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On images taken by the NEOWISE-Satellite on July 2, 2018 a comet of estimated brightness 15.0 mag and an apparent coma diameter of 1.5' was discovered in the constellation Cetus. Visual follow-up observations of comet C/2018 N1 (NEOWISE) yielded a total brightness of 11.5 mag and an apparent coma diameter of 6'. The comet passed its perihelion at a solar distance of 1.31 AU on Aug. 1, 2018 (CBET 4531/32). Its minimum distance to Earth (0.31 AU) was reached on July 27, when it was of magnitude 9.0 mag, showing a coma diameter of 8'. From mid-European locations it was briefly observable low above the southern morning horizon for a few days in mid-July, before its rapid movement took it to the southern parts of Sagittarius. It will become visible again in the evening sky from early August until mid-September, but remains at low altitudes (short of 15°) while passing through the southern parts of Ophiuchus onto Libra.
A total of 71 observations from 18 observers could be used for the analysis. It shows a steep brightness development that can be described reasonably well using the parameters
m0=9.2 mag / n=7.
Thus, a maximum brightness of 8.6 mag on July 27, 2018 results, primarily a result of the small Earth – comet distance.
Total Brightness and Coma DiameterThe development of the apparent coma diameter was comparably fast. Around July 10 measuring about 4' it increased to 12' by July 27, only to decrease again comparably fast thereafter. On Aug. 15 the observers reported 5.5' and on Sep. 10 only 1.0'. The absolute coma diameter developed slower. Around July 10 it measured only 110.000 km, but increased to a maximum of 225.000 km until Aug. 10. On Aug. 27 it had decreased to 180.000 km, but the coma shrank further, reaching 75.000 km on Sep. 10. The coma was always diffuse to very diffuse. During the first weeks the degree of condensation was estimated at DC 2-3, decreasing to DC 2 until early August and to DC 1-2 until early September to DC 1-2. No tail was observed visually.
Andreas Kammerer