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On images taken in the course of the "Asteroid Terrestrial-impact Last Alert System" (ATLAS) on Oct. 2, 2017, Aren Heinze discovered a comet of magnitude 18 in the constellation Hydra, near the border to the constellation Monoceros. Follow-up observations of comet C/2017 T1 (Heinze), undertaken in mid-October, showed a medium-condensed 12" coma of total magnitude 17.0 and a 35" tail in p.a. 245°. The comet will pass perihelion at a distance of 0.58 AU on Feb. 21, 2018, expected to reach magnitude 9.5 (CBET 4444). However, the derived absolute magnitude is near the Bortle limit, making it possible, that the comet will fizzle while approaching Sun. Prior to perihelion it will pass Earth in a distance of only 0.22 AU on Jan. 4, 2018, expected to reach magnitude 9.0. Despite its brightness it will be a difficult object, showing a diffuse coma of diameter about 8'. Mid-European observers can glimpse the comet until the opening of March in the morning sky. Until the start of January it will reach altitudes of 35° which decrease to 10° between mid-January and the start of March. Between end of December and the start of February it will be visible in the evening sky too, reaching 70° above the horizon in mid-January! Between November 2017 and the opening of March 2018 it will move through the constellations Hydra, Cancer, Lynx, Camelopardalis (minimal Earth distance), Cassiopeia, Lacerta and Pegasus. Earth will cross the orbital plane on Jan. 3, 2018.
Despite the small distance Earth-comet only 105 reports from 34 observers could be taken into account for an analysis. The small number of observations was caused by the very low surface brightness of the diffuse coma, which resulted in a large scatter of the estimates of brightness and coma diameter. In conjunction with the very diffuse coma of diameter 7', a very transparent sky was necessary for a reliable brightness estimate. However, the brightness development can be rather well simulated by the formula
m = 12.3 mag + 5×log D + 13.5×log rThus the brightness peaked at 9.8 mag during perigee, and at 9.5 mag at perihelion. Due to the rapid approach of the comet to Earth the comet brightened rapidly during the first weeks of the apparition from 14.0 mag to 9.8 mag.
Total Brightness and Coma DiameterThe apparent coma diameter increased from 1.0' to the maximum of 6.5' during perigee. Thereafter it decreased steadily, reaching short of 2.5' in mid-February. The absolute coma diameter increased steadily during the whole apparition from 35.000 km to 125.000 km at perihelion. This is rather surprising, considering the strong solar wind at 0.58 AU, which should cause a compression of the coma. Maybe because of the very diffuse character of the coma the observers did not recognize the full extent of the coma at perigee. A value of 13' (140.000 km) during those days seems more plausible. This assumption is supported by the development of the reported degree of condensation. At DC 2 at the start of the apparition it increased to DC 4 at perigee, only to decrease during the following days to DC 2 again. This could be a hint, that the observers did only recognize the more condensed inner coma. During the days of perihelion the degree of condensation was estimated at DC 4. Between Dec. 22 and Jan. 12 the tail was recognized visually, with a maximum reported length of 10' (300.000 km). During this time span the tail orientation changed from WSW towards South and eventually NE.
Andreas Kammerer