Home=Current Comets: C/2025 A6
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An asteroidal object discovered on Jan. 3, 2025 with the 1.5m reflector on Mt. Lemmon near the borders of the constellations Gemini/Cancer/Canis Minor displayed cometary morphology in follow-up observations. Comet C/2025 A6 (LEMMON) showed a coma of only 2" diameter with a total magnitude of 20.5. The comet will pass perihelion at the solar distance of 0.53 AU on Nov. 8, 2025, expected to reach magnitude 10.5. On Oct. 21 it will pass Earth at a distance of 0.60 AU at about the same brightness (CBET 5508). The comet should be brighter than 16 mag between early September 2025 and mid-January 2026. During this period it will pass through the constellations Gemini, Lynx, Leo Minor, Ursa Major, Canes Venatici, Ursa Major (perigee), Serpens (perihelion) and Scorpius. From mid-European locations it reaches its greatest altitude in the morning sky at around 40° at the end of September and disappears from it around Oct. 20. In the evening sky it can be seen between the first week of October and the first week of November, reaching a maximum altitude of 25°.
The comet became the most interesting comet in autumn and the second brightest of 2025. It was intensively observed, although the weather in Central Europe was anything but cooperative. When it reappeared in the morning sky around Aug. 10 the comet was significantly brighter than expected (CBET 5594). Based on 750 observations by 92 observers (until the start of November 2025) it appears that the high activity during the first weeks had been significantly reduced around Sep. 10.
Development of the heliocentric magnitude
The appropriate brightness parameters are as follows:
t < -60d: m0 = 5.5 mag / n = 8Only of magnitude 12.5 in mid-August, the above parameters yield a maximum brightness of 4.1 mag on Oct. 28. However, the observations clearly indicate a maximum brightness of 3.8 mag on Oct. 25, 2025. By Nov. 10 the comet had faded to 4.8 mag.
Total Brightness and Coma Diameter
Between mid-August and mid-October the apparent coma diameter increased from 1' to the maximum of 11'. By Nov. 10 it had decreased to 5'. In absolute terms the coma expanded from just under 100,000 km in mid-August to the maximum of 500,000 km around Sep. 25. Thereafter it shrank to 300,000 km around Oct. 20, a value that appears to have maintained fairly consistently until early November. As it approached the Sun the coma condensed significantly from DC 3 in mid-August to DC 7 at the end of October. By Nov. 10 the coma was still strongly condensed (DC 6-7). Observations at the Teide Observatory at the end of September showed an exceptionally active nucleus with several jets directed towards the Northeast and Southwest (ATel 17450).Development of the Tail Length
Starting at the end of August an ion tail, which was also well recognizable visually, could be observed. Around Oct. 25 it reached the maximum visual length of 5° (8 mio. km). By Nov. 10 it had shrunk to 1° (2.5 mio. km). Initially pointing WNW the tail rotated noticeably toward North starting around Oct. 10, reaching an easterly orientation around Nov. 10. In a photograph, taken with a 135 mm telephoto lens by Michael Jäger and Gerald Rhemann on Oct. 4, a tail length of 12° is documented (CBET 5621).
Andreas Kammerer
