on October 3 by Mars, and ESA’s JUICE mission will be 0.4 AU away in November.
9. The Science Driven by Technology
The collaboration between terrestrial surveys and space-based telescopes is at the heart of the endeavor. ATLAS’s rapid-scan mode, Rubin’s 3.2-gigapixel LSST camera, and JWST’s infrared sensitivity are unveiling details from volatile inventories to grain size distributions that were out of reach during the fleeting windows of ‘Oumuamua and Borisov. High-resolution spectroscopy over 300–2500 nm is constraining OH and CN production rates, while near-infrared data probe deeper compositional layers.
As 3I/ATLAS shines brighter toward perihelion, its changing coma will be a lab to study the behavior of ancient, radiation-irradiated ices when subjected to solar heating. Within a few months, it will disappear into the blackness, with a legacy of spectra, images, and orbital data a chemical and dynamical footprint from another star system, unprecedented in size and scope.
