Interstellar Object 3I/ATLAS Shifts Color and Coma as It Approaches Earth

A mysterious interstellar object known as 3I/ATLAS has changed shape and color as it moves through the inner Solar System, with new images and spectral data showing a previously reddish glow shifting to green and the emergence of a gas-rich coma.

Austrian astronomers captured a deep image of 3I/ATLAS on Sept. 7 from dark skies in Namibia during a total lunar eclipse, combining multiple exposures in blue, green and red light. "By combining multiple exposures in blue, green, and red light, we were able to clearly see the comet's gas-rich coma," astronomer Michael Jäger said. Data released by the ATLAS team also showed that the cloud of light around the object expanded more rapidly when 3I/ATLAS was farther from the Sun and slowed its growth as it approached.

Observers have proposed different explanations for the changes. The ATLAS team suggested the change in apparent color and the shift in coma growth rate can be explained by evolving surface and dust properties: as 3I/ATLAS warmed, sunlight-scattering off red dust may have given way to the release of smaller, brighter icy particles that make the surrounding plume more reflective and bluer. Jäger said deeper images show a blue-green coma and a tail consistent with a developing cometary atmosphere.

High-resolution spectroscopy reported by the Very Large Telescope and by a group of Chilean astrophysicists in late August found 3I/ATLAS emitting cyanide (CN) at roughly 20 grams per second and nickel at roughly five grams per second. Those measurements also indicate the production rates of those species rose sharply as the object drew closer to the Sun. Harvard physicist Avi Loeb highlighted the nickel and cyanide detections in a recent blog post and noted that the nickel appears to be present without detectable iron, an unusual pattern compared with typical comets. Loeb wrote that the steep rise in production with decreasing distance followed a pattern he characterized as roughly proportional to the ninth power of the object's distance from the Sun and said the combination of size, brightness and unusual spectral signatures merits consideration of non-natural possibilities.

Other researchers urged caution in drawing such conclusions. Jäger and colleagues emphasized that the coma and a tail are visible in their data, and that the object appears to have reached the threshold where gas-rich comets become active. They and other investigators pointed to plausible natural processes: 3I/ATLAS likely spent billions of years in interstellar space, where radiation can form a thick crust that later fractures as subsurface ices sublimate, releasing gas and dust. The Chilean team suggested that nickel could be liberated from dust by gentle mechanisms such as solar heating or the breakdown of small nickel-bearing compounds rather than by industrial processes.

3I/ATLAS differs in some respects from the two previously observed interstellar visitors. The first, 1I/ʻOumuamua, showed no clear signs of gas or dust; the second, 2I/Borisov, behaved like a typical comet. 3I/ATLAS has displayed an unusual combination of features including an anti-tail, pronounced color changes and a large coma, prompting active study and debate among astronomers.

The object is on track to make its closest approach to Earth on Dec. 19, 2025, at about 168 million miles, roughly the distance between Earth and Mars. Astronomers say continued monitoring across wavelengths will be required to refine production rates, test models for the origin of the metals and radicals detected, and determine whether the observed behavior can be fully accounted for by cometary physics and chemistry.

Researchers worldwide are continuing observations with ground-based telescopes and space assets to track how 3I/ATLAS evolves as it approaches and recedes from the Sun, and to place its properties in context with other interstellar and Solar System comets.