Observations of the interstellar object 3I/ATLAS conducted with the Keck II telescope’s Cosmic Web Imager (KCWI) at 0.3425–0.55 µm on August 24, 2025, captured narrow-band emissions centered on nickel (0.3865–0.3885 µm) and cyanide (0.3605–0.3625 µm) wavelengths.

The data show that nickel emission is concentrated near the nucleus, while cyanide extends farther outward, with exponential radii of about 600 km (370 miles) and 840 km (520 miles), respectively.

Unlike all known comets, including the interstellar comet 2I/Borisov, 3I/ATLAS displays strong nickel emission but no detectable iron. The Keck team proposed that this behavior may arise from a natural carbonyl process, analogous to industrial nickel refining through the intermediate compound nickel tetracarbonyl Ni(CO)₄, potentially forming and decomposing close to the nucleus.

The emission pattern also exhibits a distinct anti-tail, a narrow extension pointing toward the Sun rather than away from it. This geometry, previously recorded by the Hubble Space Telescope on July 21, 2025, suggests an anisotropic gas or dust ejection mechanism rather than solar-radiation-driven dust dynamics typical of comets.

Complementary measurements from other instruments reinforce the object’s anomalous chemistry. James Webb Space Telescope (JWST) spectroscopy revealed a CO2-dominated coma with minor components of H2O, CO, and OCS, and a CO2/H2O ratio of approximately 8 ± 1, significantly higher than in solar-system comets.

Ultraviolet observations with the Neil Gehrels Swift Observatory detected OH emission at 3.51 AU (525 million km) from the Sun, implying a water production rate of about 1.35 × 1027 molecules s⁻1 or roughly 40 kg s⁻1. A polarimetric campaign using the Very Large Telescope and other ground facilities measured a narrow negative polarization minimum (−2.7% at 7° phase) with an inversion angle of about 17°, a signature described as unprecedented among comets and asteroids.

3I/ATLAS is the third confirmed interstellar object, following 1I/‘Oumuamua and 2I/Borisov. It follows a hyperbolic trajectory and will reach perihelion at approximately 1.4 AU (210 million km) from the Sun around October 29, 2025. Current Hubble limits constrain its nucleus diameter to below 5.6 km (3.5 miles).

In his technical commentary, Avi Loeb, head of the Galileo Project, founding director of Harvard University’s Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard–Smithsonian Center for Astrophysics, and former chair of the university’s Department of Astronomy (2011–2020), indicated that the accumulating data continue to mark 3I/ATLAS as an outlier.

Loeb, a former member of the President’s Council of Advisors on Science and Technology and former chair of the Board on Physics and Astronomy of the U.S. National Academies, maintains his rating of 4 on the “Loeb scale”, a qualitative measure of anomaly he introduced to categorize the deviation of cosmic objects from established natural classes.

He noted that NASA’s Mars Reconnaissance Orbiter, using its HiRISE (High Resolution Imaging Science Experiment) camera, captured images of 3I/ATLAS on October 2, 2025, as the object passed about 30 million km (19 million miles) from Mars.

The observations were intended to provide a side view of the gas and dust envelope surrounding the object, with an image resolution of roughly 30 km (19 miles) per pixel — about three times sharper than previous Keck and Hubble images. As of October 16, 2025, the HiRISE data have not yet been publicly released.

Additional spacecraft observations are expected from ESA’s JUICE mission in November 2025 and NASA’s Juno spacecraft in March 2026, which will offer new information on the object’s structure and composition.

While some researchers have proposed speculative origins, including artificial hypotheses explored in a theoretical study by Hibberd, Crowl, and Loeb (2025), most of the astronomical community presently interprets 3I/ATLAS as a natural interstellar comet exhibiting unusual volatile chemistry and metallic emissions.

The combination of nickel dominance, iron depletion, and anti-solar morphology represents a complex case not yet explained by conventional models of cometary activity. Further spacecraft and telescope observations in late 2025 and 2026 are expected to refine these interpretations.

References:

1 New images of nickel and cyanide around 3I/ATLAS from the Keck Telescope – Avi Loeb – October 15, 2025

2 Spatial Profiles of 3I/ATLAS CN and Ni Outgassing from Keck/KCWI Integral Field Spectroscopy – W. B. Hoogendam et al. – Earth and Planetary Astrophysics – October 15, 2025 – https://doi.org/10.48550/arXiv.2510.11779

3 Comet 3I/ATLAS – NASA Science – Accessed October 16, 2025