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The old periodic comet 6P/d'Arrest (P=6.54a) will pass perihelion again on Sep. 17, 2021 in the solar distance of 1.35 AU. Between August and November 2021 it moves from the western part of Ophiuchius towards the border of the constellations Microscopium/Piscis Austrinus, and thereafter will continue to Aquarius until the end of year. Thus, for mid-European observers it will be situated very near the southern evening horizon from end of September onwards. During the apparition 2008 the comet experienced the time-dependent brightness parameters m0=11.5 mag / nt=-0.15 pre-perihelion and m0=11.5 mag / nt=+0.03 during the period 0d < t < +55d. If the comet should develop in a similar manner it should brighten from magnitude 16.0 to 11.0 between mid-August and mid-September, thereafter fading, reaching 14.0 mag in mid-November and 16.0 mag by the end of year. Earth will cross the orbital plane on Aug. 12.
This apparition can be described on the base of 73 observations from 16 observers. The extraordinary brightness development can be verified and described with the following formulae
t < +50d : m = 12.0 mag + 5×D – 0.045×|t-T|The maximum brightness of 10.0 mag was reached around Nov. 10, 2021.
Total Brightness and Coma DiameterThe apparent coma diameter increased from 1' at the start of the apparition to its maximum of 4.5' at the end of October. This value was hold quite constantly until mid-December. Thereafter the apparent coma diameter decreased, reaching 3.5' by the end of February 2022. The absolute coma diameter continuously increased from 30.000 km at the start of the apparition to 300.000 km at the turn of 2021/22 and to 400.000 km by the end of February 2022. The coma remained very diffuse, as is typical for this comet. The degree of condensation was DC 2 at the start and at the end of the apparition, peaking at DC 3 in October/November 2021. A tail was not reported.
Andreas Kammerer