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A familiar comet, known since 1858, will pass perihelion for another time on Apr. 12, 2017. However, this year, comet 41P/Tuttle-Giacobini-Kresak (P=5.42a) will also be near the Earth (perigee at 0.14 AU occurs on Apr. 1). It is expected that this very diffuse comet will peak around magnitude 5-6, exhibiting a coma of more than 20' diameter. The apparition could get even more spectacular, if the comet would show one of its large outbursts (reaching up to 10 mag!) during the days around perigee. During the most interesting weeks the comet moves through the constellations Ursa Major, Draco, Hercules and Lyra, favoring Northern hemisphere observers.

The apparition 2017 proceeded unspectacular. The comet did not show an outburst and peaked at only magnitude 6.9 around Apr. 5. Based on 415 observations from 47 observers the comet developed different, pre- and post-perihelion. Whereas the activity increased very rapidly pre-perihelion it decreased much more slowly (but nevertheless above the average) post-perihelion. The appropriate formulae are as follows:

pre-perihelion : m = 10.4 mag + 5×log D + 35×log r
post-perihelion: m = 10.9 mag + 5×log D + 12.0×log r

Between end of January and end of February 2017 the apparent coma diameter measured between 2' and 3'. Then, due to the rapidly decreasing distance comet-Earth, the diameter increased rapidly, reaching the maximum of 30' at the turn of March/April 2017. Thereafter it decreased in a steadily manner, reaching 4' at the end of June and 1.5' at the end of July. Between the end of January and the end of February the absolute coma diameter increased slowly from 25.000 km to 65.000 km. At the final days of March it peaked at 190.000 km, still measuring 160.000 km in mid-May. Thereafter it decreased more rapidly, measuring 90.000 km at the end of June and 50.000 km at the end of July.

Total Brightness and Coma Diameter

The coma was very diffuse during the whole apparition. The degree of condensation increased from DC 2 at the end of January to only DC 3 at the end of March. This degree was hold constant until the end of April. Thereafter the coma became more and more diffuse, with the degree of condensation decreasing to DC 2 in mid-May and to DC 1 at the end of June. With a telescope a brighter central condensation could be discerned, in which a false nucleus with a magnitude up to 13.0 could be seen temporarily. Visually no tail was reported.

Andreas Kammerer

FGK observations