A comet is classified as an icy body that releases gas or dust. Most comets visible from Earth appear to travel in long ovals around the sun. Comets consist of a nucleus bounded by a fuzzy or cloudy atmosphere and may have a tail or two. The gases in the comet and the comet's tail are bright and reflective, allowing us to see comets better when they are closer to the sun. Astronomers also believe that comets release the energy they store from the sun, which makes them shine. Astronomers believe that comets are leftover ice, rocks and gases from whatever shaped the planets billions of years ago. They also believe that comets are closely linked to life on Earth, because they bring some of the water and carbon-based molecules that fuel life on land. When astronomers think about the comet's nucleus, they think of a dirty snowball. It's basically a ball of ice and rock dust particles. The coma forms when the sun moves closer to the sun and some of the ice vaporizes, forming the dust cloud around the comet. Solar radiation also pushes dust particles away from the comet and this is how the tail is formed. Each time a comet orbits and arrives in the inner circle of the solar system, it melts a little more and eventually some comets will fail to melt completely. Astronomers classify comets into two groups. Short and long period comets due to the time they take to orbit the sun. Short-period comets take less than two hundred years, while long-period comets take two hundred years or more. Astronomers doubt that short-period comets originate from the Kuiper Belt, which is essentially a band of objects near Pluto's orbit. They believe that the gravitational pull of planets can cause an object to be... middle of paper ...... a short-range reflection where space probes fly a short distance from asteroids and collect data detailed. There is also a persistent theory that perhaps an asteroid crashed into the earth, causing climate and environmental changes that caused the dinosaurs to become extinct. The asteroid is believed to have created a huge depression in Mexico's Yucatan Peninsula. Asteroids are classified into several types according to their spectrum. Type C includes more than 75% of known asteroids: extremely dark; similar to carbonaceous chondrite meteorites; approximately the same chemical arrangement as the Sun minus hydrogen, helium and other volatiles; Type S, seventeen percent: relatively bright; metallic nickel-iron with iron and magnesium silicates; Type M, most others: bright; pure nickel-iron. There are also a dozen additional rare types.