How long have comets been in existence?
According to the most widely believed theory, comets are
believed to be as old as the solar system itself. They are said to
be the remnants of planet formation, i.e., whatever did not join any
planet during formation got left over as comets. Which is why comets
and planets exhibit similar patterns of movement – in an orbit
around the sun. The reason for comets having extremely elongated
orbits is attributed to the gravitational pull of the planets.
Comets have been sighted by man since time
immemorial and till 1577, they were considered an atmospheric
phenomenon. Proof of man having sighted comets as early as the
eleventh century has been found in an illustration in a Bayeaux
Tapestry which shows a group of people pointing out to a body in the
sky with a blazing tail.
Early astronomers to have worked on comets
were Danish astronomer Tycho Brahe, who proved that comets were
celestial bodies, and British scientist Sir Isaac Newton, who in the
17th century demonstrated the similarities between movement of
comets and planets.
Naming of comets
It has been customary to name comets after the astronomers
who discovered them. And the name that comes to mind when
encountering the word ‘comet’, is almost always Halley’s comet.
Halley’s comet has been named after the British astronomer
Edmond Halley who showed the comet of 1682 to be identical with
two others. The other two had appeared in 1607 and 1531. He also
predicted that the comet would reappear in 1759 and time did
prove him true. In 1986, the Halley’s comet made another
appearance. It makes a reappearance every 76 years.
Comet Tempel-Tuttle, which is seen once
every 33 years, is named jointly after two astronomers Ernst
Wilhelm Liebrecht Tempel, who saw it in December 1865, and
Horace Parnell Tuttle, who saw it in January 1866, just a month
later than Tempel. But as both the discoveries were independent
of each other the comet was named after both of them.
What are comets made of and how big are
Frozen water and fine carbon dust are the main components of a
comet. The dust and gas, which form a comet, on getting heated by
the sun, create tails that are millions of times larger than the
nucleus of the comet. These tails are often large enough to be
visible from the earth.
Photographs of the Halleys comet taken in
1986 by the Giotto spacecraft (launched by the USSR in 1986) show a
nucleus at the core of the comet. The color of the nuclei was found
to be black. The nucleus appears to be covered by a crust made up of
fine dust particles. The size of the nuclei itself is quite small.
Observations of the Halley’s comet showed
its dimensions to be 15 kilometers by 8 kilometers. Its mass was
estimated to be 1017 grams. The time taken by a comet to
orbit the sun once – of many comets have also been calculated. For
instance, the Donati’s Comet takes two thousand years, while the
Comet Hyakutake takes ten thousand years.
Different types of comets
On the basis of their life span, comets are classified as
short period comets and long period comets. The short-period comets
have a life span of less than 200 years and the long-period ones
exist for more than 200 years. The US probe known as the
International Cometary Explorer, launched in 1985, was the first
attempt at investigating comets. The comet investigated by this
probe was Comet Giacobini-Zinner.