Australian Sky & Telescope — November-December 2017

50 AUSTRALIAN SKY & TELESCOPE November | December 2017

COMETS by David Seargent

Just passing through

A faint fuzzball will visit Orion and the Hyades this summer.

T

his summer will find one

particular comet well placed

for southerner observers, and,

although it will not become bright,

there are good reasons to think that it

will be an easy object for moderate-sized

telescopes at least. This object is C/2016

R2 (PANSTARRS). Like many comets

found in recent years, it was discovered

W Comet C/2017 O1 (ASASSN) showed a

condensed nucleus and a green coma when

Rolando Ligustri took this image on July 22

as it was crossing Cetus.

by the Pan-STARRS 1 telescope at

Haleakalã as a faint (magnitude 19.1)

object, well in advance of perihelion. It

was found on September 7, 2016, but

will not come closest to the Sun until

May 9, 2018, at a distance 2.6 a.u. from

our parent star.

CCD observations suggest that the

comet is intrinsically rather bright, and

its orbit indicates that it is not a ‘new’

object on its maiden voyage into the

inner Solar System. Its early brightness

is not, therefore, likely to have been due

to the sublimation (ie. from ice to gas)

of a superficial layer of highly volatile

material on its surface.

If the early level of activity is indeed

maintained, this comet is likely to

reach magnitude 10.5 to 11 by late

November, when it will be well placed

in Orion. Drifting into Taurus in the

middle of December, the comet will

be found at the edge of the Hyades by

the end of that month, possibly a little

less than half a magnitude brighter

than the late November value. It might

still be around magnitude 11 as late as

next July

Aside from C/2016 R2, at the time

of writing the closing months of 2017

do not appear very promising with

respect to brighter visual comets. Of

course, there is always the possibility

that a new object will change this

outlook. The surprise discovery last

winter of the relatively bright C/2017

O1 (ASASSN) proves that comets can

still reach visual observability before

discovery, despite the deep search

programmes that are now in place.

Unfortunately for southern observers,

C/2017 O1 is now deep in Northern

Hemisphere skies, well out of range for

mid-southern latitudes.

■ DAVID SEARGENT is the discoverer

of comet 1978 XV. His most recent book

on comets, Snowballs in the Furnace, is

available from Amazon.com

Oops! In our last issue, we said that the

eccentricity figure for a circle is 1.0 —

it’s actually 0.0. And the lovely image of

Comet Machholz was of the wrong comet

— we showed C/2004 Q2 instead of 96P.

Don Machholz has 11 comets to his name.