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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.
The comet developed surprisingly positive between September and November 2025. So positive, in fact, that instead of the initially predicted maximum brightness of 12-13 mag it is actually of magnitude 9.5 at the beginning of November – and thus being visually detectable even in smaller instruments. Based on 136 observations from 32 observers (until the start of November 2025) the comet showed a rather low activity parameter until around Aug. 30, but thereafter this changed to a far above-average increase in heliocentric brightness.
Development of the heliocentric magnitude
The appropriate brightness parameters are: t < -60d: m0 = 11.8 mag / n = 2It was just of magnitude 17.5 at the beginning of July and only 16.0 mag by Aug. 30. But then a rapid brightening began, so that by the time it disappeared into the evening twilight in early October the comet was already of magnitude 12.0. Finally, the comet peaked at magnitude 9.4 on Nov. 1, 2025. I expect that the activity post-perihelion will not decrease as rapidly as it increased during the last weeks pre-perihelion (n = 4...5 is more likely).
Total Brightness and Coma Diameter
The coma diameter increased from 0.3' at the beginning of July to just 0.5' in mid-September (absolute: constant 40,000 km). Thereafter it expanded much more rapidly, measuring 1.0' (125,000 km) at the end of September and 2.5' (250,000 km) at the beginning of November. Estimates of the degree of condensation have only been available since mid-September. These show that the comet has had a constant DC value of DC 5 since then. A tail could be documented on CCD images since the beginning of September, reaching a length of 6' (2 mio. km) at the beginning of November. Pre-perihelion the tail was pointing towards ESE, post-perihelion towards WNW.Until mid-February 2026 the comet will move from the constellation Virgo to the constellation Gemini, thus switching from the morning sky into the night sky. It is expected to fade to 16.0 mag during this period. The Earth will be close to the comet's orbital plane in the first half of February.
Andreas Kammerer
