Might an interstellar comet bring the chemical signature of industry? That is the question raised by fresh spectroscopic findings from the European Southern Observatory’s Very Large Telescope, which have identified an unprecedented find: gas-phase nickel in the coma of 3I/ATLAS without any associated iron. In natural astrophysical settings, the two elements are alloyed together in supernova ejecta and are found together in cometary plumes. Lack of iron, scientists point out, is a characteristic of the nickel carbonyl process a industrial refining method on our planet so the discovery represents a possible technosignature.
The anomaly comes against a background of other chemical surprises. Measurements by NASA’s SPHEREx mission and the James Webb Space Telescope indicate that the coma is carbon-dioxide dominated, constituting about 95 percent of detected volatiles with only about 5 percent water vapor. This water-poor, CO₂-rich profile is the reverse of most Solar System comets, which release water vapor as they enter the Sun. Even more bizarre, high-resolution Hubble images detected no detectable dust tail in spite of the bright coma, contradicting models assuming a small, dusty nucleus that reflects sunlight.
The nickel emission lines at a heliocentric distance of 2.8 astronomical units were recorded with an inferred mass-loss rate of nickel at around 5 grams per second, increasing steeply with solar distance following a power-law index of –8.43. Cyanide (CN) was also seen at ~20 grams per second, with an even more precipitous index of –9.38. Such high dependencies indicate unstable release mechanisms sensitive to solar heating yet the mixture away from water-powered processes characteristic of ordinary comets.
Spectroscopic identification of these elements depended upon high-dispersion methods able to isolate weak atomic lines against the continuum of scattered light. VLT’s X-shooter, for example, can resolve nickel’s neutral atomic transitions in the visible and near-infrared while searching for iron lines at the same time. The non-detection of Fe I features, if at solar Ni/Fe ratios the detection is possible, is the basis for the argument of a real compositional anomaly.
Trajectory analysis introduces another level of mystery. 3I/ATLAS has a retrograde orbit inclined only 4.886 degrees from the ecliptic plane, a very precise alignment which some have suggested might be compatible with intentional targeting of the inner Solar System. Its nearly 58 km/s hyperbolic excess velocity and eccentricity of 6.2 establish its interstellar origin, probably from the thick disk of the Milky Way. The coincidence is statistically unlikely for a random incoming object of this size, fueling speculation regarding non-natural origins.
The argument for deliberate observation is supported by the object’s imminent close flyby of Mars on October 3, 2025, when the HiRISE camera on board the Mars Reconnaissance Orbiter will capture it at ~30 km per pixel. These observations might untangle the relative roles of the coma and nucleus in their contribution to brightness, placing limits on the size and mass of the nucleus. If the nucleus has a size comparable to the 46 km from initial SPHEREx photometry, it would be a million times more massive than 2I/Borisov a highly unlikely first detection if such objects are typical and have random distributions.
In light of the nickel anomaly, radio astronomers are also exploring technosignature searches. As Nobel winner David Gross has hinted at during a recent Copenhagen conference, sites might be assigned to scan 3I/ATLAS for narrowband signals. Radio arrays today are capable of picking up signals orders of magnitude weaker than the historic 1977 “Wow! Signal,” and networks of tiny dishes, like in the Wow@Home experiment, could offer round-the-clock coverage. The centimeter-wavelength sensitivity of great single-dish and interferometric arrays would facilitate the detection of even modest-power emissions if beamed into Earth.
The dominance of CO₂ in the comet is a technological mystery in itself. In Solar System comets, CO₂ sublimates at ~100 K and triggers activity beyond Mars’ orbit, but is normally correlated with water and sometimes carbon monoxide. Lack of CO in SPHEREx and JWST spectra implies either exotic primordial composition or significant volatile loss over tens of billions of years in interstellar space. If nickel is in fact derived from nickel carbonyl decomposition, CO₂ might be a product of processing, although none of the currently known natural cometary processes creates this mixture without water and iron.
As the object moves away from perihelion, coordinated multi-wavelength efforts from optical spectroscopy to radio SETI searches will be required. High-resolution spectrographs will be able to continue tracking the Ni/Fe ratio, and thermal infrared instruments will be able to search for remaining water ice or other volatiles. The overlap of chemical, dynamical, and morphological anomalies in 3I/ATLAS makes it one of the most technologically intriguing interstellar visitors so far seen, and quite possibly the first to blur the line between comet science and the search for extraterrestrial technology.
