A team of astronomers has reported a notable increase in the brightness of the interstellar comet 3I/ATLAS as it nears its closest point to the Sun. On October 29, 2025, the comet reached perihelion, coming within approximately 210 million kilometers of the Sun. Despite this significant event in its trajectory, further observations will not be possible until early December 2025, when the comet re-emerges from behind the Sun and becomes visible to ground-based telescopes again.
A key moment will occur on December 19, 2025, when 3I/ATLAS will reach its closest approach to Earth, positioned about 1.8 astronomical units, or around 270 million kilometers away. This timeframe will be crucial for the study and analysis of this third-known interstellar comet, which originates from deep space and traverses our solar system.
In the lead-up to perihelion, astronomers have been intrigued by a particular detail regarding the comet”s brightness. Due to its position behind the Sun, optical observations from Earth were hampered in the preceding month. Researchers Qicheng Zhang from the Lowell Observatory and Karl Battams from the U.S. Naval Research Laboratory reported their findings in a study submitted to the Astrophysical Journal Letters on October 28, 2025. They noted that the brightness of 3I/ATLAS “far exceeded the brightness increase rate of most Oort cloud comets,” utilizing coronagraphs aboard spacecraft such as STEREO-A, SOHO, and the GOES-19 satellite from the National Oceanic and Atmospheric Administration (NOAA).
The researchers presented photometric data collected with the SECCHI HI1 and COR2 instruments from STEREO-A, LASCO C3 from SOHO, and CCOR-1 from GOES-19 between September and October 2025, which illustrated a rapid increase in the comet”s brightness. Additionally, color photometry from LASCO indicated that the comet appears significantly bluer than the Sun, consistent with the emission of gas that contributes a substantial portion of the visible brightness near perihelion.
Experts suggest that the enhanced brightness is likely due to a sudden increase in water sublimation from the comet”s nucleus. “Our preliminary analysis of these data indicates that the comet is likely to emerge from solar conjunction considerably brighter than when it entered,” they stated.
3I/ATLAS is the third object identified as originating from outside our solar system, classified as interstellar due to its hyperbolic orbital trajectory. Astronomers worldwide are investigating the size and physical properties of this intriguing comet, which has captured significant attention due to its unique characteristics, including its speed and shape.
