What are Comets?

Comets are the most wonderful and dreadful members of the solar system. In the ancient times Comets were considered as warning of God and cause of death of monarch, disasters, plague and famine etc. Comet frightened the people for many centuries but actually Comets are very harmless and powerless.

Comets are small solar system bodies and orbit the sun in highly elliptical orbits. Comets are generally made up rocks along with ice and dust. Comet consists of three parts a small nucleus, a tenuous and bright coma and a long tail. Nucleus of comet is very small and ranges from 100 m to 40 km in size and due small size of comets they are unable to make their shape spherical and found in irregular shapes.

The nucleus of comet is composed of rock, dust, water ice and frozen gases like carbon monoxide, carbon dioxide, ammonia and methane. Observation of rocky surface suggests that the water ice is hidden beneath the crust. The nucleus of a comet appears dark and has very low reflectivity. The dark color of the nucleus of comet is due to the presence of organic materials on the surface of nucleus. It was confirmed by NASA from the dust obtained from the comet that comet contains organic materials like methanol, hydrogen cyanide, formaldehyde, ethanol and ethane, and also some complex compounds like amino acids (glycine) and long chain hydrocarbon etc. Comets also contain the components of DNA and RNA (adenine, guanine and related organic molecules).

As the Comet is consist of water in the form of ice and also other volatile compound which remain in frozen state in outer solar system. When comet approaches the inner solar system, the volatile compounds start evaporating due to solar radiation and streams out of nucleus carrying away dust with them. The streamed out dust and gas form tenuous atmosphere around the nucleus which is called coma of a comet. The size of coma of comet may be very large up to the size of the Sun.

The coma of comet starts stretching due to pressure of solar radiation and solar wind, in this process an enormously large tail is usually created. Comet consists of two tails, one is dust tail and other is tail made up of ionised gases.

The dust tail of comet usually pointed in the direction opposite to the direction of velocity of Comet. The ionised gas tail of comet is pointed in the direction opposite to the sun due to pressure applied by the solar radiation and interaction of solar wind with ionised gases. The dust tail reflects the sunlight and the gas tail glows due to ionised gases.

Comet Lovejoy can be seen by naked eyes in the first week of January 2015. Credit : NASA/MSFC/MEO/Aaron Kingery

The length of comet tail may be very large up to 1 AU which is the distance between the sun and the planet Earth. As the brightly glowing ionised tail always point opposite to the sun, the comet tail point backward when the comet is approaching the sun and the comet tail point forward when the comet is going back.

Comets can be periodic or non-periodic. The non-periodic comet comes only one time and then goes back in deep space forever in hyperbolic or parabolic paths. The periodic comets are generally grouped in two categories, short period comets and long period comets.

Short period comets generally come from the region after the orbit of Neptune called Kuiper belt. The time period of these comets are usually less than 200 years and they are orbiting the sun in elliptical orbits the same plane in which planet moves. For example Halley Comet comes from the region of orbit of Neptune and has orbital period of 76 years.

Long periods comets come from the region called ootz cloud which is very far from the Sun at the distance of 50000 AU. The time periods of these Comets range from 200 to millions of years. Long period comets orbit the sun in highly eccentric orbits.

The comets orbits around the sun at these far places but do not come inside the planetary orbits but sometimes due to gravitational attraction of the planets, they get disturbed and start moving towards the sun and come inside the planetary orbits, As they come closure to the sun, comet tails start building up due to evaporation of volatile materials like water. The comets usually comes very close to sun at perihelion and then goes back towards their aphelion which is usually present beyond the orbit of the planet Neptune.

Comets cannot be observed easily due to their small size and low reflectivity of nucleus when they are in the outer solar system. Comets become visible by telescopes only when they enter into the inner solar system and they tail starts glowing. Most of the comets which do not come close to the Earth cannot be viewed with naked eyes. The Comets flying nearby Earth can be viewed with naked eye for some nights during their approach.