The latest on comet 3I/ATLAS

JWST observation of Interstellar comet 3I/ATLAS.

(This article forms a part of the Science for All newsletter that takes the jargon out of science and puts the fun in! Subscribe now!)

3I/ATLAS is the third confirmed interstellar object to pass through the Solar System, after 1I/ʻOumuamua in 2017 and 2I/Borisov in 2019. It was first spotted on July 1, 2025, by the ATLAS telescope in Río Hurtado, Chile. The Minor Planet Centre issued the designation C/2025 N1 (ATLAS) and, on July 2, confirmed its interstellar nature and assigned the permanent interstellar prefix 3I.

The comet’s path through the solar system and its hyperbolic orbit are together clear that 3I/ATLAS isn’t gravitationally bound to the sun.

Trajectory solutions from NASA have also indicated that 3I/ATLAS poses no hazard to the earth. Its closest approach to our planet is about 1.8 astronomical units (AU). Its perihelion, i.e. the point at which it got closest to the sun, occurred around October 29-30, 2025, at roughly 1.4 AU — just inside the orbit of Mars. Because the object was near solar conjunction as it approached perihelion, it was poorly placed for ground-based observatories on the earth to track; it only emerged into the dawn sky in early November.

The comet has been faint by amateur standards and is never expected to become a naked-eye target, although its behaviour around perihelion did draw intense professional attention. As it came off conjunction, astronomer Qicheng Zhang reported the first post-perihelion observations with the Lowell Discovery Telescope in the USA, including continued brightening and a gaseous, bluish appearance in images taken after October 31.

The comet is proving unusual in terms of its chemical composition. Early spectroscopy using the James Webb Space Telescope (JWST) detected a coma dominated by carbon dioxide. In fact its ratio of carbon dioxide to water was around 8 — among the highest measured in any comet. Carbon monoxide, water vapour, carbonyl sulphide, water ice, and dust were also present. The dominance of carbon dioxide suggested that the comet may have formed near a carbon dioxide ice line in its original system.

Follow-up studies have since advanced a different explanation, however. In one recent preprint paper, scientists from Belgium and the US argued that long exposure to galactic cosmic rays during interstellar travel could have processed the outer tens of meters of the comet’s nucleus, converting carbon monoxide to carbon dioxide, creating an organic-rich crust. If this possibility is true, it could have implications for a larger issue: scientists often study such interstellar objects for clues about their environs from a long time ago. But such an object’s outer shell has become transformed by cosmic rays, scientists will have to wait until the shell is completely eroded and exposes the less processed material below, to conduct studies.

The size and age of 3I/ATLAS are also unclear. Early estimates suggested that its nucleus could be many kilometres wide. Dynamical models have suggested that the comet originated in the Milky Way’s older star populations, meaning its age could exceed that of the Solar System. However, this conclusion is uncertain and needs more study to refine.

There has been some public discussion that included speculation that comet 3I/ATLAS has artificial origins. But NASA scientists have explicitly rejected these claims, noting that 3I/ATLAS’s observed dynamics and coma activity are consistent with that of a natural comet.

As of mid-November, the observing situation has improved for those watching with telescopes of a moderate aperture in the northern hemisphere. Astronomers have also called for people to not confuse 3I/ATLAS and a newly found “nearly interstellar” comet called C/2025 V1.

The next several months of spectroscopy and photometry will test whether the erosion of the comet’s outer layers will expose the inner ones. Scientists will also hope to refine its size and activity, and — by comparison with 2I/Borisov and more interstellar visitors in future — begin to map the diversity in comets from beyond the solar system.