Comet 45P/Honda-Mrkos-Pajdusakova (P=5.25a) will pass perihelion at the end of September 2011, after its close approach to Earth (0.060 AU) on Aug. 15. Expected to be of magnitude 7 at perigee it will be situated at declinations near -70°. Mid-european observers will have two observing windows with the comet at low altitudes in the morning sky. The first window closes at the end of July. In July the comet should brighten from magnitude 15 to 10 (based on my analysis) or 17 to 12 (ICQ), moving from Aquarius into Piscis Austrinus. The second window will open around Sep. 20, with the comet in Leo, moving eastward. Thus the altitudes will increase, while the comet will fade from 7.5 mag to 8.5 mag (based on my analysis and ICQ) during the following three weeks.

The comet reached the predicted maximum brightness. According to 6 observations by 3 members of the German Comet Section and 70 international observations (until early November 2011) the comet peaked at 6.8 mag around Sep. 23. As indicated in the diagram the comet showed two maxima during this apparition: one due to its perigee in mid-August and the main maximum due to its perihelion passage. Currently the time near perigee is documented by only a small number of observations, but a brightness of 8.0 mag is confirmed. Another result: based on the mentioned observations the brightness evolution can be described by one formula for the whole apparition:

During previous apparitions the comet showed a significantly slower brightness evolution post-perihelion. Therefore it seems likely that additional post-perihelion observations will make it necessary to differentiate between pre- and post-perihelion. In the diagram the upper dashed line marks the brightness parameters m₀=9.5 mag / n=3, which are possibly too optimistic. The further evolution may be observed in the morning sky in the constellation Virgo.

Total Brightness and Coma Diameter

The coma diameter seems to have reached 12' (40.000 km) at perigee, with the dramatic decrease during the following days, as shown in the diagramm, being not plausible. The post-perihelion coma diameter estimates are in the order of 2' (70.000 km). While the coma appeared quite diffuse pre-perihelion (DC 2-3), it was strongly condensed around perihelion (DC 7), only to become more diffuse again until mid-October (DC 5). Despite the low altitude a visual tail up to 10' (400.000 km) was reported in the days around perihelion.

Andreas Kammerer

FGK observations