The brighter comets of 2018

This report describes and analyses observations of the brighter or more interesting comets at perihelion during 2018, concentrating on those with visual observations. Magnitude parameters are given for all comets with observations. Any evolution in the magnitude parameters of those periodic comets with multiple returns is discussed. Additional information on the comets discussed here, and on other comets seen or at perihelion during the year, may be found on the Section visual observations web pages.



57 comets or potential comets were assigned year designations for 2018, and 43 previously numbered periodic comets returned to perihelion. 193 comets detected by the SOHO satellite and 15 from STEREO were credited during 2018, including one returning object. 186 were members of the Kreutz group, 10 were of the Meyer group, seven were of the Marsden group (one a return), none were of the Kracht group and five were not associated with any known group. None of these objects were given a designation. There were three possible amateur discoveries (2018 E2, V1 and Y1), for which João Ribeiro de Barros, Don Machholz, Shigehisa Fujikawa and Masayuki Iwamoto may gain the Edgar Wilson Award, though there has been no formal announcement to date. 12 periodic comets were numbered during the year. One comet was reported as visible to the naked eye. Overall, 2018 was another disappointing year for visual comet observers, though brightened by the return of two periodic comets making close passes of Earth.

The remainder of this report covers only the comets that were at perihelion during the year. When periodic comets have visual or electronic observations at five or more returns and have not previously been analysed in detail over the past decade, the secular behaviour of the comet is considered, even though it may not qualify as a ‘brighter’ comet during the present return. Any evolution in behaviour is of interest, as is observation of a steady state.

Orbital elements for all the comets discovered and returning during the year can be found on the JPL Small-Body Database Browser,1 which will also generate ephemerides. Discovery details and further information for the other comets found or returning during the year are available on the Section visual observations web pages,2 which also contain links to additional background information. The raw visual observations for the year are on the Section visual observations web pages in ICQ format, and in the Comet Observations database (COBS).3 The full dataset from COBS is used for the multi-return analyses presented here, but otherwise only those submitted to the Section are included. Additional images of the comets are presented in the Section image archive.4


The comets given a discovery designation

2016 M1 (PanSTARRS)

Pan-STARRS discovered this comet on 2016 Jun 22.5 as a 20th-magnitude object. A few days later, Kevin Hills imaged it at 19th magnitude. He and a few other electronic observers continued to monitor it as it approached the Sun. Visual observers picked it up in 2018 February and in the northern hemisphere it was followed until June, but it was a southern-hemisphere object when at its brightest at the end of the month. Southern-hemisphere visual observers continued to follow it until early in 2019. Despite only reaching 9th magnitude, it was one of the better-followed non-periodic comets of the year, saying something about the paucity of decent comets in this class.

Unusually, the observations were initially not well fitted by a single light curve, with the comet being distinctly brighter than indicated by the mean curve at the first two oppositions post-perihelion. However, at the third opposition in 2021 they were once again a good fit, showing that a partial light curve may give misleading predictions. The mean curve is shown in Figure 1, with magnitude parameters also being provided for pre- and post-perihelion fits in Table 3.

Figure 1. The observations of 2016 M1 with a standard light curve fitted to them. The dashed lines show the 95% confidence limits.

Figure 2. The observations of 2016 M1 with a standard light curve fitted to them. The dashed lines show the 95% confidence limits.


2017 S3 (PanSTARRS)

Pan-STARRS discovered this comet on 2017 Sep 23.2 as a 21st-magnitude object. A few days later, Kevin Hills imaged it at 20th magnitude. Visual observers suddenly picked it up at 9th magnitude in early July of 2018, when it was in outburst, several magnitudes brighter than expected. A second outburst took place later in the month, bringing it up to 7th magnitude. By late in the month it was best seen in the morning sky, so the number of observers declined. It was then at a high northern declination and passed close to open cluster NGC 2281.

It entered solar conjunction in early August and then remained too close to the Sun for further observation until October. It was intrinsically faint and did not survive its perihelion passage at 0.2au. The sequence of events leading to its demise is discussed by Zdeněk Sekanina & Rainer Kracht (2019).5 Although there are some visual observations from October and November, these are much brighter than expected and were not confirmed by other observers. They could be illusory or might relate to the dispersing debris cloud of the disrupted comet nucleus.


2017 T1 (Heinze)

Aren (Ari) Heinze discovered an 18th-magnitude comet in images taken with the ATLAS (Asteroid Terrestrial-impact Last Alert System) 0.5m Schmidt at Mauna Loa, Hawaii on Sep 28.6. It was another intrinsically faint comet. However, it passed close to Earth on its way to perihelion and reached 10th magnitude when it did so. Closest approach was at 0.22au on Jan 4, when it was at high northern declination and convenient for viewing in the early evening. The orbit has a miss distance of 0.014au, and therefore a meteor shower might be possible, though no meteor shower was reported. The comet moved rapidly across the sky and became poorly placed after early February.


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