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C/2016 U1 (NEOWISE)

On infrared images, taken by the NEOWISE satellite on Oct. 21, 2016, a comet was discovered near the border of the constellations Lynx / Ursa Major, with the optical-wavelength magnitude roughly estimated as 19. Terrestrial follow-up observations of comet C/2016 U1 (NEOWISE) showed a diffuse, 6" coma of total magnitude 20.0. The comet will pass perihelion (at the solar distance of 0.32 AU) in mid-January 2017 (CBET 4335 / MPEC 2016-V116). Due to the fact that this comet possesses a very faint absolute magnitude it is expected that the comet will peak at only magnitude 11.5. However, the absolute magnitude of this comet is well below the Bortle-limit. Thus it is extremely probable that this comet will dissipate on its way toward perihelion. It should become brighter than 16 mag at the start of December, then positioned in Bootes. From mid-European sites it could be visible in the morning sky until the first week of January (then positioned in Ophiuchus), then expected to be of magnitude 12 (if it should not have dissipated).

This comet surprised the observers at the turn of the years 2016/17 in a very positive manner. Instead of being a faint telescopic object, probably doomed to disintegration, it transformed into a brighter binoculars object, thereby exhibiting a strongly condensed coma and a photographically prominent gas tail.

A total of 83 observations from 27 observers indicate that the comet more probably turned active in December than having its brightness being underestimated at discovery. After it turned active the brightness increased at an average rate, according to the formula

m = 11.3 mag + 5×log D + 9.7×log r

This yields a maximum brightness of 6.8 mag at perihelion and of 7.2 mag during the last day of the apparition prior to the solar conjunction (Jan. 7). The last astrometric observation dates of Jan. 12 two days prior to perihelion. However, during the following months the comet could not be observed from any location. It should reappear for Southern hemisphere observers in May/June 2017, alas only as an object of magnitude 17-18.

Total Brightness and Coma Diameter

During the first days of the visual apparition the coma diameter was short of 2' (80.000 km), increasing rapidly to 6' (210.000 km) until Dec. 20. Thereafter it shrunk - due to the increasing solar wind - comparably fast, measuring only 1.5' (70.000 km) around Jan. 5, 2017. The degree of condensation increased steadily from DC 1 at the beginning of the visual apparition to DC 8 at the date it disappeared into the dawn. Visually the south-pointing gas tail reached a length of 8' (250.00 km).

Andreas Kammerer

FGK observations