Analysis of Comet Apparitions

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C/2017 O1 (ASASSN)

On images taken with the All-Sky Automated Survey for Supernova (ASASSN) on July 19, 2017, a comet was detected in the eastern part of the constellation Cetus, showing a 40" coma of magnitude 15.5. Follow-up observations of comet C/2017 O1 (ASASSN) showed a diffuse 4.5' coma of magnitude 11.5 with a small central condensation. The comet will reach perihelion at a distance of 1.51 AU on Oct. 14, 2017, expected to peak at magnitude 8.0 - or may become even a bit brighter (CBET 4414). It should be brighter than 14 mag until April 2018. Until then it will move through the constellations Taurus, Perseus, Camelopardalis and Cepheus, where it will reverse direction at a declination of +85°, thereafter moving through Camelopardalis again, heading towards Ursa Major. Thus the comet will be an object in the morning sky during the first months, thereafter an object for the whole night. Earth will cross its orbital plane on Oct. 19, 2017.

Based on 265 observations from 43 observers the comet remained significantly fainter than expected, peaking at only magnitude 8.9. Starting in mid-November 2017 the brightness decreased rapidly, reaching 13.5-14.0 mag at the end of January 2018. The brightness evolution can be represented quite well by the following formulae:

The pre-perihelion formula clearly states that the comet experienced an outburst, enabling it to reach magnitude 10. After the outburst peaked the activity decreased, despite the comet approaching the Sun, expressed by the negative value of the activity parameter. This negative value limits the application of the pre-perihelion formula to the period between the outburst and the perihelion. The fact that the apparent brightness did not peak until mid-October is a result of the then decreasing distance to Earth. Following perihelion the activity rapidly decreased further, this time in accordance with the apparent brightness, due to the then increasing distance of the comet to Earth and Sun.

Until the end of August the apparent coma diameter decreased slightly from 9.5' to 8.5', despite the decreasing distance of the comet to Earth. During the following two weeks the approach to Earth resulted in a short-lived increase of the apparent coma diameter, which peaked at 10' in early September. Thereafter the decreasing activity plus the now increasing distance to Earth yielded to a shrinkage with the apparent coma diameter reaching 1.5' in mid-January 2018. The absolute coma diameter confirms an outburst just before the discovery. It decreased - rapidly at first, with the slope of the shrinkage decreasing steadily - through the whole apparition. Measuring 600.000 km by the end of July 2017, it decreased to only 100.000 km in mid-January 2018. During the whole apparition the coma was quite diffuse. The degree of condensation was reported at DC 2-3 during the first weeks, DC 2 during October/November and DC 1-2 thereafter. No visual tail sightings have been reported.

Andreas Kammerer

FGK observations