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In the course of the Mt. Lemmon survey an asteroidal object of magnitude 20 was discovered on Oct. 7, 2021 in the constellation Taurus. Follow-up observations showed cometary morphology. Comet C/2021 T4 (Lemmon) showed a strongly condensed nearly stellar coma of total magnitude 20.0. It will pass perihelion on July 31, 2023 in the solar distance of 1.48 AU and could reach magnitude 9.5. It will pass Earth on July 20, 2023 at the distance of 0.54 AU and could then peak at magnitude 9.0 (CBET 5063). The comet should be brighter than 16 mag between October 2022 and April 2024. During this period it will move through the constellations Cetus (showing an opposition loop), Sculptor, Grus, Indus, Telescopium (perigee), Ara, Norma (perihelion), Lupus, Libra, Virgo, Bootes and Coma Berenices. It should be brighter than 12 mag between June and September 2023, moving from Cetus to Libra. At mid-European latitudes the comet can be observed until mid-February 2023 in the evening sky (then expected to be of magnitude 15) and starting in December 2023 in the morning sky (expected magnitude: 14).
At the end of October 2023 a total of 108 observations from 27 observers can be used for the analysis of this comet. These show a steady brightness development, which can be represented quite well by the formulam = 8.4 mag + 5×log D + 8.4×log r
yielding a maximum brightness of 8.5 mag during the days of perigee (July 20, 2023). By mid-October 2023 the comet had faded to 12.5 mag.Total Brightness and Coma Diameter
The apparent coma diameter measured constantly 1' in 2022, but starting at the end of May 2023 (1.5') increased significantly, reaching a maximum of 10' during the days of perigee. By the beginning of October 2023, it had decreased to 1.5'. The absolute coma diameter increased from 125,000 km in 2022 to 250,000 km at the time of perihelion, thereafter decreased, reaching 150,000 km by the beginning of October 2023. The degree of condensation was constant at DC 4-5 in 2022, but decreased from DC 4 to DC 2-3 in 2023.
In 2022 CCD observers were able to detect a tail. In 2023 surprisingly few tail sightings have been reported with lengths up to 25' (2 mio. km). While the tail was constantly oriented towards NE in 2022, it changed direction from SSW to SE in 2023.