Comets - relics of the Solar System
For a long time, comets were feared visitors to the night scene. Their sudden
unexpected arrival, their hazy mysterious appearance, and their long sweeping tails
bewildered and frightened superstitious people. Even educated people in the seventeenth
century believed comets to be bearers of ill-fortune. A comet apparition would bring
on dire warnings of famine, pestilence, war and witchcraft.
Sometimes a new comet gets very bright and is visible to the naked eye (such as
Comet McNaught in early 2007). New comets
are being discovered all the time - perhaps as many as 30 per year. Most of them are
visible only to observers with substantial telescopes. From the viewpoint of low-power
telescopes or binoculars, a bright comet has two main parts: the head and the tail. The
tail always points away from the Sun.
The comet's head consists of the "nucleus" and
the "coma". The nucleus is the actual solid object from which the coma and the tail
originate. It is believed that comet nuclei range in size from a few kilometres to a
few tens of kilometres, and resemble "dirty snowballs" made up of ices, dust, and
fragments of rocky material. The nucleus is so small that we do not see it from Earth.
The fuzzy head that we can see is the coma ("hair"): it is made up of gas and dust
boiled off the nucleus by the heat of the Sun. The width of the coma can be as much as
100,000 km. A comet can have two tails - the dust tail and the gas (or ion) tail; ions
are charged atoms such as CO+ and H2O+. The tail or tails can be millions of kilometres
in length!
Comets are part of our Solar System recently named small Solar System
bodies and are believed to come from a region
far beyond the orbits of Neptune and Pluto. From time to time a
comet will head in towards the Sun - we don't really know why. Some, like
Comet Halley go into periodic orbits: it orbits the Sun once every 76 years.
The most frequent cometary visitor is Comet Encke, which passes
by the Sun every 3.5 years. Unfortunately, it is too faint for naked-eye
observation.
Faint, non-periodic comets occur quite frequently, and pass through the
inner Solar System only once before vanishing back out into the
void of space. The study of comets gives vital clues to scientists
as to the composition of the Solar System at the time of its formation,
as most comets have never before been close to the Sun, where the
action of heat effects molecules that have remained unchanged for millions
of years.
