Astronomers are sounding the alarm after reporting unusual behavior from the interstellar object 3I/ATLAS as it nears the Sun.
The leading scientists tracking the 3I/ATLAS have reported two significant new developments after moving behind the Sun, out of view from Earth.
Behind the Sun, the comet appears to show signs of acceleration beyond what is expected by gravity.
And for reasons not yet clear, it appears to have changed color for a second time as the object approached perihelion.
The second unexplained color change and signs of non-gravitational acceleration are sending shockwaves through the astronomy and astrophysics communities.
The findings have just been revealed in a study published on the pre-print server arXiv.
3I/ATLAS, first identified on July 1 by the Asteroid Terrestrial-impact Last Alert System (ATLAS), is only the third confirmed interstellar visitor to pass through our Solar System, following 1I/ʻOumuamua and 2I/Borisov.
Its unusual trajectory, with an eccentricity above 6, confirmed its extrasolar origin.
Since its discovery, astronomers have debated the object’s composition and behavior, noting characteristics that suggest it may be an exceptionally old remnant from a much earlier era of the universe.
Researchers have also struggled to determine which region of the Milky Way it originated from.
As the object approached perihelion on October 29, it moved into a position directly behind the Sun from Earth’s perspective.
The maneuver has limited observational opportunities during a key phase of its journey.
In the new study, scientists used imagery from Solar TErrestrial RElations Observatory (STEREO), SOlar and Heliospheric Observatory (SOHO), and the GOES-19 weather satellite to monitor the object despite the line-of-sight challenges.
Their findings indicate that 3I/ATLAS brightened far more rapidly than typical Oort-cloud comets and underwent a marked shift in color.
The team reports the object brightened to approximately magnitude 9, making it visible through small telescopes.
They note that “color photometry shows the comet to be distinctly bluer than the Sun.”
This is the second known color change, and researchers say they currently lack a definitive explanation for the behavior.
The brightening rate is described as “far in excess” of what is normally observed in comets originating from the outer reaches of the Solar System.
Evidence of Non-Gravitational Acceleration
In separate observations, Davide Farnocchia of NASA’s Jet Propulsion Laboratory reported signs of non-gravitational acceleration, meaning the object’s motion cannot be explained by gravity alone.
Harvard University theoretical physicist Avi Loeb summarized the findings, writing that the acceleration was detected at a distance of 1.36 AU from the Sun, roughly 203 million kilometers.
He notes a “radial acceleration away from the Sun of 135 kilometers (=9×10^{-7}au) per day squared” and a “transverse acceleration relative to the Sun’s direction of 60 kilometers (=4×10^{-7}au) per day squared.”
Scientists caution that non-gravitational acceleration does not prove the object is maneuvering; many comets experience additional acceleration due to outgassing, the release of gas and dust as surface ices are heated by the Sun.
Loeb writes that, assuming typical thermal ejection speeds, “the evaporation half-life of 3I/ATLAS is 6 months,” adding that the object could lose around ten percent of its mass over the month surrounding perihelion.
If so, researchers expect a detectable plume of gas during November and December 2025.
The European Space Agency’s JUICE spacecraft may be able to help confirm whether this mass loss is occurring.
Brightening Continues, but Causes Remain Unclear
In their pre-print analysis, the observing team reports that “the comet will likely emerge from conjunction considerably brighter than when it entered,” projecting a geocentric V magnitude of ~9 at perihelion.
They note this may be “driven by prominent, visible gas emission.”
However, they emphasize that major questions remain:
“The reason for 3I’s rapid brightening… remains unclear.”
They note that earlier cooling effects from CO₂ sublimation could have suppressed water-ice sublimation at greater distances, contributing to the unusual brightness surge as the object approached the Sun.
They also note that 3I/ATLAS is moving toward the Sun more rapidly than most comets, which may be influencing its activity.
What Comes Next
Researchers stress that 3I/ATLAS’s post-perihelion behavior is unpredictable.
“Without an established physical explanation, the outlook for 3I’s postperihelion behavior remains uncertain,” they conclude.
They add that the object could plateau, continue brightening briefly, or begin to fade as it moves away from the Sun.
Continued observations in the coming weeks may help resolve the questions surrounding this rare interstellar visitor.
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