Home=Current Comets: 3I
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On July 1, 2025 the ATLAS team discovered an asteroidal object of magnitude 17.5 in the constellation Sagittarius, which moves on a highly hyperbolic orbit (e=6.14!). Additional observations showed a strongly condensed, tiny coma and a 4" tail in p.a. 280°. Comet C/2025 N1 (ATLAS) will pass perihelion on Oct. 29, 2025 at the solar distance of 1.36 AU and could reach a maximum brightness of 12.5 mag in mid-November (CBET 5578). It is the 3rd discovered interstellar object, which is why this second interstellar comet was given the permanent designation 3I/ATLAS. It is expected to be brighter than 16 mag between the beginning of August 2025 and the end of January 2026. During this period, it will move through the constellations Bootes, Scorpius, Libra, Virgo (perihelion), Leo and Cancer. From Central European locations it appears in the morning sky in mid-November and quickly gains altitude during the following weeks. At the end of January it can be observed throughout the night.
At the end of September 2025 118 observations (mostly by CCD) from 27 observers can be used for the analysis, with the scatter in the estimates now acceptable. Until around Sep. 10 (when the comet was of magnitude 15.0) the brightness increased fairly steadily according to the parameters
m0 = 10.3 mag / n = 3.0.This would have meant that the comet would have peaked at magnitude 13.0 at the start of November 2025. However, after Sept. 10 a rapid brightening was observed, which was not caused by a change in instrument size or observation method, but was confirmed by photographs.
Development of the heliocentric magnitude
The further development is currently unclear, but the comet could ultimately become significantly brighter than predicted by the above formula.Total Brightness and Coma Diameter
By Sep. 10 the diameter of the significantly condensed (DC 5-6) coma had reached 0.6' (80,000 km), but thereafter a significant increase was observed, that cannot yet be clearly defined (Michael Jäger and Gerald Rhemann have detected a faint outer coma with a diameter of 6' in their images). CCD images show a short tail extending towards East.Andreas Kammerer
