Comet 3I/ATLAS, confirmed as the third interstellar object to visit Sistema Solar, was detected on July 1, 2025 by the ATLAS system telescope at Rio Hurtado, at Chile.
This discovery occurred when the object was about 676 million kilometers from Sol, moving at 220 thousand kilometers per hour.
Astronomers from agencies such as Nasa and Agência Espacial Europeia (Esa) immediately began observations to map its hyperbolic orbit, which indicates origins outside our star system.
The trajectory predicts perihelion, the closest point to Sol, on October 31, 2025, followed by a safe passage of around 274 million kilometers from Terra on December 19.
Early observations and confirmation
Ground-based telescopes confirmed cometary activity shortly after initial detection.
On July 2, 2025, Telescópio Óptico Nórdico in Ilhas Canárias recorded a diffuse appearance, with a reddish coma typical of organic dust.
Other equipment, such as the Telescópio Canadá-França-Hawaii, detected an elongation of 3 seconds of arc, suggesting an incipient tail.
- The initial coma measured 13,000 kilometers in diameter in June.
- Growth to 18 thousand kilometers occurred by July.
- Reddish color points to irradiated organic compounds, similar to D-type asteroids.
These initial measurements helped refine the speed and rule out collisions with planets.
Chemical analyzes reveal solar similarities
The Nasa SPHEREx observatory examined 3I/ATLAS between August 7 and 15, 2025, when the comet was 466 million kilometers from Sol.
The data indicated an abundance of water ice and carbon dioxide in the coma, a gaseous cloud surrounding the nucleus.
This chemical composition resembles native Sistema Solar comets, suggesting similar galaxy formation processes.
Further research with Telescópio James Webb confirmed the presence of methanol and hydrogen cyanide, with a methanol/HCN ratio of up to 124, within ranges observed in comets such as Hale-Bopp.
We’ve just released the latest images of the interstellar comet 3I/ATLAS, as observed by eight different spacecraft, satellites, and telescopes.
— NASA (@NASA)November 19, 2025
Here’s what we’ve learned about the comet — and how we’re studying it across the solar system:https://t.co/ZIt1Qq6DSp pic.twitter.com/ITD6BqVlGn
Detailed images from ground-based telescopes
Telescópio Gemini Sul, at Chile, captured the first zoomed-in views in August 2025, near La Serena.
The September photos showed an extended coma of dust and gas, with an elongated tail indicating increased activity as the comet approached Sol.
Astronomers noted that the elongation of the tail was more pronounced than in previous observations, reflecting accelerated sublimation of volatiles.
These images, processed by international teams, provided data on the size of the nucleus, estimated at between 440 meters and 5.6 kilometers in diameter.
The core remains solid and icy, releasing material in the form of teardrops or dust drops.
Contributions from orbiters on Marte
Sonda Trace Gas Orbiter of Esa, in Martian orbit, observed the comet for a week starting October 1, 2025.
The minimum distance to the probe was 30 million kilometers on October 3, allowing unprecedented viewing angles.
The images revealed the nucleus as a fuzzy white dot against the starry background, improving orbital predictions by a factor of ten.
Data from the Mars Express probe complemented it, measuring gaseous emissions and adjusting trajectory models for planetary defense.
This collaboration demonstrated the use of interplanetary spacecraft to track fast interstellar-like objects.
Nasa mission records in September
Several Nasa probes recorded 3I/ATLAS at the end of September 2025, despite delays due to government shutdowns.
The Lucy probe captured stacked images on September 16, showing the comet as a bright spot with a faint tail, on its way to Marte.
The PUNCH satellites, in low Earth orbit, observed from September 28 to October 10, when the object was 372 to 378 million kilometers away.
The MAVEN spacecraft and the Perseverance rover at Marte added surface views, with Mastcam-Z recording the comet as a faint spot on October 4, 29.9 million kilometers away.
- PUNCH detected interactions with the solar wind.
- SOHO monitored passage behind Sol in October.
- Psyche recorded four observations on September 8 and 9, 53 million kilometers away, refining the trajectory.
These collective records formed a global catalog of images.
Hubble sightings and recent updates
Telescópio Espacial Hubble of Nasa revisited 3I/ATLAS on November 30, 2025, using Câmera of Campo Largo 3.
286 million kilometers from Terra, the comet appeared as a bright point, with background stars as streaks, confirming orbital movement.
This was the second observation of Hubble, after July, when it revealed the teardrop shape of the dust cocoon.
Esa released photos of Juice, en route to Júpiter, in November, after approaching 66 million kilometers on November 4.
The images highlighted the coma halo and suggested two tails: ionized plasma extending upward and dust downward to the left.
Additional data from Juice arrives in February 2026, expanding the composition study.
Preparations for public observation
Comet 3I/ATLAS remains visible with small telescopes in the pre-auroral sky until spring 2026.
On December 12, 2025, it reaches more than 90 degrees from Sol, facilitating middle-of-the-night observations.
Opposition occurs on January 22, 2026, when it will be visible all night, although fainter.
Applications like Eyes on the Solar System da Nasa allow you to track your departure from the
Astronomers encourage amateurs to contribute data to refine models, without risk of terrestrial impact.
