The third confirmed visitor from outside our solar system continues to baffle astronomers. Comet 3I/ATLAS reached its perihelion—its closest point to the Sun—around October 29, exhibiting behavior that once again defies scientific expectations and reveals clues about its ancient interstellar journey. In this new chapter of its voyage, what has most surprised researchers is the speed at which the comet has brightened as it approaches the Sun. Between mid-September and late October, as 3I/ATLAS approached from approximately twice the Earth-Sun distance to 1.36 astronomical units, its brightness increased much more dramatically than predicted, displaying atypical behavior even for such an exotic object. Observations reveal unprecedented brightness increase. From Earth, the comet has remained virtually invisible for the past month, as it has been almost directly behind the Sun. However, an ingenious team of astronomers found a way to continue observing it. during this critical period.

Qicheng Zhang of the Lowell Observatory and Karl Battams of the US Naval Research Laboratory used solar monitoring satellites—STEREO-A, SOHO, and GOES-19—to capture the comet's spectacular transformation, as reported by Universe Today.

The result was astonishing: according to their calculations, as reported by the space-focused publication, the comet's brightness increased in inverse proportion to the heliocentric distance raised to the power of 7.5, approximately twice as fast in its increase in brightness as a typical comet.

“The reason for 3I's rapid increase in brightness, which far exceeds the rate of increase in brightness of most Oort cloud comets at a similar radial distance, remains unclear,” Zhang and Battams state in a paper published on the preprint server arXiv, which is still awaiting peer review.

In comparison, comets Common stars tend to gradually increase their brightness as they approach. the Sun, when heat turns ice into gas. However, this interstellar visitor is shining at approximately twice the usual speed, indicating that something unusual is happening on its surface.

Unique Chemical Composition

Observations showed that comet 3I/ATLAS shines with a bluish hue, distinct from the golden reflection of the Sun. That color indicates that its luminosity comes not only from dust, as is usually the case with comets, but also from gases that are being actively released as it heats up.

Interestingly, in previous observations, the comet's dust showed a reddish hue. The shift to a bluer color suggests that its surface is undergoing some change, probably—although still unconfirmed—caused by the release of molecules such as cyanogen or ammonia, responsible for that cooler, brighter hue. Furthermore, images from the GOES-19 satellite confirmed that the comet is surrounded by a large envelope of gas and dust—the so-called coma—which extends several arcminutes across the sky. This is a clear sign that 3I/ATLAS is very active and that solar heat is causing its surface to release material at high speed. During its closest point to the Sun, its brightness reached approximately magnitude 9, enough to be visible with small telescopes. The research team is considering several possible mechanisms to explain the unusual and rapid increase in brightness. This could be related, in part, to the speed at which the comet approaches the Sun, although it could also reflect characteristics of its internal composition. This is particularly exciting because, if the internal composition of 3I/ATLAS differs from that of comets in the Oort cloud, it could suggest that the planetary system from which it originates also has a different chemical composition. Furthermore, astronomers believe that its sublimation process—the direct conversion of ice into gas—occurs in an unusual way: 3I/ATLAS would continue to expel carbon dioxide even at a relatively close distance to the Sun—about three times the Earth-Sun distance—whereas normally water vapor would predominate. This behavior could have altered its heating rate and explain its unexpectedly intense brightness.

Cosmic rays transformed its surface

Beyond its unusual luminous behavior, 3I/ATLAS hides invisible scars: a crust profoundly altered by cosmic rays during their billions-of-year journey through the galaxy.

This is suggested by another recent study, based on observations from the James Webb Space Telescope and still pending peer review, which indicates that the comet has accumulated so much interstellar radiation that its outer layer has been chemically transformed to a depth of between 15 and 20 meters.

“It's very slow, but over billions of years, the effect is very strong,” explains Romain Maggiolo of the Royal Belgian Institute for Space Aeronomy, Live Science.

According to the experts, cosmic rays would have converted the carbon monoxide on its surface into carbon dioxide, significantly altering its original composition.

The authors believe that these findings could represent a “paradigm shift” in the study of interstellar objects, since, if confirmed, the comet would no longer show the pristine material of its star system of origin, but would instead be a product of its long journey.

“We have to be careful and take into account the aging processes,” warns Maggiolo.

Even so, there remains hope that The erosion caused by solar heat during its passage through perihelion may remove the surface layer and reveal original materials, allowing for a better understanding of what these cosmic travelers are made of and, consequently, inferences about the conditions of the star systems from which they originate.

Upcoming observations of the interstellar comet

Discovered on July 1 by the ATLAS network in Chile, the comet is now traveling at more than 210,000 km/h – about 68 km per second – following an unusually flat and straight trajectory.

Some studies suggest that 3I/ATLAS could be one of the oldest comets ever observed, formed about 3 billion years before the solar system.

Now that it is emerging from behind the Sun, ground-based observatories will be able to study it in detail during November and December. Meanwhile, ESA's JUICE mission, en route to Jupiter, will observe it from deep space between November 2 and 25, with its closest approach on November 4, at a distance of 64 million kilometers. According to IFL Science, the full JUICE data will not be published until February 2026 due to the slow transmission speed, but they could reveal whether solar erosion has exposed pristine material from the comet's nucleus. While scientists remain puzzled by its rapid increase in brightness—which could be due to its unique composition, its speed, or peculiarities acquired during its interstellar journey—one thing is clear: this third visitor from beyond our solar system could offer valuable clues about the formation of star systems and the chemical diversity of the cosmos.