http://www.skyandtelescope.com.au 49A
utumn of 2016 is a time for
some interesting cometary
anniversaries. Those of us
who were already active observers
at the time may find it hard to
believe, but 30 years have now
passed since that most celebrated
of comets, 1P/Halley, made its
most recent passage through the
inner Solar System, swinging by
the Sun at a minimum distance of
0.59 a.u. on February 9, 1986, and
approaching Earth to just 0.42 a.u.
on April 10.
Although the comet did not
become as bright as it does on most
returns, the same observational
geometry which precluded a really
close approach to Earth and a
strong brightness peak was also
responsible for an unprecedented
period of naked-eye visibility: from
November 8, 1985 right through
until May 30, 1986. This length of
naked-eye visibility has only been
exceeded by the Great Comet of 1811
(9 months) and Hale-Bopp of 1997
(nearly 21 months!).
Prior to Hale-Bopp, Halley’s 1986
was the most thoroughly observed
comet in history and was the first
to be imaged close-up from space
as well as from the ground. March
2016 marks the 30th anniversary of
four encounters between the comet
and spacecraft; Japan’s Suisei on the
8th and Sakigake on the 10th, the
European Giotto probe on the 13th
(giving us the first close-up images
of a comet’s nucleus) and the long-
distance ICE probe on March 20,
the latter having also approached
P/Giacobini-Zinner the previous
year and monitored its effect on the
solar wind.
March 25 also marks 20 years
since C/1996 B2 (Hyakutake), the
magnificent Great Comet of 1996,
passed just 0.1 a.u. from Earth,
sporting a tail measured by some
as over 110 degrees in length.
We long-term comet observers
certainly have plenty of interesting
memories this autumn!This month
April’s most interesting comet
will be the dynamically new object
C/2013 X1 (PANSTARRS), which
will emerge from the morning
twilight ready for its extended
period of visibility during the
autumn and winter months,
during which it will pass just 0.7
a.u. from our planet in June.
As we saw in the previous issue,
this comet was discovered back
on December 4, 2013, and initially
seemed to hold little prospect for a
bright telescopic display. However,
during the closing months of last
year it brightened considerably
and magnitude estimates close to
9 were already being reported by
Christmas. This rapid increase is
unlikely to continue, but prospects
for a relatively bright binocular
object during the coming months
nevertheless appear encouraging.
April will see C/2013 X1
moving slowly in a south-westerly
direction through Pisces, arriving
at perihelion (1.31 a.u. from the
Sun) on the 20th. Although
brightness predictions must
necessarily be uncertain, it is
unlikely to be fainter than about
8.5–9.0 early in the month and
is expected to brighten by up to
one magnitude or thereabouts by
April’s end, all the while becoming
higher and easier to observe in theearly morning sky.
Interestingly, it looks as though
2017 will also see a relatively
bright telescopic comet well-
placed through the cooler months.
On November 3 last year, Jess
Johnson found a 17th-magnitude
object during the course of the
Catalina Sky Survey which, if
early indications are correct,A morning comet in Pisces
Comet C/2013 X1 becoming higher and easier to see in April’s early morning sky. DAVID SEARGENT
C/2013 X1
(PANSTARRS),
photographed
in the northern
sky by Gerald
Rhemann on
January 26, 2016.
The comet will
appear in our
morning skies in
April.should become an object for large
binoculars while at large solar
elongation during the middle of
next year. Perihelion will occur on
June 12, 2017 at 1.6 a.u. from the
Sun, when the comet will be in
the constellation Bootes, south of
Arcturus and probably close to 8th
magnitude... but more about this as
the time draws closer.David Seargent’s book on comets,
Snowballs in the Furnace, is available
from Amazon.com