The interstellar comet 3I/ATLAS, detected in July 2025, surprised astronomers by changing color three times — red, green and blue — during its passage through the Solar System. Observations carried out by solar satellites indicated an increase in luminosity twice as high as expected, especially close to perihelion, on October 29th. The phenomenon, described in a study published on arXiv, raises questions about the chemical composition of the comet. The expectation is that new observations in December will reveal more details.
- Interstellar origin: The comet comes from outside the Solar System, with a hyperbolic orbit.
- Unique composition: High concentration of carbon dioxide, eight times greater than water.
- Record speed: Travels at more than 210 thousand km/h, surpassing other interstellar objects.
Color phenomenon
The comet showed a reddish light in July, due to the dust in its coma. The hue changed to green in September, possibly due to dicarbon or cyanide. In October, a bluish color was observed, associated with cyanogen and ammonia emissions. The variation in tones reflects chemical changes in the surface heated by the Sun.
Exceptional brightness
The brightness of 3I/ATLAS reached magnitude 9 at perihelion, visible in amateur telescopes. The increase, 50% greater than projections, challenges traditional models of interstellar comets. The intensity suggests intense ejections of gas and dust, still unexplained.finitive. New analyzes await greater visibility of the comet in the Northern Hemisphere in December.
Spatial Observations
STEREO-A, SOHO and GOES-19 satellites captured data during its passage by the Sun. The observations were crucial, as the comet was hidden from Earth in October. ESA’s JUICE probe plans to study the comet’s coma in November. Images from Hubble and James Webb confirm a diffuse, carbon dioxide-rich tail.
Intriguing Chemistry
The James Webb Space Telescope revealed a coma with eight times more carbon dioxide than water, an unprecedented proportion. This composition suggests formation in a star system with extreme conditions. The presence of carbon monoxide and nickel reinforces the uniqueness of the 3I/ATLAS. Astronomers speculate that the comet, about 7 billion years old, preserves traces of the early universe.
Trajectory and future
3I/ATLAS follows a hyperbolic orbit, with no gravitational link to the Sun. After perihelion, it approaches Earth on December 19, 168 million kilometers away. Two ESA probes will cross its tail before it leaves the Solar System in 2026. The comet poses no risk to Earth, but its passage offers a unique opportunity tostudy interstellar objects.