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Comet 45P/Honda-Mrkos-Pajdusakova (P=5.26a) will pass Earth at the very small distance of only 0.084 AU on Feb. 11, 2017, expected to reach 5.5 mag during these days. Thus it should be observable in binoculars. However, the coma should have a diameter comparable to that of the Full Moon. Furthermore, whereas this comet exhibits a strongly condensed coma near perihelion (on Dec. 31, 2016), it becomes rapidly more diffuse during the following weeks. Thus, 6 weeks after perihelion, it should exhibit a very to extremely diffuse coma, requiring a rather dark sky to see it in its whole extent. For mid-European observers it will appear post-perihelion during the first days of February above the eastern morning horizon, climbing rapidly higher during the following days. Getting only slightly brighter until perigee, it will rapidly fade thereafter. Until the first week of March it will move through the constellations Aquila, Hercules, Corona Borealis, Bootes, Canes Venatici and Leo / Leo Minor.

The comet showed a surprisingly continuous brightness development during the apparition 2016/17, based on 212 observations from 27 observers. The estimates can be represented rather well by the formula

m = 11.4 mag + 5×log D + 12.8×log r

A distinction between pre- and post-perihelion is not necessary – contrary to previous apparitions. However, the maximum brightness is not well documented by the observations. According to the formula the comet would have reached magnitude 6.8 a few days prior to perihelion and 5.8 mag at perigee. Actually the maximum brightness pre-perihelion, reported on Jan. 8, was magnitude 6.1, whereas at perigee a maximum brightness of only 6.5 mag was reported. However, it has to be considered, that the comet was very low above the horizon around perihelion (with the Full Moon brightening the background). At perigee it is plausible, that the observers did not recognize the coma in its full extent due to the very small contrast between the sky background and the very diffuse coma.

Total Brightness and Coma Diameter

Pre-perihelion the coma diameter increased in an ever more rapid fashion from 1.5' in early December 2016 to 6' around Jan. 8, 2017. Post-perihelion it increased – due to the shrinking distance Earth–comet – rapidly during the first weeks (10' at the opening of February, 25' around Feb. 17). Thereafter it decreased rapidly, measuring 12' on Feb. 25, 5.5' around March 10 and 2' at the end of March. The absolute coma diameter increased – rapidly at first, gently at the end - between the opening of December and Jan. 8 from 80.000 km to 150.000 km. On Feb. 10 it measured 80.000 km, thereafter increasing to the maximum of 170.000 km around Feb. 25, then decreasing similarly rapidly (mid-March: 100.000 km, end of March: 30.000 km). However, it is plausible to assume that the coma diameter was underestimated at the start of February due to the then low altitudes. Whereas the degree of condensation increased from DC 2-3 to DC 6 between the opening and the end of December it decreased slightly to DC 5 until Jan. 8. Between start of February and end of March the coma was rather diffuse with the degree of condensation decreasing from DC 2 to DC 1.

Visual tail reports have been published between mid-December and mid-March, with the length reaching 12' (450.000 km) pre-perihelion and 25' (1 Mill. km) post-perihelion. Pre-perihelion the tail was oriented towards East, post-perihelion towards West.

Andreas Kammerer

FGK observations