Halley's Comet

A comet is a bright object with a long tail that travels around the sun. Comets are two types- short period comets and long period comets. Short period comets are those whose period of revolution around sun is less than 200 years. Among short period comets Halley’s Comet is the most famous one. This comet is visible from earth for every 75 to 76 years. This is clearly visible to naked eye maximum twice in a human lifetime. Other comets are brighter and spectacular but only appear for thousands of years.

There were recordings of Halley’s Comet even from 240 BC. People of that period predicted that it was a messenger sent by god. Some scholars who made research in early ages concluded that it was an object which is far beyond moon and believed that it travels in a straight line through solar system.

As time passed in 1705 Edmond Halley, scientist and a friend of Sir Isaac Newton stated that the comet which appeared in 1682 would again appear in 1758. He also concluded that this comet orbits around sun periodically and calculated its timings using Newton’s laws of motion which were recently published at that time. The comet was named after him as HALLEY’S COMET which did appear in 1758 as he mentioned.

The recent sightings of Halley’s Comet are recorded in the years 1910 and 1986. In 1986 several spacecraft’s were sent into space to have clear view of the comet. The main solid body of the comet is called nucleus. These space craft’s succeeded

in obtaining photographs of the nucleus. After this research scientists discovered and understood several interesting facts the composition, orbit and journey of Halley’s Comet.

Halley’s Comet is large and active. It has well defined and regular orbit. It orbits the sun in clockwise direction from above the North Pole. This orbit is highly elliptical. Due to its high eccentricity it has one of the highest velocities relative to earth or any other object in solar system. Its Perihelion, the distance of nearest orbital point from sun is 0.6 AU and its Aphelion, the distance of farthest orbital point from sun is 35 AU, which is almost equal to the distance of Pluto.

The researches on the appearance of the comet revealed that it is largely composed of dusty, non volatile materials and small portion of volatile compounds like dusty ice, carbon dioxide, carbon monoxide and water. When the Halley’s Comet nears the sun its volatile components sublimes from surface.The evaporation of this dirty ice releases dust particles which travel with gases away from the nucleus in the opposite direction of its motion and it appears as a tail. That is the reason why the length of the tail increases when it moves towards the sun and decreases while moving away from sun.

Over centuries the period of appearances of Halley’s Comet varied from 75 to 79 years. Its average orbital period is 76 years. But it is still very difficult to calculate the dates when it returns. Since its orbit is affected by the gravity of other bodies in the solar system. Scientists concluded that it will return to inner solar system in 2061 and its closest approach to sun (perihelion passage) will be early in 2062 as the recent appearance was in the year 1986.

The projected lifetime of Halley’s Comet could be 10 million years. Scientists even predicted the future of this comet. They suggested that Halley’s Comet will get evaporated or split into two in next thousands of yearsor even it can get ejected from solar system as its mass is reducing by revolutions.

- P. PRATIBHA

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