2017 September 6

Autumn comets

Nick James

As the nights draw in for northern hemisphere observers there are two interesting comets which will be well placed for observation from the UK. Neither are particularly bright and both are telescopic objects but they are worth making an effort to observe.

C/2017 O1 (ASASSN)

This is a fairly recent discovery picked up by the All-Sky Automated Survey for Supernovae. It was discovered on 2017 July 19 and appeared to be in outburst at the time. The comet is very diffuse there has been a large scatter in total magnitude estimates however the comet has been brightening slowly. At the beginning of September it was around 9th magnitude and it is expected to brighten to around 8th magnitude in October by which time it will be very well placed in Perseus. It continues to move north and by the end of December will be a 9th magnitude object a few degrees from the pole.

C/2017 O1 (ASASSN) at 5d intervals from September through December

Latest images.

C/2017 O1 on 2017 September 2 (Denis Buczynski)

C/2015 ER61 (PANSTARRS)

This comet has been around for a long time, having spent much of the last year as a good object from the southern hemisphere. It was discovered  by PANSTARRS at magnitude 21, 9 au from the Sun on 2015 March 15. It was originally thought to be asteroidal although it had a very eccentric orbit. By 2015 June the object showed an apparent coma and it was reclassified as a comet in 2016 January. The orbit is highly elliptical with a perihelion passage 1.04 au from the Sun on 2017 May 10. The comet is now well placed from the UK and it made a close approach to M45 in Taurus in late August. It will spend the next two months moving slowly through Taurus and Aries at about mag 12.

C/2015 ER61 chart

The comet shows a reasonable tail in the Latest images.

C/2015 ER61 on 2017 August 26 (Nick James)

As always, if you observe these comets please submit your observations and images to the Comet Section.