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°.
At the end of September 2025 the analysis can be based on 138 observers from 43 observers. These show an exceptionally high activity parameter since its reappearance in the morning sky. Between end of August and end of September the brightness increased according to the parameters
m0 = 6.3 mag / n = 6.By Sep. 20, 2025 the comet had already reached 8.0 mag. If it could sustain this development until perihelion it would reach magnitude 2.0 in the period between perigee and perihelion! According to my empirical formulae the comet would show a visual coma diameter of about 25' during perigee and a maximum visual tail length of about 15°!
Total Brightness and Coma Diameter
However, the observation period of one month is still a little too short to be able to predict these values with certainty. It would not be surprising if the activity would decline on its way towards perihelion (in consequence the comet would not get as bright as mentioned with the maximum coma diameter smaller and the tail length shorter).The coma diameter and degree of condensation also showed a rapid increase during the one-month observation period. The coma diameter increased from 1.5' (150,000 km) around Aug. 25 to short of 8' (500,000 km) on Sep. 20, and the degree of condensation from DC 2-3 to DC 5-6. A westward-oriented tail has been observed by CCD observers since the end of August. On Sep. 20 it had reached a length of 25' (2 mio. km).
Andreas Kammerer
