50 AUSTRALIAN SKY & TELESCOPE April 2018
A
lthough its position low in the
northwesternskyduringtwilight
makesC/2016 R2 (PANSTARRS)
averydifficultobjectforobservers
in middle southern latitudes, it may
stillbeaccessible(possiblyatabout
magnitude10–11)forthoselivingin
thefarnorthofAustralia,asitslowly
drifts through Andromeda.
C/2016R2turnedouttobefarmore
interesting than initially suspected.
As mentioned in the previous issue,
therewasatimeduringthespring
months of last year when it seemed
that early predictions of magnitudes
around 10 or thereabouts had been too
optimistic. Early brightening trends
appeared to slow or even stagnate. Yet,
toward the end of last year, the comet’s
activity once more picked up and by the
middle of December, visual brightness
estimatesmatching—orevenexceeding
—originalforecastswerebeing
reported. At least one observer spotted
the comet with binoculars at that time.
Nevertheless,itwasnotC/2016R2’s
increase in intrinsic brightness that
aroused the greatest excitement. As its
activity increased, the comet developed
awell-definedgastailthatdisplayed
remarkable activity, even though it was
still more than three astronomical units
fromtheSun!Imagesrevealedthis
featuretobecomplexandturbulent,
with disconnected sections and cloud-
likefeatures,givingtheappearance
of sporadic eruptions of gas from the
comet’s head.Unusual beasts
This sort of behaviour reminded a
number of observers of the remarkable
performances of comets C/1908
R1 (Morehouse) and C/1961 R1
(Humason). These objects remain
the prime examples of rare carbon
monoxide comets, and it has now beenCOMETS by David SeargentRare cometary beasts
C/2016 R2 defied forecasts to put on a remarkable display.
confirmed that R2 is another of these
unusual beasts.
Astronomers back in 1908 hardly
knew what had hit them as they saw
Morehouse. Even today, a sequence
of photos of this remarkable comet
stands as a classic account of extreme
tail turbulence. Tail disconnections,
cloud-like structures and all manner
of contortions made this an incredible
object, as did brightness outbursts and
changes in appearance so dramatic that
some observers remarked that it was
difficult to believe that it was really
the same object being observed on
consecutive nights.
As for Humason, when still out
near the orbit of Jupiter — more than
a year before perihelion — the coma
displayed a pinwheel-like appearance
and gaseous matter extended in all
directions. A tortuously contorted gas
tail at least 25 arcminutes long was a
remarkable feature, considering the
comet’s relatively large solar distance
at that time. Another odd piece of
behaviour was an outburst in brightnessamounting to around six magnitudes
in June 1964, about eighteen months
after perihelion. During this outburst,
Humason’s intrinsic brightness rivalled
the most luminous of comets such
as Hale-Bopp (in 1996) and Sarabat
(in 1729). In fact, even without the
brightness flare, Humason ranked
high on the list of intrinsically bright
comets. Despite a perihelion distance of
2.1 a.u., it still became bright enough to
be observable without optical aid.
As C/2016 R2 is another member of
this exclusive cometary clan, it is to be
hoped that it will be carefully followed
by observers for as long as possible.■ DAVID SEARGENT is a long-time
comet observer, and author of Weird
Comets and Asteroids (Springer
2017) and Visually Observing Comets
(Astronomer’s Pocket Field Guide series,
Springer 2017).S These three images, taken on January 8,
9 and 10, 2018, show the evolution of comet
C/2016 R2’s tail. Courtesy Gerald Rhemann.