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On Nov. 3, 2018 comet 64P/Swift-Gehrels (P=9.41a) will pass perihelion in the solar distance of 1.39 AU. Between August and November it should brighten from 15.0 mag to 11.0 mag, only to fade in a similar manner thereafter. Between August 2018 and January 2019 it moves through the constellations Pegasus, Pisces, Andromeda, Triangulum, Aries and Taurus, thus will be observable the whole night through.
The comet showed a short-lived outburst of almost 3 mag in mid-August 2018. On Aug. 13 it was of magnitude 16.0, but 32 hours later the comet had brightend to 13.4 mag. During the following days the brightness decreased, reaching nearly the pre-outburst level (CBET 4544), only to increase again - continuously now - soon thereafter. The comet peaked at 8.7 mag on Dec. 20, 2018, and was thus clearly brighter than expected. Based on 382 observations from 47 observers the brightness development can be best described by time-dependent formulae with the maximum brightness reached 50 days past perihelion. In addition, the comet's absolute brightness before and after Dec. 20, 2018 was very different as is shown by the following formulae:
t < +50d: m = 11.5 mag + 5×log D – 0.038×(t-T) t > +50d: m = 7.9 mag + 5×log D + 0.034×(t-T)Total Brightness and Coma Diameter
Between mid-August and early October 2018 the apparent coma diameter increased only from 1.5' to 2.5', followed by a period of rapid increase with the diameter reaching 8' in early November. Thereafter it increased on a significantly lower rate, reaching the maximum of 11' around Dec. 10, 2018. This value remained constant for 3 weeks, followed by a rapid decrease to 6' in mid-January 2019 and to only 2' in mid-March. The absolute coma diameter remained constant at 55.000 km until early October 2018, but thereafter increased steadily, reaching 280.000 km in early December. This value remained constant until the beginning of January 2019. Thereafter the coma shrunk rapidly, reaching 150.000 km in mid-March 2019. Throughout the apparition the coma was only slightly condensed. Between mid-August and early December 2018 the degree of condensation increased from DC 3 to DC 4. Thereafter the coma got more diffuse, with the degree of condensation reaching DC 1-2 at the beginning of March 2019. Sporadically a tail of maximum length 7' (500.000 km) could be glimpsed between mid-October 2018 and mid-January 2019, pointing towards Southwest.
Andreas Kammerer