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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 239 observations from 29 observers its brightness developed very continuously according to the formulam = 6.9 mag + 5×log D + 12.0×log r
resulting in a maximum brightness of 7.6 mag during the first week of May 2017. However, the comet experienced a short-term outburst of nearly 2 magnitudes between Apr. 4 and 28 (not included in the formula), resulting in an actual peak magnitude of 6.1 mag on Apr. 6, 2017. The reason for this outburst was detected on June 13, when images showed a fragment of magnitude 15.5-16.0 about 12" behind the nucleus (in the tail-ward direction), with the distance slowly increasing (CBET 4409).Total Brightness and Coma Diameter
Until the start of March 2017 the apparent coma diameter increased only slowly from 0.4' to 2'. During the following weeks it increased more rapidly with the diameter reaching the maximum value of 7.5' between the beginning of April and the end of May. Thereafter it decreased slowly to 5' at the end of May, short of 3' at the end of July to 0.8' at the end of October. The absolute coma diameter measured 75.000 km at the start of the apparition, increasing to 175.000 km until the start of March 2017. The maximum diameter of 380.000 km was held constant during April. Thereafter it decreased, reaching 300.000 km at the end of May, 190.000 km at the end of July and 60.000 km at the end of October.
Whereas the coma was very diffuse (DC 2) at the beginning of the apparition it steadily condensed while approaching the Sun, reaching DC 5 at perihelion (with the degree of condensation shortly peaking at DC 6 during the outburst). Post-perihelion it grew more and more diffuse, reaching DC 3 at the end of July and DC 0-1 at the end of October.
Visually the tail could be recognized between start of March and the opening of July 2017. It constantly pointed towards West. It reached its maximum length of 40' (2.8 Mill. km) during the outburst, measuring 25' (1.8 Mill. km) during the days around perihelion.