48 AUSTRALIAN SKY & TELESCOPE July 2019
COMETS by David SeargentA very intriguing comet
Did C/2017 T2 (PANSTARRS) undergo an outburst earlier this
year, and can we expect an impressive show by late-winter?O
n October 2, 2017, the
Pan-STARRS 1 telescope at
Haleakala, Hawai‘i, discovered
a 19.9 magnitude comet. Nothing
unusual about that these days as this
telescope now finds the lion’s share
of such objects and most of them do
not become bright enough to draw
the attention of visual observers using
small telescopes. However, this comet —
C/2017 T2 (PANSTARRS) — was a little
different as, at the time of its discovery,
it was still 9.3 astronomical units (a.u.)
from the Sun and 8.6 from Earth and
not due to arrive at perihelion until
May 4, 2020.
Despite its faint magnitude, to
have reached even that brightness
at such a vast distance means that,
intrinsically speaking, C/2017 T2
must be a relatively luminous comet.
Moreover, subsequent refinement of its
orbital elements indicated that this is a
dynamically evolved object of very long
period and not one making its maiden
voyage inward from the Oort Cloud.
Consequently, its activity level while
still far from the Sun is more likely
to imply a large nucleus rather thanan abundance of the type of unstable
surface ‘frosting’ that drives the distant
activity of objects coming in fresh from
the Cloud.
Nevertheless we should not get too
excited, as the 1.62 a.u. perihelion
distance of this comet will keep C/2017
T2 well away from both Earth and Sun
and prevent it from becoming really
bright. Moreover, its path through
the constellations does not favour
the Southern Hemisphere, although
it should be easily within range of
small telescopes and large binoculars
relatively low in the evening sky from
mid-southern latitudes from the latter
part of June next year as it emerges
from evening twilight. It will then have
faded only a little from its maximum
brightness.
The reason for drawing attention
to C/2017 T2 so far in advance,
however, concerns several interesting
observations made by an experienced
comet observer located at high altitude
within the mountains of Spain. As
the comet drifted toward evening
twilight during March and April this
year, brightness predictions based onearly CCD observations indicated that
it should have increased in lustre to
around magnitude 15. Nevertheless,
the visual reports from Spain made it
close to 12 in March and, at the very
beginning of April, a faint and extended
outer coma gave a total brightness just a
little fainter than 9!
Unfortunately, no other visual
reports are known and the observer
suggested that he might have been
witnessing a brief outburst during
March, leaving in its wake the extended
coma of early April. ‘Extended’ is,
indeed, the right word for it as C/2017
T2’s estimated diameter of 6 minutes of
arc indicates a real diameter of almost
1.5 million kilometres at the distance
of the comet from Earth. This is
substantially larger than the Sun! Such
dimensions are not unknown although
they are relatively rare.
C/2017 T2 will emerge from
morning twilight during late winter
and early spring 2019. In late July it
will be located within Taurus in the
region of Aldebaran and is predicted
to be approximately magnitude 14.
Nevertheless, recalling of the Spanish
reports of March and April, it is possible
that it could be brighter than this
value. Even if these observations were
of an outburst, it is conceivable that
the comet did not fully return to ‘base’
intrinsic magnitude and may still be
somewhat brighter than predicted.
Observers having good eastern skies
and access to large telescopes might
like to try for it as early as possible. All
observations — positive or negative,
visual or CCD — will be useful in
providing more information about the
development of this intriguing comet.■ DAVID SEARGENT is the discoverer
of comet 1978 XV. His latest book on
comets, Snowballs in the Furnace, is
available from Amazon.com The intriguing comet C/2017 T2
(PANSTARRS) underwent an apparent
brightness surge in March and April 2019.
This image by Michael Jaeger was taken on
October 11, 2018.