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On images taken by the ATLAS project on Sep. 17, 2020 Nicolaus Erasmus at the South African Astronomical Observatory discovered a comet of magnitude 18 in the constellation Canis Minor. Follow-up observations of comet C/2020 S3 (Erasmus) showed a moderately condensed 15" coma of total magnitude 17.5 and a broad 20" tail pointing towards p.a. 300°. The comet will pass its perihelion at a solar distance of only 0.40 AU on Dec. 12, 2020 (CBET 4855) and could get as bright as 11.0 mag. Unfortunately it will be on the far side of the Sun at that time and therefore too close to the Sun for observations. From mid-European locations the comet may be observable until the end of November in the morning sky, when it should be of magnitude 12.0. During the weeks before it moves through the constellations Hydra, Sextant, Crater and Corvus to the border of Virgo and Hydra. However, the absolute brightness of this comet is clearly below the Bortle-Limit, making it prone to disintegration on its way to perihelion.
The comet survived its perihelion and reached a maximum brightness of 5.3 mag, based on terrestrial observations. During the weeks following the perihelion the comet remained so close to the Sun that only observations by the SOHO probe were possible in the period 5 to 20 days after perihelion. During these two weeks the brightness decreased from 3.8 mag to 6.2 mag. It remains unclear whether the comet was actually 1.5 mag brighter post-perihelion – maybe due to forward-scattering - or if systematic errors by deriving the SOHO-magnitude must be taken into account. Based on 172 observations from 38 observers the following formulae result:
pre-perihelion: m = 8.9 mag + 5×log D + 11.2×log r post-perihelion: m ≈ 7.5 mag + 5×log D + 12.5×log rThe coma diameter increased from 2' (150.000 km) at first to the maximum value of 7.5' (340.000 km) in mid-November, only to begin to decrease right after that date, reaching almost 1' (55.000 km) by mid-December (influenced by twilight). Approaching the Sun the coma got more compact with the degree of condensation increasing from DC 2 to DC 7.
Total Brightness and Coma DiameterStarting at the end of October, tail observations were reported, with the tail reaching a maximum length of 1° at the end of November (and also in the SOHO images). This results in a pre-perihelion tail length of 2 Mio. km. Post perihelion the tail was of 8 Mio. km length – due to perspective for-shortening. The tail was oriented westward throughout the whole apparition.
Andreas Kammerer