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A Comet For Christmas

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Darrell Heath | Astronomy Columnist

Since the dawn of history
we humans have marveled
in awe, wonder, and even
fear at the sight of a comet
in our night sky. Once we
clawed our way out of the
dark ages we began to understand
that these amazing
celestial spectacles are not
omens sent by god(s) but
are instead dirty chunks of
ice cruising around in space.
With further knowledge we
have come to realize that
these dirty snowballs are the
left over fragments from the
birth of the Sun and planets
and that they reside on the
outer fringes of the solar
system. For billions of years
they are content to remain in
their lonely orbits far from
the Sun, but on occasion
they get bumped out of their
normal paths and plunge
inwards towards the center
of the solar system. As they
get nearer and nearer to the
Sun their ices begin to vaporize
and the comet’s nucleus
becomes surrounded by a
shell of gas and dust (called
the “coma”). The Sun’s
energy often causes the gas
and dust from the coma to
fan out in long streamers
to create a tail millions of
miles long. Usually there
are two tails: one is formed
from the gas and dust while
the other is a stream of ionized
gas (“ionized” means
that electrons have been
stripped from the atoms of
the various gases converting
a neutrally charged atom
into one that carries an electrical
charge). Sometimes
there may even be multiple
tails. The dust tail follows
behind the comet’s direction
of travel but the ion tail is
always pointed away from
the Sun as it interacts with
the solar wind.
Comets are notoriously
unpredictable when it
comes to forecasting how
bright they may become to
our eyes here on Earth. On
rare occasions they may
become so bright they may
even be seen during the
day, at other times a comet
predicted to become bright
enough to be seen with the
unaided eye simply falls
apart as it thaws out near
the Sun and we never see
anything at all.
The good news is that
we now have a comet literally
on our horizon and
just in time for the holiday
season! Its name is Comet
Catalina (C/2013 US10) and
was discovered as part of
the University of Arizona’s
Catalina Sky Survey project
on Halloween of 2013.
On November 15th of this
year Comet Catalina made
a spin around the Sun and,
after travelling for millions of
years into the inner reaches
of the solar system, it is now
on a trajectory that will take
it out into interstellar space,
never to be seen by human
eyes again.
We don’t know the size
of the nucleus but it is estimated
to be anywhere from
2 to 12 miles across and is
barreling along at around
103,000 mph. Right now
Comet Catalina is an object
in our predawn sky, low
upon the eastern horizon, after
the New Year it will move
into our evening sky. It has
been hovering right around
naked eye visibility and currently
requires binoculars or
a small telescope to be seen.
We hope that it will brighten
up to where it can be seen
with the unaided eye but,
honestly, it could go either
way at this point. To keep
track of the comet’s status
and whereabouts be sure
to check with web sites like
EarthSky.org and skyandtelescope.
com. These sites will
also provide you with maps
to help locate it.
So, Comet Catalina will
not become one of the Great
Comets like Halley’s or Hale-
Bopp and if you are lucky
enough to spot it then it
will look like a fuzzy smear
of light. But the important
thing is to enjoy the thrill of
the hunt and the pleasure
that you take in understanding
what it is that you are
looking at: a frozen relic
from the creation of the solar
system, surrounded in a
tenuous atmosphere of gas
and dust as it thaws out near
the Sun, and slowly making
its long and lonely voyage
out among the stars.