Nakshatra: Asteroids,Comets,and Meteoroids

. Asteroid 951

Asteroids,comets,and meteoroids are all debris remaining from the nebula in which the Solar System formed 4.6 billion years ago. Asteroids are rocky bodies up to about 1,000 kilometres in diameter,although most are much smaller. Most of them orbit the Sun in the asteroid belt,which lies between the orbit of Mars and Jupiter. Comets may originate in a huge cloud (called the Oort cloud) that is thought to surround the Solar System. They are made of frozen gases and dust,and a few kilometres in diameter. Occasionally,a comet is deflected from the Oort cloud Sun in a long,elliptical path As the comet approaches the Sun,the comet's surface starts to vaporize in the heat,producing a brightly shining coma (a huge sphere of gas and dust around the nucleus),a gas tail,and a dust tail. Meteoroids are small chunks of stone or stone and iron,some of which are fragments of asteroid or comets. Meteoroids range in size from tiny dust particles to objects tens of metres across. If a meteoroid enters the Earth's atmosphere,it is heated by friction and appears as a glowing streak of light called a meteor (also known as a shooting star).Meteor showers occur when the Earth passes through the trail of dust particles left by a comet. Most meteor burn up in the atmosphere. The few tht are large enough to reach the Earth's surface are termed meteorites. As we see about the comets its tail is the most distinctive feature. As a comet approaches the Sun it develops an enormous tail of luminous material that extends for millions of kilometers away from the Sun. When far from the Sun, a comet's nucleus is very cold and its material is frozen. Water ice, as well as other compounds such as carbon dioxide and carbon monoxide ice, may be found in the nucleus. This icy nucleus changes radically when a comet approaches the Sun. The intense solar wind from the Sun transforms the solid nucleus directly into a vapor, bypassing the liquid phase. This process is called sublimation. The vapor helps stir things up in the nucleus, forcing the core to form a cloud-like mixture of gas and dust around it, called the coma. There, sunlight and the solar wind interact with the ingredients, creating the tails. The ingredients in the coma determine the types and number of tails.

Some comets may appear to have no tails, but they really do. They are simply very faint. Scientists can identify these tails by using special filters that are sensitive to dust or gas emissions. Other comets, like Hale-Bopp, which could be seen from Earth in 1997, have very prominent tails. Although Hale-Bopp's tails could be seen visibly from Earth, scientists using sensitive cameras identified a much more complicated tail structure. One of these images revealed a long, curving dust tail. Other pictures showed dust and gas ion tails. There was even an image of a dust tail and two gas ion tails. The different tails provide scientists with important information about the internal chemistry and structure of a comet's nucleus.