Creators & Destroyers Of The Universe
Long ago, people thought that comets were bad omens that brought bad luck. They were terrified when a comet appeared in the sky; bright comets were considered signs of war, disease, and other disasters. Today, we recognize comets as members of our solar system. They travel along orbits, controlled by the Sun' s gravity. But comets are still puzzling, and no one can predict when a new one will appear. Billions of comets probably orbit the Sun beyond Pluto. Occasionally, a passing star may tug on a comet and send it plunging toward the Sun. If the comets pass near Jupiter, that planet' s strong gravity may put it into a new orbit. A brilliant comet with a shining head and a tail stretches halfway across the sky in an awesome sight. Indeed, comets caused considerable alarm in ancient times, as they were often regarded as harbingers of doom. Yet in comparison with a planet, a comet in flimsy and insubstantial: on several occasions the Earth has been know to pass unharmed through a comet' s tail. Comets have been aptly described as dirty ice-ball . The main nucleus is composed of rocky fragments held together with various frozen materials such as methane, ammonia, carbon dioxide and water. Surrounding the nucleus is the coma, or head, which is made up of dust and gas. The sun makes the comet visible by floodlighting the coma. When the comet nears the sun, the radiation pressure of the sunlight produces a tail that usually consists of both dust and gas. However, many faint comets do not develop a tail. A comet is very cold and shines by reflecting sunlight. A comet may appear in the sky any time. Some appear on predictable schedules. A spectacular comet may shine for weeks.
Comets probably come from the Oort Cloud and the Kuiper Belt. These aren' t places in a science fiction book! The Oort cloud, named after astronomer Jan Oort, is the oretically a loose shell of leftover matter from the solar system's formation. Approximately 98,000 astronomical units (Aus) thick, it encircles the sun beginning about 2,000 Aus away. The Kuiper Belt, theorized by astronomer Gerald Kuiper, is purported to be a band of planetesimals from the solar nebula that lies just beyond Neptune' s orbit. Some comets may originate there.
Comet Halley is the most famous. It has appeared in our sky every 76 years since at least 240 B.C. In 1985-1986, scientists from more than 50 countries used instruments on the ground and aboard spacecraft to gather more data than ever before. Comet Halley's nucleus measures about 9 by 5 miles (15 by 8 kilometers). About 5 new comets are discovered every year, but you can' t see most o f them without a telescope. When first seen in the distance, a comet looks like a faint blob of light. Then it brightens and grows a tail as it gets closer to the Sun.
The head of a comet. ( Below )
The head of a comet contains a nuclues,made of ice , rock and dust. The nuclues is surrounded by a cloud of dust and gas, called coma. Many comets develope two tails The gas tail always point away from the sun, while the dust tail always curves away.
Comet Shoemaker-Levy 9 discovered in 1993, was extremely unusual. The gravitional pull of the giant planet, Jupiter trapped this comet into an elongated orbit round Jupiter. On a close approach to Jupiter in 1992, tidal forces tore this comet into more than twenty separate pieces , each of which continued on its own orbit. The broken comet looked like a string of pearls. Towards the end of 1994, the fragments crashed head on into Jupiter, causing a spectacular changes in the atmosphere. This was the first time the astronomers could witness the effects of an impact between a comet and planet.
THe effects
of an impact between a comet and a planet.