Letter to the Editor

Perihelion observations of interstellar comet 3I/ATLAS with the IRAM 30-m telescope^★

¹ LIRA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Université, Université de Paris, 5 place Jules Janssen, F-92195, Meudon, France
² IRAM, Avd. Divina Pastora, 7, 18012, Granada, Spain
³ IRAM, 300, rue de la Piscine, F-38406, Saint Martin d’Hères, France
⁴ NASA Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD, 20771, USA
⁵ Department of Physics, Catholic University of America, 620 Michigan Ave. NE, Washington, DC, 20064, USA
⁶ Department of Physics, American University, 4400 Massachusetts Ave NW, Washington, DC, 20016, USA

^★★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 4 February 2026
Accepted: 23 March 2026

Abstract

Comet 3I/ATLAS is the third interstellar comet identified as passing through the Solar System. Its high outgassing activity and favourable perihelion passage on October 29, 2025 UT provided an excellent opportunity to investigate the composition of its coma gases through millimeter spectroscopy. We present observations undertaken with the IRAM 30-m telescope on November 1–3, 2025 at an heliocentric distance of 1.36–1.37 au. Lines of HCN, CH₃OH, CO, and H₂CO are well detected, and ∼4σ detections are obtained for CS and CH₃CN. The search for H₂S was unsuccessful. Abundances of CO, H₂CO, CH₃OH, and CH₃CN relative to HCN are in the upper ranges of values measured in Solar System comets. The sulfur-to-carbon abundance ratio in 3I/ATLAS’s coma is at most the minimum value observed in comets. The unusually low expansion velocity of coma gases suggests a near-nucleus gas flow driven by heavy molecules such as CO₂, and/or a large fraction of the gaseous production coming from subliming icy grains.