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On Nov. 2, 2021 comet 67P/Churyumov-Gerasimenko (P=6.43a) will pass perihelion at the solar distance of 1.21 AU, expected to reach magnitude 9. It will appear as an morning object of magnitude 15-16 at the opening of July. Between September 2022 and March 2023 it will move through the constellations Taurus, Gemini and Cancer. Between November 2021 and February 2022 it will show an opposition loop in the northern part of Cancer, thus being well-placed for Northern hemisphere observers. Earth will be situated near the orbital plane of the comet between mid-October and mid-November.
The comet showed a slower development than expected and could thus be seen longer in smaller instruments. Based on 625 observations from 70 observers the comet's brightness development can be best described with time-dependant formulae, using three different phases. The appropriate formulae are as follows:
t < +20d: m = 11.4 mag + 5×log D - 0.025×|t-T|After the first phase of brightness increase the comet showed a nearly three month phase with a constant heliocentric magnitude. Thereafter it faded, even a bit more rapid than the brightness increase took place. The comet peaked at magnitude 9.0 at the end of November 2021.
Total Brightness and Coma DiameterThe apparent coma diameter increased very continuously from 0.5' in mid-July 2021 to its maximum of 6.5' at the beginning of November, remaining almost constant at this value for the following 6 weeks. Not before the turn of 2021/22 did it decrease, still measuring 4' in early February 2022 and reaching short of 1' in mid-April. The absolute coma diameter increased from 25.000 km in mid-July 2021 to 120.000 km by the start of November, thereafter increasing more slowly, reaching a maximum of 140.000 km in early March 2022. Thereafter it decreased, reaching 60.000 km in mid-April. Until mid-December 2021 the degree of condensation was recorded at DC 5, but thereafter got more and more diffuse with DC 3 in early February 2022 and DC 2 in mid-April. Between mid-August 2021 and mid-April 2022 a tail was reported, which reached a maximum length of 0.5° (about 1.5 Mio. km) The tail pointed towards West to Southwest. In November, while Earth was close to the comet's orbital plane, a faint and thin anti-tail could be recorded photographically.
Andreas Kammerer