Interstellar comet 3I/ATLAS methane shows 3.18 percent heavy hydrogen, 13 times the solar system record measured by Webb
A study published in Nature Astronomy reports that methane in the interstellar comet 3I/ATLAS contains heavy hydrogen at a ratio of 3.18 percent, with an uncertainty of 0.30 points, 13 times the level measured in the solar system. The team said the ratio shows the comet was born in an incredibly cold, unknown star system.
The comet was reported on July 1, 2025, by the ATLAS survey telescope in Río Hurtado, Chile, and had been known for less than six months. The James Webb Space Telescope observed it on December 22, 2025, under General Observer program 5094. Before that, school groups in Upminster and Bridgend in Britain, Alcarràs in Spain and Zagreb in Croatia sent in position measurements through the Comet Chasers program, helping astronomers refine the comet's predicted location.
The paper is led by Nathan X. Roth of NASA's Goddard Space Flight Center, with 17 co-authors. At the time of observation the comet was about 360 million kilometres from the Sun and 269 million kilometres from Webb. The paper does not name a home star for 3I/ATLAS.
Source: Nature Astronomy
Published by GSM · USA (original Chinese edition)
Photo: 图中是詹姆斯韦布空间望远镜的艺术想象图，该望远镜对星际彗星3I/ATLAS作了光谱观测。ESA, CC BY 4.0, via Wikimedia Commons
