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On Jan. 8, 2020 Masayuki Iwamoto discovered a comet in the constellation Ophiuchus, utilizing a 400mm telephoto lens and a Canon EOS 6D camera. Due to the faintness of the comet the positional uncertainty was rather large. Although Iwamoto's observations had been published on the PCCP webpage, only after three long days did Gennady Borisov report the independent discovery of this comet. At discovery comet C/2020 A2 (Iwamoto) showed a diffuse 40" coma of total magnitude 14.5, but no tail. Observations in mid-January showed a tailless comet with a 2' coma of total magnitude 13.5. The comet passed perihelion in the solar distance of 0.98 AU just at the date of discovery (CBET 4714/15). Because the comet will approach Earth until Feb. 21, the maximum brightness of 13.0 mag was expected to occur during the first week of February. Until mid-February the comet should show only a slight fading. Thereafter, however, it will fade rapidly, expected to reach magnitude 16 at the beginning of April. During this interval it will move from Ophiuchus through the constellations Hercules, Lyra, Draco, Cepheus, Cassiopeia and Auriga. Thus until the end of February it will be better positioned in the morning sky, then in the evening sky.
The comet was observed until the end of March 2020. Based on 65 observations from 23 observers the brightness development can be described by the formula
m = 10.4 mag + 5×log D + 9.5×log rThis yields a maximum brightness of 10.9 mag around Feb. 10, 2020, followed by a very rapid fading.
Total Brightness and Coma DiameterThe coma diameter increased from 1.5' (90.000 km) at the start of the apparition to short of 3' (120.000 km) at the end of February. Until the end of March it decreased to 1.0' (65.000 km). During the apparition the coma grew ever more diffuse; the degree of condensation decreased from DC 4 to DC 1-2. No tail was detected.
Andreas Kammerer