Starting in astronomy
beginner’s guide to stargazing
By Mike Barrett
Comets are unpredictable beasts
The media was full of
dislodged from their orbits
talk of comets with the
by gravitational forces
inaugural visit of Comet
or collisions with other
ISON C/2012 S1. It was
bodies and start their
pre-hyped as being “The
journey diving towards
Comet of the Century”,
the Sun. Some comets
being visible in daylight
will only visit the sun once
etc, etc. The reality of the
before being catapulted
matter is that comets are
out of the solar system,
strange beasts and are
others will have periodic
rarely predictable unless
elliptical orbits returning
they have been round a
to pass the Sun at regular
few times.
intervals.
Comet
ISON
was
In the outer Solar System,
spotted a long way out
where the temperature is
in the solar system and
very cold, the comet will
had been tracked all the
travel as a frozen ball. As
time it was approaching
the comet nears the Sun it
the Sun. This meant that
starts to melt and consists
it was reasonably easy to
of four distinct parts: the
predict its trajectory as it
nucleus; a coma; an
came flying in towards
ionised gas tail and a dust
the Sun. The first thing
tail. The nucleus is the
that was realised was This image from NASA’s Solar Dynamics Observatory shows the Sun, ‘dirty snowball’ of rock,
that this would be a Sun- but no Comet ISON was visible. A white plus sign shows where the ice and dust. The coma is
Image: NASA/SDO an ‘atmosphere’ formed
grazing comet coming Comet should have been had it survived.
within one Sun’s width of
blocks of life were deposited by a
from the evaporation
our primary star. This was special passing comet. Generally there is of the ices into liquids which will
for a number of reasons, but most more ice than solid components contain suspended dust particles.
especially because we now have in a comet, though some comets The dust tail is a stream of dust
so many instruments that are able can contain more solids than ice. A particles that have been liberated
to observe the Sun and have the suitable analogy to an earthbound from the coma and are left trailing
capability to view the interaction structure would be that of a dirty behind the comet pushed away
between the comet and the Sun.
snowball and these comets are by the solar winds. The ion tail is
But we are getting ahead often described as such.
formed from electrically charged
of ourselves, first we need to
A comet can range in size from gas molecules that are ejected
understand what comets are and a few hundred feet to a tens of from the coma. The two tails take
where they came from: A comet miles. Comet ISON is believed to be slightly different paths as the
is a small icy body composed between one and two miles across, ionised gasses are lighter and more
of rock, ice, and dust. The ice is but it is very difficult to determine as susceptible to the solar wind than
created not only from water, but the coma obscures the nucleus.
the dust tail. Thus the ion tail will be
also from gasses such as oxides of
Comets are believed to originate straight and narrow where the dust
carbon, etc and many organic in either the Oort Cloud or the tail will curved and wide.
compounds. It has been proposed Kuiper Belt where the residue of
Getting back to Comet ISON.
that some of the water on Earth planetary construction materials As the comet approached the
and also possibly the building are now found. These are inner Solar System it started to
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