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On Dec. 18, 2018 Japanese Masayuki Iwamoto discovered a comet on CCD images taken near the borders of the constellations Hydra/Libra/Virgo. Follow-up observations of comet C/2018 Y1 (Iwamoto) using CCD equipment showed a 2.9' coma with a total brightness of 12.5 mag and a prominent central condensation, but no tail. Alan Hale estimated the comet to be of magnitude 11.5, showing a 3.1' coma on Dec. 20. The comet will pass its perihelion on Feb. 7, 2019 and may become as bright as 8 mag (CBET 4588 / MPEC 2018-Y69). It will pass Earth on Feb. 13 at only 0.30 AU, expected to reach a brightness of 7.0 mag during those days. According to my empiric formula a coma diameter of 15' may be expected. From mid-European locations it may be observable well into April 2019. During this interval it will pass through the constellations Virgo, Leo (maximum brightness), Cancer, Gemini, Auriga and Perseus. Earth will cross the comet's orbital plane on Feb. 16.
The comet evolved to an interesting binoculars object in spring 2019. It peaked at 6.3 mag on Feb. 12, the day of closest approach to Earth. Due to this special circumstance the coma was nearly as large as the Moon, which made it useful to have a really dark sky for seeing its whole extent.
Based on 371 observations from 55 observers the comet showed a high activity parameter prior to perihelion, while it was unusually small post-perihelion. Superimposed to this intrinsic development was the large variation in distance to Earth. As a result, the apparent brightness developed in an almost identical way before and after closest approach. At magnitude 12.5 mag during the Christmas days 2018, the comet increased rapidly in brightness during the following weeks, reaching 6.3 mag at the date of perigee. Thereafter the brightness decreased rapidly, reaching 10.5 mag in the early days of April. It seems unlikely that the comet will continue to show the small post-perihelion activity parameter. The last two observations may already show a break in this development. The following formulae describe the development well (omitting the last two observations):pre-perihelion: m = 5.7 mag + 5×log D + 31×log r post-perihelion: m = 8.4 mag + 5×log D + 5×log r
The apparent coma diameter increased from 1' during the Christmas days of 2018 to the maximum value of 25' at the time of closest approach to Earth, only to shrink again at a comparably fast rate. At the end of April 2019 the observers again recorded an apparent diameter of 1'. The absolute coma diameter measured 110.000 km at the start of the apparition, increasing rapidly thereafter, reaching 360.000 km on Feb. 10, 2019. This value remained constant until mid-March. Thereafter the coma shrunk rapidly, reaching 110.000 km at the end of April. The coma appeared diffuse to moderately condensed during the whole apparition. The degree of condensation increased from DC 2-3 to DC 3-4 during the days around perihelion and closest approach to Earth, only to decrease thereafter, reaching DC 2 in mid-April.Total Brightness and Coma Diameter
Visual tail observations were rare, spanning only the last three weeks of February. The maximum length of the tail reached 15' (750,000 km). During its short visibility it rotated from WNW to E.