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C/2015 ER61 (PanSTARRS)


An asteroidal object of magnitude 21, discovered already on Mar. 14, 2015 near the border of the constellations Sextans/Leo, showed cometary morphology when observed in January 2016. Comet C/2015 ER61 (PanSTARRS) exhibited a 3" coma of total magnitude 18.0 and a 2" tail in p.a. 120°. It will pass its perihelion very near the Earth orbit (1.04 AU) at the opening of May 2017 and assuming a standard evolution - may then reach magnitude 8 (CBET 4248/49)! However, for mid-European observers it will be less than 5° above the horizon between end of April and the start of June, and will reach an altitude of more than 20° not prior to mid-July, when it is expected to have faded to magnitude 10.

The comet surprised in a positive manner in spring 2017. According to 103 observations from 18 observers (until the end of April 2017) its brightness developed according to the formula

m = 6.8 mag + 5×log D + 12.8×log r

resulting in a maximum brightness of 7.5 mag during the first week of May 2017. However, the comet experienced a short-term outburst of about 2 magnitudes between Apr. 3 and 20 (not included in the formula), resulting in an actual peak magnitude of 6.0 mag on Apr. 6, 2017.

Total Brightness and Coma Diameter

The coma diameter increased between the end of January and early April from 1.5' (150.000 km) to 4.5' (275.000 km), thereafter increasing rapidly to 8' (400.000 km). This value remained nearly constant during the outburst, thereafter decreasing to 5' (275.000 km) until the end of April. During the first weeks the coma condensed slowly (DC 2-3 to DC 4-5), sharply condensing to DC 5-6 during the first days of the outburst. Thereafter the degree of condensation decreased rapidly again to the value prior to the outburst, but started slowly to increase to DC 5 until end of April.

A visually discernible tail, pointing westward, was observed from the first week of March onwards. Its maximum length of 0.5° (1.8 Mio. km) was reached during the outburst. Shortly thereafter it got invisible.

The comet will reappear in the morning sky during the last decade of July for mid-European observers. It will be positioned near the border of Pisces and Taurus. Its brightness will decrease between June and early September from 10.5 mag to 12.0 mag.

Andreas Kammerer


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