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After receiving an anonymous report from an online forum about a moving object having motion similar to that of comet 58P the Australian amateur Michael Mattiazzo looked carefully at online SWAN images and found an appropriate object in images between Mar. 20 and Apr. 5, 2020. On Apr. 7 he then performed a photographic search and found the comet near the border of the constellations Aquarius/Pisces despite full moonlight and an altitude of only 11°, as an object of magnitude 12 with a 1' coma. Follow-up observations of comet 58P/Jackson-Neujmin (P=8.25a) showed a 1' coma of total magnitude 12.0 and a 1' tail in p.a. 255°. Thus this comet – which was last observed in 1996 – was 6 mag brighter than expected! It will pass perihelion in a solar distance of 1.38 AU on May 27, 2020, but will be situated near the Sun (CBET 4747). From mid-European locations the comet will appear at the opening of August above the eastern morning horizon, situated in the northern parts of Orion. It is unclear, if it will be still observable at this time because it very likely is in a state of outburst. Typically the comet's brightness develops with n≈10 pre- and with n≈6 post-perihelion.
Based on 55 observations from 8 observers the brightness estimates clearly show a different development pre- and post-perihelion. Pre-perihelion the comet showed a constant heliocentric brightness after the large outburst, which implies a constant decrease in activity. Post-perihelion the activity dropped rapidly – the gas and dust supplied during the outburst were exhausted. At the end of July the comet had faded to 14.0 mag. The development can be described by the following formulae:
pre-perihelion: m = 9.1 mag + 5×log D post-perihelion: m = 2.9 mag + 5×log D + 45×log rThus the comet displayed an almost constant maximum brightness of 10.6 mag between the end of April and the end of May 2020.
Total Brightness and Coma DiameterThe coma diameter remained almost constant at 2.7' (250.000 km) between mid-April and the end of June. Thereafter it shrunk rapidly, measuring only 1.2' (90.000 km) at the end of July. The coma showed a degree of condensation of DC 3 until early July, which dropped to DC 2 by the end of July. A tail was not observed.
Andreas Kammerer