2018 produces three comets within the range of binocular viewing, two of them short period comets and one Halley type comet. Both of the short period comets have the potential to enter the realms of naked eye visibility.
21P/Giacobini-Zinner
Discovered by Michael Giacobini in December 1900 at the Nice observatory, this comet caused some confusion when Ernst Zinner unknowingly observed its return 6.5 years later. The comet’s orbital period was thought to be 6.8 years up until that point.
Giacobini-Zinner’s return in 2018 is a promising one as it passes the Earth at a distance of 0.39 au. In June, 2018, 21p will be observable from modest sized telescopes sitting in the constellation of Cygnus with a brightness of magnitude 11. It will then brighten rapidly through July and August by which time the nucleus may reach naked eye visibility. Rising in the late evenings, 21p remains well placed for observation for most of the summer.
On the night of 03 September, the comet will lie just over 1 degree from bright star Capella in Auriga making it much easier to locate. The comet reaches closest approach on the night 10/11 September, 2018 after which it rises after midnight. Current predictions peak at magnitude 6, so only just within the naked eye threshold. Through the summer months, comet 21p passes within close visual proximity to quite few deep sky objects during its journey through Cygnus, Camelopardalis, Auriga and Monoceros.
38P/Stephan-Oterma
Despite the name, this comet was discovered by Jerome Coggia at the Marseilles
Observatory in January 1867, but the name was awarded to E J M Stephan, the Observatory Director as he first calculated the position with accuracy. This comet has an orbital period of 38 years defined as a Halley-type comet hence why it was last observed in 1980/1.
Perihelion occurs on 26 August, 2018 by which time it should become sufficiently bright enough for backyard telescopes. On the night of 08/09 November, Comet 38p will lie very close to deep sky object NGC 2392 (The Eskimo Nebula) separated by just 8 arcmins, a good photographic opportunity. The comet will remain well placed in the evening skies for the remainder of the year and on in to 2019 when it becomes almost circumpolar in the constellation of Lynx.
According to NASA’s JPL orbit program, the point at which the Earth is closest to the comet is not actually until January 2019 as the Earth ‘catches up’ with the comet during it’s orbit around the Sun. How bright the icy rock will be will depend on how it reacts to its perihelion passage.
46P/Wirtanen
Comet 46P/Wirtanen is a small short-period comet with an orbital period of 5.4 years. Discovered by Carl A. Wirtanen in 1948 at the Lick Observatory, California, USA. The comet reaches perihelion on 12 December, 2018 and is expected to reach magnitude 3, well within naked eye visibility. This would make it the brightest comet since C/2011 L4 (PANSTARRS) reached a similar magnitude during the twilight hours in April 2013. 46P will be well placed in December residing in Taurus. The ice rock will then pass between the Pleiades and Hyades star clusters just before closest approach where it will reach a distance of 0.078 AU (11.6m kilometers) from the Earth on 18 December. Given the relatively close approach to the Earth, the nucleus of Wirtanen should appear reasonably large in the sky.