Comet 3I ATLAS remains visible from many spacecraft and is confirmed a comet. Despite what you might read in the media it’s not hiding from us. The comet reached perihelion October 30, 2025, and is not visible from earth-based telescopes at this time but astronomers have been tracking the comet with the CCOR-1 coronagraph on NOAA’s GOES-19 geostationary weather satellite, the PUNCH satellite quartet, the Solar and Heliospheric Observatory (SOHO), and the STEREO-A spacecraft.
Comet 3I/ATLAS is on its long eliptical orbit around the sun and out of our solar system but on its way back it will pass closest to Earth on December 19, 2025, at a distance of 269 million kilometers (1.80 au), or nearly twice the Earth-Sun distance. We are all looking forward to its return to get a look at it with our telescopes. If I get a chance to see Comet 3I ATLAS it will be my first look at an interstellar comet. In early October, the comet was only magnitude 11.5, but space-based instruments indicate that it has continued to brighten quite a bit. Astronomers Qicheng Zhang at the Lowell Observatory and Karl Battams from the U.S. Naval Research Laboratory posted an article indicating that the comet had a rapid brightness surge on its approach to perihelion. The astronomers used data from the space-based instruments I just mentioned to extrapolate a perihelion brightness of magnitude 9, which is considerably brighter than originally forecast. They suspect the increase may be driven by “visible gas emissions” and that 3I ATLAS will likely emerge from solar conjunction “considerably brighter than when it entered.”
The reason for the rapid brightening of this comet, which far exceeds the brightening rate of most Oort cloud comets at similar distances from the sun, remains unclear,” the authors write. They suggest that the cooling effect of sublimating (vaporizing) CO2 ice, which held water-ice vaporization in check for a time as the comet approached the Sun.
The comet was originally expected to have a magnitude of 11.5 when it returns in mid-November. That would make it a good target with an 8-inch telescope from a dark-sky site. Hopefully, the surge in brightness will continue making this interstellar visitor visible in smaller telescopes.
Here is how to see it. November 9, 2025, the interstellar comet will reach an altitude of 10° just before the start of dawn for observers at latitude 40° north. The Moon will be in waning gibbous phase and make observing difficult but by the November 16, the comet’s altitude will reach 20° with the Moon a thin crescent located 4° to its south. On November 15 and 16, Comet 3I/ATLAS will come within ½° of the 11th-magnitude barred spiral galaxy NGC 4691. From November 16 through December 2, the Moon will no longer interfere with observations as 3I/ATLAS climbs the morning sky coming close to Virgo. It will then cross into Leo on December 7–8, and pass 1.8° south of Regulus on December 27.
3I/ATLAS is expected to fade from magnitude 11.5, in mid-November to 12.5 by the November 30, then another 2 magnitudes by the end of December. However, with comets you never know; it could brighten suddenly or dim significantly. If you want to see this interstellar comet, take advantage of every clear morning you can in November. This is a cloudy month in many areas of the Northern Hemisphere but it could be a very long time before we get another opportunity to see a comet visiting from a star far away from our solar system.
