WHAT JUST LANDED
On February 15 — the same day NASA quietly confessed to the TESS “contingency” — a team of astronomers at the Shanghai Astronomical Observatory dropped a paper onto arXiv that should have made every newsroom in the world pick up the phone.
It didn’t. Because the headline they wrote was designed not to.
The paper is called “Pre-perihelion Volatile Evolution of Interstellar Comet 3I/ATLAS Indicating Significant Contribution from Extended Source in the Coma.” Four researchers — Juncen Li, Xian Shi, Man-To Hui, and Jianchun Shi — used two Chinese radio telescopes to listen to 3I during August and September 2025, as it barreled toward the Sun. They were listening for two things: water vapor and carbon monoxide. The standard fingerprints of a melting comet.
They found both. And then they found something they didn’t expect.
The official conclusion? “It’s a comet with unusual chemistry from a cold star system.”
Pre-perihelion Emergence of the CN Gas Coma in 3I/ATLAS Temporally and Spatially Resolved by the 7-Dimensional Telescope Gregory S. H. Paek, Myungshin Im, Mankeun Jeong, Hyeonho Choi, Yoonsoo P. Bach, Masateru Ishiguro, Bumhoo Lim, Seo-Won Chang, Ji Hoon Kim, Jooyeon Geem, Willem B. Hoogendam
We present time-series medium-band (R~20-40) observations of the third interstellar object 3I/ATLAS (C/2025 N1) obtained with the 7-Dimensional Telescope (7DT), enabling spatially resolved monitoring of its gas and dust activity from 2025 July to September. The m400-band image (lambda_c = 400 nm, Delta lambda approx 25 nm) reveals the emergence of pronounced and spatially extended CN emission at heliocentric distances r_h < 3 au. This onset is consistently identified across multiple diagnostics, including a break in the light-curve evolution, excess reflectance, inward expansion of annular excess beyond 10,000-20,000 km, growth of the coma half-light radius from ~11,000 to ~19,000 km, and a rapid rise in the CN production rate Q_CN relative to Af rho. We further separate the CN-emitting and dust-scattered components through two-dimensional surface-brightness fitting into inner (dust) and outer (gas) components. The outer component preserves a nearly constant profile shape, varying only in normalization, implying relatively fast expansion of CN-bearing molecules. Together, these results reveal a transition in the optical from dust-dominated scattering at large heliocentric distances to volatile-driven, gas-dominated activity as 3I/ATLAS enters the inner Solar System. The timing and characteristics of the CN activation resemble the volatile enhancement observed in 2I/Borisov, suggesting that both known active interstellar objects exhibit comparable activation behavior at heliocentric distances of ~2-3 au.Comments: 16 pages, 6 figures. Accepted for publication in The Astrophysical Journal Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Astrophysics of Galaxies (astro-ph.GA) Cite as: arXiv:2602.12930 [astro-ph.EP] (or arXiv:2602.12930v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2602.12930 Submission history From: Gregory S.H. Paek [view email] [v1] Fri, 13 Feb 2026 13:40:19 UTC (1,343 KB)