A sudden and rapid surge in the brightness of interstellar comet 3I/ATLAS was observed by spacecraft on Wednesday, October 29, as it reached its closest point to the sun. The comet’s unusual behaviour has got the attention of scientists worldwide.
Earlier this week, Harvard professor Avi Loeb told Newsweek that the comet’s behaviour at perihelion, the stage of its orbit when it comes nearest to the sun, would serve as an “acid test” to determine whether it might display characteristics of an extraterrestrial spacecraft, a possibility he has previously suggested.
Meanwhile, scientists studying the interstellar object, discovered in July this year, noted its brightening in a preprint report. The study has not yet been peer-reviewed. The researchers said the reason for 3I’s sudden increase in brightness is still unclear.
According to researchers, data obtained from STEREO-A’s SECCHI HI1 and COR2, SOHO’s LASCO C3, and GOES-19’s CCOR-1 instruments between September and October 2025 recorded a sharp increase in the comet’s brightness.
The scientists also reported that colour measurements conducted by LASCO show the comet to appear bluer than the Sun, indicating that gas emissions during perihelion play a significant role in its visual brightness. Their analysis concluded that the comet will be brighter after conjunction than before, likely due to the powerful and visible emission of gas.
Loeb said that the comet’s appearance of bluer than the Sun is extremely surprising. He explained that under normal conditions, dust particles redden sunlight, and because the surface of this object is much cooler than the Sun’s 5,800-degree Kelvin photosphere, its colour should normally be red.
He added that its blue colour at perihelion should be considered the ninth unusual characteristic of this mysterious interstellar object, reported Newsweek.
Loeb reported that based on eight previously recorded unusual properties, he gave Comet 3I/ATLAS a score of 4 out of 10 on his “Loeb Scale,” pointing to its possible technogenic origin. These abnormalities include its path being within only five degrees of the ecliptic plane, and its visible jet (anti-tail) directed towards the Sun during July-August 2025, which is not the result of an optical illusion but a real phenomenon.
It was also observed that the comet’s nucleus is significantly larger than other known comets. Its arrival was timed such that it passed very close to Mars, Venus, and Jupiter, while remaining invisible from Earth at its perihelion. Some other unexpected properties have also been observed in this object.
NASA has clarified that Comet 3I/ATLAS poses no threat to Earth and will remain at a safe distance, reported Newsweek.