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An interesting object promises comet 38P/Stephan-Oterma (P=37.94a) to get in fall/winter 2018/19. Normally this comet can only be observed twice in the lifetime of an observer. The previous perihelion passage took place in December 1980. The current perihelion passage will take place on Nov. 10, 2018 at a solar distance of 1.59 AU. Between August and November it will be visible in the morning sky, moving from Taurus into Gemini, predicted to brighten from magnitude 13.5 to the maximum of 9.0 mag, showing a coma diameter of about 5'. Between December 2018 und March 2019 it will be visible during the whole night, moving through Cancer and Lynx. However, during this period it will fade, expected to reach magnitude 15-16 by the end of March.
The comet did not live up to the expectations for this apparition. Instead of reaching the predicted maximum brightness of 9.0 mag it peaked at 9.7 mag. Based on 376 observations from 50 observers the brightness developed differently pre- and post-perihelion. Whereas the comet brightened rapidly pre-perihelion it faded in an average manner post-perihelion. It peaked at 9.7 mag on Dec. 1, 2019. The brightness development can be well-described by the formulae:pre-perihelion: m = 4.4 mag + 5×log D + 28.5×log r post-perihelion: m = 7.5 mag + 5×log D + 13.3×log r
Total Brightness and Coma Diameter
The apparent coma diameter increased quite steadily from 0.3' in mid-July 2018 to the maximum value of 6.0' at the end of December, when the comet reached perigee. Thereafter the apparent coma diameter decreased rapidly, reaching 2.5' in early February 2019 and 1.5' at the end of April. The absolute coma diameter increased steadily between mid-July and mid-November 2018 from 50.000 km to the maximum value of 190.000 km, which remained constant until early January 2019. Thereafter the coma shrunk rapidly, reaching 140.000 km in early February and 120.000 km at the end of April. Pre-perihelion the degree of condensation remained remarkably constant at DC 3-4, but decreased slowly thereafter, reaching DC 2-3 in mid-April 2019. A tail was visually observed between early October 2018 and early March 2019, which never exceeded 9' (900.000 km). It mainly pointed westward, rotating towards SW near the end of its visibility.