The interstellar comet 3I/ATLAS, the third object confirmed to come from outside Sistema Solar, shows a significant increase in brightness and a greenish hue in recently captured images. Observações carried out on November 26 by the Gemini North telescope, at Havaí, show the comet in a phase of high activity after its closest passage to Sol, in October. The closest approach to Terra occurs on December 19, at a safe distance of around 270 million kilometers.
These changes indicate intense sublimation of ices in the core, releasing gases and dust that form a bright coma and an extensive tail. The green color arises from the emission of diatomic carbon molecules excited by solar radiation.
Astronomers monitor the comet’s behavior to understand its composition and origin in another star system.
Origin of the greenish glow
The green hue observed in the 3I/ATLAS coma results from the presence of diatomic carbon (C2).
This molecule, composed of two carbon atoms, emits light in the green range of the spectrum when exposed to intense sunlight.
The phenomenon occurs in several Sistema Solar comets when heated by Sol.
Interstellar visitors from beyond our solar system include 2017 Oumuamua in 2017, 2I/Borisov in 2019, and 3I/ATLAS in 2025. radio signal.#3IATLAS pic.twitter.com/k4dtn5Fe4N
— 永倉政司 nagakura seiji (@uBF2fV1cVQxRjQo)December 16, 2025
Registered color change
Previous images, obtained by Gemini South in August, showed the comet with a reddish color, dominated by dust.
After perihelion, the release of new gases changed the appearance to greenish.
This transition reveals evolution in the composition of the coma as the core heats up and expels volatile materials.
Observations with colored filters confirmed the faint but distinct presence of the green hue.
Intense post-perihelion activity
Solar heating causes direct sublimation of ices into gas, expelling dust and forming a hazy atmosphere around the core.
The tail stretches for tens of thousands of kilometers, pointing away from the Sol.
Astronomers anticipate possible additional bursts of material, common in comets with a delay in conducting heat to the interior.
Brightness has increased significantly since passing through perihelion, facilitating terrestrial observations.
Approximation with Terra
On December 19, 3I/ATLAS reaches the closest point to Terra, 270 million kilometers away.
This distance is equivalent to around 1.8 astronomical units, without any risk of impact.
The comet follows a hyperbolic trajectory, with speeds exceeding 200,000 km/h, and will leave Sistema Solar permanently.
- Minimum distance: 270 million km
- Date: December 19, 2025
- Relative speed: around 210 thousand km/h
- Trajectory: hyperbolic, no return
Interstellar Visitor Features
Discovered in July 2025 by the ATLAS system, 3I/ATLAS is the third confirmed interstellar object, after ‘Oumuamua and Borisov.
It has an active icy core, a coma rich in gases such as carbon dioxide and water vapor, as well as dust.
It is estimated to be one of the largest and oldest observed interstellar objects.
Its composition offers clues about the formation of planetary systems in stars far from Via Láctea.
Observations at multiple wavelengths
Telescopes such as Hubble, James Webb and X-ray observatories recorded the comet in different phases.
Missions on Marte captured close images during a pass by the red planet in October.
The combined observations reveal emissions of hydrogen, carbon and other elements in the coma.
Educational programs, such as Shadow the Scientists, allowed public monitoring of the sessions at Gemini North.
Uncertain future behavior
Astronomers wait for the comet to cool as it moves away from Sol.
Delayed reactions to solar heat can trigger evaporation of additional compounds or flare-ups.
Monitoring continues with dozens of ground and space observatories.
This data contributes to understanding objects formed billions of years ago in distant regions of the galaxy.
