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Analysis of Comet Apparitions


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C/2020 M3 (ATLAS)


An asteroidal object of magnitude 19 was discovered by the "Asteroid Terrestrial-Impact Last Alert System" (ATLAS) on June 27, 2020 in the constellation Sculptor. Follow-up observations of comet C/2020 M3 (ATLAS) showed a compact 10" coma of total magnitude 18.5 and a 12" tail in p.a. 245°. The comet will pass perihelion in the solar distance of 1.27 AU on Oct. 25, 2020 and could peak at magnitude 14.0 in early November (CBET 4808). It should be brighter than 16 mag between September and the end of 2020. During this interval it will move through the constellations Eridanus, Lepus, Orion, Taurus and Auriga. It will appear above the morning horizon at mid-European locations in October, shifting towards midnight during the best observation time.

The comet developed much better than expected. At the end of August it brightened from 16 mag to 12 mag within one week, showing a constant high state of activity during the following weeks. However, although it reached magnitude 8, it never was an easy binocular object due to its rather diffuse, and during the days of Earth approach large coma. Based on 350 observations from 61 observers the comet peaked at magnitude 7.8 on Nov. 8, 2020 - two weeks after perihelion passage and one week prior to closest approach to Earth at 0.36 AU. After perihelion the brightness decreased much slower as the following formulae indicate:

pre-perihelion: m = 7.1 mag + 5×log D + 28×log r
post-perihelion: m = 8.6 mag + 5×log D + 13×log r

The apparent coma diameter increased steadily from 3' in early September 2020 to 15' at closest approach to Earth. Thereafter it decreased rapidly during the first weeks, then more slowly December onwards, reaching 2' by the end of January 2021. The absolute coma diameter increased between early September and mid-October from 70.000 km to 260.000 km. This value remained almost constant until Nov. 20 when it started to decrease steadily, reaching just 90.000 km at the end of January 2021.

Total Brightness and Coma Diameter

The comet displayed a quite diffuse coma throughout the apparition. The degree of condensation developed quite symmetric to the perihelion. It showed a value of DC 2 at the beginning and the end. The maximum value of DC 3-4 was reached by the end of October. No visual tail observations have been reported.

Andreas Kammerer

FGK observations


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