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On images taken on Sep. 7, 2018 with the 1.5m-reflector atop Mt. Lemmon a 19 mag comet was discovered near the border of the constellations Aquarius/Capricornus. Comet C/2018 R3 (Lemmon) showed a strongly condensed 8" coma and a 4" tail pointing towards East. The comet will pass perihelion in the solar distance of 1.29 AU on June 7, 2019, expected to reach magnitude 13-14 (CBET 4556). It should be brighter than 16 mag between April and August 2019, thereby moving through the constellations Pegasus, Andromeda, Cassiopeia, Camelopardalis, Lynx (maximum brightness) and Leo. However, for mid-European observers the comet will be low above the horizon, attaining no more than 25° at the start of May in the morning sky and no more than 20° at the opening of June in the evening sky.
The comet surprised in a positive way and got considerably brighter than expected. On May 10, 2019 at only magnitude 13.5 it brightened rapidly during the following weeks, reaching magnitude 10.3 at perihelion. Thereafter the comet faded slower with the brightness still at 10.8 mag on July 5, 2019. Due to the unfavorable observing conditions the brightness formulae can be based on only 34 international reports:
pre-perihelion: m = 8.7 mag + 5×log D + 0.12×|t-T| post-perihelion: m = 8.1 mag + 5×log D + 7.5×log rTotal Brightness and Coma Diameter
Between May 10 and perihelion the coma diameter increased from 1.0' (85.000 km) to 3.8' (325.000 km), only to decrease to 3.0' (275.000 km) by July 30. However, these data need to be handled carefully, due to the unfavorable observing conditions. Between the end of May and the end of June the degree of condensation decreased from DC 4 to DC 2. No tail was reported.
Very few observers succeeded in the detection of the comet between October and the opening of December 2019, however at magnitudes of only 18-19 mag – 5 mag fainter then the formula above predicts. Thus the comet either faded much more rapidly (according to the parameters m0=6.5 mag / n=9) or its gas/dust deposits exhausted during the period it could not be observed, resulting in a sudden fading. I tend to the first scenario because the formula above represents the brightness development during the first weeks after perihelion very well.
Andreas Kammerer