A Comet Primer for 2018

With 2017 passing by relatively quiet for comet activity, 2018 on the other hand will be a very busy year for the comet observer. At least 1 comet should reach naked eye visibility in December, 46P Wirtanen, and a host of other objects, 15 in total, may appear brighter than magnitude 12.

No doubt there will be additions to this list as new comets are discovered throughout 2018.

C/2016 R2 PANSTARRS

Closest to Earth 2017 Dec 22 at 2.05AU
Closest to Sun 2018 May 9 at 2.60AU
Maximum Mag 10-11 in Jan 2018
Period approx. 20,326 years

This comet is well situated in evening skies during January, trekking slowly northwards through the constellation of Taurus. Unfortunately perihelion occurs in May, when the Earth is situated on the opposite side of the Sun. This comet could potentially appear brighter than magnitude 11, as it has shown unusual activity for such a distant comet.

C/2017 T1 Heinze

Closest to Earth 2018 Jan 4 at 0.22AU
Closest to Sun 2018 Feb 21 at 0.58AU
Maximum Mag 9 in Jan 2018
Period = dynamically new.

This dynamically new comet has a particularly close passage to the Earth in early January that may see it reach magnitude 9, but viewing will be restricted to the northern hemisphere. The comet is intrinsically faint and may not survive its perihelion passage of 0.58AU in February. If the comet survives, it may become visible for southerners from late March in the hour before dawn, situated in Aquarius and fading from magnitude 11.

C/2013 CU129 PANSTARRS

Closest to Sun 2018 Jun 24 at 0.80AU
Closest to Earth 2018 July 18 at 0.23AU
Maximum Mag 10 in July 2018
Period 4.88 years

This intrinsically faint short period comet, discovered in 2013, generally favours southern hemisphere observers at perihelion. It will be picked up in the south-western evening sky at the start of July, then make its way through solar conjunction mid month, well south of the Sun. It will then be seen in the south-eastern morning sky, near the vicinity of the brighter C/2017 T3.

C/2017 T3 ATLAS

Closest to Sun 2018 July 19 at 0.82AU
Closest to Earth 2018 Aug 1 at 1.35AU
Maximum Mag 8 in July 2018
Period approx. 41,266 years

This comet may outperform its predicted peak of magnitude 8, as it is a returning long period object. The apparition is unfavourable as the comet remains close to the Sun, however southerners will get the best view of perihelion passage. It should be picked up in late June as it emerges from morning twilight, trekking southwards through Orion, Monoceros and Canis Major, remaining relatively low to the horizon. By mid-August, the comet reaches solar conjunction, well south of the Sun, and is then observable in the evening sky, fading from magnitude 9.

C/2017 S3 PANSTARRS

Closest to Earth 2018 Aug 6 at 0.76AU
Closest to Sun 2018 Aug 15 at 0.21AU
Maximum Mag 3 in Aug 2018
Period = dynamically new.

Of the comets in 2018, "S3" has the smallest perihelion distance at 0.21AU. At first glance you might think the comet has naked eye potential. However, it is at risk of disintegrating as the orbit appears to be dynamically new and the comet is intrinsically rather faint. Pre-perihelion viewing will be virtually restricted to the northern hemisphere, when the comet may reach magnitude 6 before being lost in twilight by the first week of August. In the very unlikely chance the comet exceeds expectation, southerners may get an opportunity in mid August, before sunrise, when the comet is situated only 11 degrees away from the Sun. Then on August 25, the comet enters the field of view of the SOHO LASCO C3 coronagraph, fading from magnitude 7, if it still exists.

66P du Toit

Closest to Earth 2018 May 17 at 0.89AU
Closest to Sun 2018 May 19 at 1.29AU
Maximum Mag 11 in May 2018
Period 14.86 years

66p was discovered in South Africa in 1944 and is a “short” period comet that generally favours southern hemisphere observers at perihelion. I made several observations at its last appearance in 2003 where it peaked at magnitude 11. This comet has rarely been seen visually. This year will see 66P at a similar peak brightness as it treks slowly eastwards through Sculptor during May, and is best observed in the morning sky.

C/2016 M1 PANSTARRS

Closest to Earth 2018 June 24 at 1.29AU
Closest to Sun 2018 Aug 10 at 2.21AU
Maximum Mag 9-10 in July 2018
Period approx. 99,716 years

This relatively distant but intrinsically bright long period comet will be having a favourable apparition during July, especially for southerners. Opposition occurs at the end of June, when the magnitude 10 comet rises after sunset.

During July, it will then be seen trekking slowly south-westward through Sagittarius, Corona Australis, Ara and Norma.

C/2016 N6 PANSTARRS

Closest to Sun 2018 Jul 18 at 2.67AU
Closest to Earth 2018 Dec 23 at 2.36AU
Maximum Mag 11 in Jul 2018
Period approx. 81,156 years

This distant long period comet may reach magnitude 11 when it is at opposition during December, rising in the east after sunset. It will be seen trekking south-westwards through Hydra, Puppis and Canis Major.

48P Johnson

Closest to Sun 2018 Aug 12 at 2.00AU
Closest to Earth 2018 Aug 20 at 1.01AU
Maximum Mag 11 in Aug 2018
Period 6.55 years

48P had an encounter with Jupiter in 2014 which reduced its perihelion distance from 2.3 to 2.0AU. This apparition will be highly favourable one as the comet is at opposition and perihelion around the same time during August. The magnitude 11 comet will rise in the east after sunset, well situated in Aquarius, then trek southwards into Pisces Austrinus.

21P Giacobini-Zinner

Closest to Earth 2018 Sep 10 at 0.39AU
Closest to Sun 2018 Sep 10 at 1.01AU
Maximum Mag 7 in Sep 2018
Period 6.55 years

21P is another periodic comet having a very favourableapparition in 2018 as it is closest to the Earth and Sun on the same day of September 10. During August, the comet is restricted to northern observers. The magnitude 7 comet will come into view for southerners by the first week in September, trekking southwards through Auriga in the hour before dawn. The comet treks through Gemini, Monoceros and Canis Major, fading to magnitude 9 by the end of October.

64P Swift-Gehrels

Closest to Earth 2018 Oct 27 at 0.44AU
Closest to Sun 2018 Nov 3 at 1.39AU
Maximum Mag 10 in Nov 2018
Period 9.4 years

Yet another periodic comet having a very favourable apparition in 2018. 64P is closest to the Earth and Sun at the same time in early November. The magnitude 10 comet will be rising after sunset, but better situated for northerners, as it treks slowly eastwards through Andromeda.

38P Stephan-Oterma

Closest to Sun 2018 Nov 10 at 1.59AU
Closest to Earth 2018 Dec 17 at 0.76AU.
Maximum Mag 9 in Nov-Dec 2018
Period 37.9 years.

This “Halley type” comet reached magnitude 8 at its previous return in 1980. Observing circumstances are slightly worse off in 2018, but the comet is likely to reach magnitude 9 by late November, as it treks slowly north-eastwards through Gemini and Cancer, in the morning sky. It will be better situated for northern viewers. It should be noted that the actual discoverer of this comet in 1867 was Jerome Coggia but credit was taken by the observatory director EJM Stephan.

46P Wirtanen

Closest to Sun 2018 Dec 12 at 1.05AU
Closest to Earth 2018 Dec 17 at 0.0776AU
Maximum Mag 3 in Dec 2018
Period 5.4 years

Will be having an exceptional apparition in 2018 as it passes a mere 11.6 million kms from Earth on December 17, appearing visible to the naked eye as a 3rd magnitude circular haze over a degree across. It initially favours viewers in the southern hemisphere and may reach naked eye visibility after the full moon of November 23 when the comet is situated in Fornax, in the evening sky. During December, the comet moves rapidly north-eastwards through Cetus, Eridanus, Taurus, Perseus and Auriga. The comet is lost to southerners by Christmas, whilst northerners get the best view post perigee, with the comet fading beyond naked eye range by mid January.