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On images taken by the "Asteroid Terrestrial-Impact Last Alert System" (ATLAS) team on Sep. 12, 2020 a comet of magnitude 19 was discovered near the border of the constellations Cetus/Eridanus. Follow-up observations of comet C/2020 R4 (ATLAS) showed a significantly condensed 12" coma of total magnitude 18.8 and a 15" tail in p.a. 275°. The comet will pass perihelion in the solar distance of 1.03 AU on Mar. 1, 2021 (CBET 4849), expected to reach magnitude 15.5. It will approach Earth until Apr. 23, when it will be at a minimum distance of 0.46, expected to reach magnitude 13.5. It should be brighter than 16 mag between February and May 2021. During this weeks it will move through the constellations Capricornus, Aquila, Hercules, Corona Borealis (maximum brightness), Bootes, Canes Venatici and Leo. Observers at mid-European locations can follow the comet between mid-March and end of May.
The comet was not an easy binoculars object. Although the brightness parameters yield a peak brightness of 8.9 mag on Apr. 21, the rather diffuse coma was of diameter 7' due to perigee, which occured three days later. In consequence the scatter in the estimates is accordingly large. Based on 201 observations from 47 observers the brightness parameters can be derived as follows:
m0 = 9.4 mag / n = 3.7About Mar. 10, 2021, when the comet reappeared in the morning sky, it was of magnitude 10.5. During March and April the brightness estimates show small oscillations. It is uncertain if the brightness was actually 0.7 mag below the values the brightness parameters indicate around the days of perigee. This could also be explained by assuming, that the observers could not detect the whole extent of the diffuse coma during these days. In consequence the observed maximum brightness of 9.1 mag occured on Apr. 25. At end of June the comet had already faded to 14.5 mag.
Total Brightness and Coma DiameterThe apparent coma showed a diameter of 1.5' in mid-December 2020. In mid-March 2021 it had increased to 2.5', peaking at 8' on Apr. 30. After perigee the apparent coma diameter decreased rapidly, measuring just 1.0' at the opening of June. The absolute coma diameter measured 125.000 km in December 2020. Around Mar. 10, 2021 it had increased to 175.000 km. During perigee the observed coma yields an absolute diameter of 150.000 km, but this again could have been caused because the observers could not detect the whole extent of the diffuse coma during these days. Then during the first week of May the maximum coma diameter of 190.000 km was reached. Thereafter it shrunk rapidly, measuring just 80.000 km in mid-June. The comet presented a rather diffuse coma. The degree of condensation was DC 3 to DC 3-4 until the start of May 2021; thereafter decreasing to DC 2. A tail of maximum length 10' (500.000 km) could be detected between Apr. 10 and May 10, 2021. During this period its orientation changed from Southwest towards Southeast.
Andreas Kammerer