Cloud made of comets, Science

A Cloud Made of Comets

The cold outer areas beyond Pluto are the regions of  comets, those visitors that dash, around the Sun, seldom to be seen again. Comets are of grei3t  interest because they are the relics of the early history of the Solar System. When solar matter was churning and the Sun had just  been ignited, its heat drove  ,the  lighter elements into the outer reaches of  the Solar Syst.em. Hydrogen, oxygen, nitrogen and carbon collected into something like snowy cotton balls and they still float as a "cloud",  at a disiance of 100.000 times the distance of the Earth from the sun! This thin cloud of comets  reaches out to a distance halfway to  the nearest star, i.e. almost a few billion kilometers. Comets are made mainly of  'ices',  that is ordinary water ice mixed with frozen gases such as methane, carbon dioxide and ammonia.

The ices are mixed with specks of dust that makes them look like dirty snowballs. Billions of comets stay in their great cloud, moving slowly in enormous orbits around the Sun. While still in this cloud, comets do not shine. Once in a while the gravity of a passing star disccrbs this cloud. A few comets then move into interstellar space and are  lost to the Solar System. Others move towards the Sun. Seen from  the Earth, the comets shinc more brightly  than anything in the sky, except the Sun and the Moon. You may wonder how dirty looking snowballs turn into bright long-tailed comets. As a comet moves towards the Sun, its surface is warmed by the strengthening sunlight. Some of  its frozen material  turns into gas, forming a rapidly growing cloud called its head or coma, around its centre. On getting nearer to  the Sun, more gas evaporates and its head becomes bigger and brighter. Also, a brilliant  tail, made of  dust and gas, is pushed out of the head by  the pressure of  the sunlight and the solar wind.

This tail extends in a direction away from the Sun. If comets pass very close to the Sun,  they acquire enormous speeds, more than a million kilometers per hour, and move off  into space with their tails pointing ahead. Most comets depart on long elliptical orbits, billions of kilometres into deep space, ana remain there for thousands of years. But a few do  not escape from the Sun so easily. If they happen to pass near a large planet, particularly Jupiter, its gravitation  pushes them into short-period orbits around the Sun. One of  the most famous comets is Halley's comet which returns once every 74 to 79 years (Fig.). It was last seen in  1986. Sometimes fragments from the comets fall on the Earth producing meteors. Comets returning repeatedly  lose their gases each time. When  all their ices melt,  comets disintegrate, leaving a stream of small particles  that spreads out thinly and loses its identity.  

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Posted Date: 9/28/2012 2:43:59 AM | Location : United States

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