Home=Current Comets: C/2023 H2
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Using the 1.5m reflector on Mt. Lemmon an asteroidal object was discovered on Apr. 23, 2023 near the border of the constellations Bootes/Canes Venatici, which showed cometary morphology in the course of follow-up observations. Comet C/2023 H2 (Lemmon) showed a 12" coma of total magnitude 20.0 with a central condensation, but no tail. The comet will pass perihelion at the solar distance of 0.89 AU on Oct. 29, 2023, expected to be of magnitude 13 around that date. During the following weeks it approaches Earth, passing it at a distance of only 0.193 AU on Nov. 11, predicted to reach magnitude 11, showing a coma diameter of about 4' (according to my empiric formula) (CBET 5264). It should be brighter than 16 mag between mid-September and late December 2023. During this period it moves through the constellations Ursa Major, Canes Venatici (perihelion), Bootes, Hercules (perigee), Aquila, Capricornus, Piscis Austrinus and Grus. From central European locations it can be observed both in the morning and evening skies until the first week of November, having greater altitudes in the morning sky until end of October. Around Nov. 25 it will disappear above the southern evening horizon. Earth will cross the comet's orbital plane on Oct. 31.
This comet is noticeably brighter than initially assumed, primarily due to its absolute brightness, whereas its activity is only slightly above average. On the basis of 35 observations from 19 observers the brightness estimates can be well represented by the parameters
m0 = 10.5 mag / n = 7indicating a maximum brightness of 7.0 mag during the days of perigee (Nov. 11, 2023). According to my empiric formula the coma diameter should reach about 15'. Around Oct. 25 the comet was of magnitude 9.5.
Total Brightness and Coma DiameterWhile the apparent coma diameter increased from 1.5' to 5.5' between mid-September and Oct. 25, the absolute coma diameter only increased from 130,000 km to 190,000 km. During this period the coma condensed noticeably. The degree of condensation increased from DC 2-3 to DC 4-5. Until now no tail sightings have been reported.
Andreas Kammerer